A composite material is a material made from two or more constituent materials with different physical or chemical properties that, when combined, produce a material with characteristics different from the individual components. The individual components remain separate and distinct within the finished structure, differentiating composites from mixtures and solid solutions; the new material may be preferred for many reasons: common examples include materials which are stronger, lighter, or less expensive when compared to traditional materials. More researchers have begun to include sensing, actuation and communication into composites, which are known as Robotic Materials. Typical engineered composite materials include: Reinforced concrete and masonry Composite wood such as plywood Reinforced plastics, such as fibre-reinforced polymer or fiberglass Ceramic matrix composites Metal matrix composites and other Advanced composite materialsComposite materials are used for buildings and structures such as boat hulls, swimming pool panels, racing car bodies, shower stalls, storage tanks, imitation granite and cultured marble sinks and countertops.
The most advanced examples perform on spacecraft and aircraft in demanding environments. The earliest man-made composite materials were straw and mud combined to form bricks for building construction. Ancient brick-making was documented by Egyptian tomb paintings. Wattle and daub is one of the oldest man-made composite materials, at over 6000 years old. Concrete is a composite material, is used more than any other man-made material in the world; as of 2006, about 7.5 billion cubic metres of concrete are made each year—more than one cubic metre for every person on Earth. Woody plants, both true wood from trees and such plants as palms and bamboo, yield natural composites that were used prehistorically by mankind and are still used in construction and scaffolding. Plywood 3400 BC by the Ancient Mesopotamians. Cartonnage layers of linen or papyrus soaked in plaster dates to the First Intermediate Period of Egypt c. 2181–2055 BC and was used for death masks. Cob Mud Bricks, or Mud Walls, have been used for thousands of years.
Concrete was described by Vitruvius, writing around 25 BC in his Ten Books on Architecture, distinguished types of aggregate appropriate for the preparation of lime mortars. For structural mortars, he recommended pozzolana, which were volcanic sands from the sandlike beds of Pozzuoli brownish-yellow-gray in colour near Naples and reddish-brown at Rome. Vitruvius specifies a ratio of 1 part lime to 3 parts pozzolana for cements used in buildings and a 1:2 ratio of lime to pulvis Puteolanus for underwater work the same ratio mixed today for concrete used at sea. Natural cement-stones, after burning, produced cements used in concretes from post-Roman times into the 20th century, with some properties superior to manufactured Portland cement. Papier-mâché, a composite of paper and glue, has been used for hundreds of years; the first artificial fibre reinforced plastic was bakelite which dates to 1907, although natural polymers such as shellac predate it. One of the most common and familiar composite is fibreglass, in which small glass fibre are embedded within a polymeric material.
The glass fibre is strong and stiff, whereas the polymer is ductile. Thus the resulting fibreglass is stiff, strong and ductile. Concrete is the most common artificial composite material of all and consists of loose stones held with a matrix of cement. Concrete is an inexpensive material, will not compress or shatter under quite a large compressive force. However, concrete cannot survive tensile loading. Therefore, to give concrete the ability to resist being stretched, steel bars, which can resist high stretching forces, are added to concrete to form reinforced concrete. Fibre-reinforced polymers s include glass-reinforced plastic. If classified by matrix there are thermoplastic composites, short fibre thermoplastics, long fibre thermoplastics or long fibre-reinforced thermoplastics. There are numerous thermoset composites, including paper composite panels. Many advanced thermoset polymer matrix systems incorporate aramid fibre and carbon fibre in an epoxy resin matrix. Shape memory polymer composites are high-performance composites, formulated using fibre or fabric reinforcement and shape memory polymer resin as the matrix.
Since a shape memory polymer resin is used as the matrix, these composites have the ability to be manipulated into various configurations when they are heated above their activation temperatures and will exhibit high strength and stiffness at lower temperatures. They can be reheated and reshaped without losing their material properties; these composites are ideal for applications such as lightweight, deployable structures. High strain composites are another type of high-performance composites that are designed to perform in a high deformation setting and are used in deployable systems where structural flexing is advantageous. Although high strain composites exhibit many similarities to shape memory polymers, their performance is dependent on the fibre layout as opposed to the resin content of the matrix. Comp
A molecule is an electrically neutral group of two or more atoms held together by chemical bonds. Molecules are distinguished from ions by their lack of electrical charge. However, in quantum physics, organic chemistry, biochemistry, the term molecule is used less also being applied to polyatomic ions. In the kinetic theory of gases, the term molecule is used for any gaseous particle regardless of its composition. According to this definition, noble gas atoms are considered molecules as they are monatomic molecules. A molecule may be homonuclear, that is, it consists of atoms of one chemical element, as with oxygen. Atoms and complexes connected by non-covalent interactions, such as hydrogen bonds or ionic bonds, are not considered single molecules. Molecules as components of matter are common in organic substances, they make up most of the oceans and atmosphere. However, the majority of familiar solid substances on Earth, including most of the minerals that make up the crust and core of the Earth, contain many chemical bonds, but are not made of identifiable molecules.
No typical molecule can be defined for ionic crystals and covalent crystals, although these are composed of repeating unit cells that extend either in a plane or three-dimensionally. The theme of repeated unit-cellular-structure holds for most condensed phases with metallic bonding, which means that solid metals are not made of molecules. In glasses, atoms may be held together by chemical bonds with no presence of any definable molecule, nor any of the regularity of repeating units that characterizes crystals; the science of molecules is called molecular chemistry or molecular physics, depending on whether the focus is on chemistry or physics. Molecular chemistry deals with the laws governing the interaction between molecules that results in the formation and breakage of chemical bonds, while molecular physics deals with the laws governing their structure and properties. In practice, this distinction is vague. In molecular sciences, a molecule consists of a stable system composed of two or more atoms.
Polyatomic ions may sometimes be usefully thought of as electrically charged molecules. The term unstable molecule is used for reactive species, i.e. short-lived assemblies of electrons and nuclei, such as radicals, molecular ions, Rydberg molecules, transition states, van der Waals complexes, or systems of colliding atoms as in Bose–Einstein condensate. According to Merriam-Webster and the Online Etymology Dictionary, the word "molecule" derives from the Latin "moles" or small unit of mass. Molecule – "extremely minute particle", from French molécule, from New Latin molecula, diminutive of Latin moles "mass, barrier". A vague meaning at first; the definition of the molecule has evolved. Earlier definitions were less precise, defining molecules as the smallest particles of pure chemical substances that still retain their composition and chemical properties; this definition breaks down since many substances in ordinary experience, such as rocks and metals, are composed of large crystalline networks of chemically bonded atoms or ions, but are not made of discrete molecules.
Molecules are held together by ionic bonding. Several types of non-metal elements exist only as molecules in the environment. For example, hydrogen only exists as hydrogen molecule. A molecule of a compound is made out of two or more elements. A covalent bond is a chemical bond; these electron pairs are termed shared pairs or bonding pairs, the stable balance of attractive and repulsive forces between atoms, when they share electrons, is termed covalent bonding. Ionic bonding is a type of chemical bond that involves the electrostatic attraction between oppositely charged ions, is the primary interaction occurring in ionic compounds; the ions are atoms that have lost one or more electrons and atoms that have gained one or more electrons. This transfer of electrons is termed electrovalence in contrast to covalence. In the simplest case, the cation is a metal atom and the anion is a nonmetal atom, but these ions can be of a more complicated nature, e.g. molecular ions like NH4+ or SO42−. An ionic bond is the transfer of electrons from a metal to a non-metal for both atoms to obtain a full valence shell.
Most molecules are far too small to be seen with the naked eye. DNA, a macromolecule, can reach macroscopic sizes, as can molecules of many polymers. Molecules used as building blocks for organic synthesis have a dimension of a few angstroms to several dozen Å, or around one billionth of a meter. Single molecules cannot be observed by light, but small molecules and the outlines of individual atoms may be traced in some circumstances by use of an atomic force microscope; some of the largest molecules are supermolecules. The smallest molecule is the diatomic hydrogen, with a bond length of 0.74 Å. Effective molecular radius is the size; the table of permselectivity for different substances contains examples. The chemical formula for a molecule uses one line of chemical element symbols and sometimes al
Hygroscopy is the phenomenon of attracting and holding water molecules from the surrounding environment, at normal or room temperature. This is achieved through either absorption or adsorption with the adsorbing substance becoming physically changed somewhat; this could be an increase in volume, boiling point, viscosity, or other physical characteristic or property of the substance, as water molecules can become suspended between the substance's molecules in the process. The word hygroscopy uses combining forms of hygro- and -scopy. Unlike any other -scopy word, it no longer refers to a viewing or imaging mode, it did begin that way, with the word hygroscope referring in the 1790s to measuring devices for humidity level. These hygroscopes used materials, such as certain animal hairs, that appreciably changed shape and size when they became damp; such materials were said to be hygroscopic because they were suitable for making a hygroscope. Though, the word hygroscope ceased to be used for any such instrument in modern usage.
But the word hygroscopic lived on, thus hygroscopy. Nowadays an instrument for measuring humidity is called a hygrometer. Hygroscopic substances include cellulose fibers, caramel, glycerol, wood, sulfuric acid, many fertilizer chemicals, many salts, a wide variety of other substances. If a compound absorbs enough moisture so that it dissolves it is classed as hydrophilic. Zinc chloride and calcium chloride, as well as potassium hydroxide and sodium hydroxide, are so hygroscopic that they dissolve in the water they absorb: this property is called deliquescence. Not only is sulfuric acid hygroscopic in concentrated form but its solutions are hygroscopic down to concentrations of 10% v/v or below. A hygroscopic material will tend to become cakey when exposed to moist air; because of their affinity for atmospheric moisture, hygroscopic materials might require storage in sealed containers. When added to foods or other materials for the express purpose of maintaining moisture content, such substances are known as humectants.
Materials and compounds exhibit different hygroscopic properties, this difference can lead to detrimental effects, such as stress concentration in composite materials. The volume of a particular material or compound is affected by ambient moisture and may be considered its coefficient of hygroscopic expansion or coefficient of hygroscopic contraction —the difference between the two terms being a difference in sign convention. Differences in hygroscopy can be observed in plastic-laminated paperback book covers—often, in a moist environment, the book cover will curl away from the rest of the book; the unlaminated side of the cover absorbs more moisture than the laminated side and increases in area, causing a stress that curls the cover toward the laminated side. This is similar to the function of a thermostat's bi-metallic strip. Inexpensive dial-type hygrometers make use of this principle using a coiled strip. Deliquescence is the process by which a substance absorbs moisture from the atmosphere until it dissolves in the absorbed water and forms a solution.
Deliquescence occurs when the vapour pressure of the solution, formed is less than the partial pressure of water vapour in the air. While some similar forces are at work here, it is different from capillary attraction, a process where glass or other solid substances attract water, but are not changed in the process; the amount of moisture held by hygroscopic materials is proportional to the relative humidity. Tables containing this information can be found in many engineering handbooks and is available from suppliers of various materials and chemicals. Hygroscopy plays an important role in the engineering of plastic materials; some plastics are hygroscopic. The seeds of some grasses have hygroscopic extensions that bend with changes in humidity, enabling them to disperse over the ground. An example is Needle-and-Thread, Hesperostipa comata; each seed has an awn. Increased moisture causes it to untwist, upon drying, to twist again, thereby drilling the seed into the ground. Thorny dragons collect moisture in the dry desert via nighttime condensation of dew that forms on their skin and is channeled to their mouths in hygroscopic grooves between the spines of their skin.
Water collects in these grooves when it rains. Capillary action allows the lizard to suck in water from all over its body. Deliquescence, like hygroscopy, is characterized by a strong affinity for water and tendency to absorb moisture from the atmosphere if exposed to it. Unlike hygroscopy, deliquescence involves absorbing sufficient water to form an aqueous solution. Most deliquescent materials are salts, including calcium chloride, magnesium chloride, zinc chloride, ferric chloride, potassium carbonate, potassium phosphate, ferric ammonium citrate, ammonium nitrate, potassium hydroxide, sodium hydroxide. Owing to their high affinity for water, these substances are used as desiccants an application for concentrated sulfuric and phosphoric acids; these compounds are used in the chemical industry to remove the water produced by chemical reactions. Many engineering polymers are hygroscopic, including nylon, ABS, polycarbonate and poly. Other polyme
Moisture is the presence of a liquid water in trace amounts. Small amounts of water may be found, for example, in the air, in foods, in various commercial products. Moisture refers to the amount of water vapour present in the air. Control of moisture in products can be a vital part of the process of the product. There is a substantial amount of moisture in. Ranging in products from cornflake cereals to washing powders, moisture can play an important role in the final quality of the product. There are two main aspects of concern in moisture control in products: allowing too much moisture or too little of it. For example, adding some water to cornflake cereal, sold by weight, reduces costs and prevents it from tasting too dry, but adding too much water can affect the crunchiness of the cereal and the freshness because water content contributes to bacteria growth. Water content of some foods is manipulated to reduce the number of calories. Moisture has different effects on different products, influencing the final quality of the product.
Wood pellets, for instance, are made by taking remainders of wood and grinding them to make compact pellets, which are sold as a fuel. They need to have a low water content for combustion efficiency; the more moisture, allowed in the pellet, the more smoke that will be released when the pellet is burned. The need to measure water content of products has given rise to a new area of science, aquametry. There are many ways to measure moisture in products, such as different wave measurement, electromagnetic fields, capacitive methods, the more traditional weighing and drying technique. Damp Liquid Dry matter Meteorology EMF measurements Microbiology Water content Humidor Electromagnetic Mixing Theory
Sulfuric acid known as vitriol, is a mineral acid composed of the elements sulfur and hydrogen, with molecular formula H2SO4. It is a colorless and syrupy liquid, soluble in water, in a reaction, exothermic, its corrosiveness can be ascribed to its strong acidic nature, and, if at a high concentration, its dehydrating and oxidizing properties. It is hygroscopic absorbing water vapor from the air. Upon contact, sulfuric acid can cause severe chemical burns and secondary thermal burns. Sulfuric acid is a important commodity chemical, a nation's sulfuric acid production is a good indicator of its industrial strength, it is produced with different methods, such as contact process, wet sulfuric acid process, lead chamber process and some other methods. Sulfuric acid is a key substance in the chemical industry, it is most used in fertilizer manufacture, but is important in mineral processing, oil refining, wastewater processing, chemical synthesis. It has a wide range of end applications including in domestic acidic drain cleaners, as an electrolyte in lead-acid batteries, in various cleaning agents.
Although nearly 100% sulfuric acid can be made, the subsequent loss of SO3 at the boiling point brings the concentration to 98.3% acid. The 98.3% grade is more stable in storage, is the usual form of what is described as "concentrated sulfuric acid". Other concentrations are used for different purposes; some common concentrations are: "Chamber acid" and "tower acid" were the two concentrations of sulfuric acid produced by the lead chamber process, chamber acid being the acid produced in the lead chamber itself and tower acid being the acid recovered from the bottom of the Glover tower. They are now obsolete as commercial concentrations of sulfuric acid, although they may be prepared in the laboratory from concentrated sulfuric acid if needed. In particular, "10M" sulfuric acid is prepared by adding 98% sulfuric acid to an equal volume of water, with good stirring: the temperature of the mixture can rise to 80 °C or higher. Sulfuric acid reacts with its anhydride, SO3, to form H2S2O7, called pyrosulfuric acid, fuming sulfuric acid, Disulfuric acid or oleum or, less Nordhausen acid.
Concentrations of oleum are either expressed in terms of % SO3 or as % H2SO4. Pure H2S2O7 is a solid with melting point of 36 °C. Pure sulfuric acid has a vapor pressure of <0.001 mmHg at 25 °C and 1 mmHg at 145.8 °C, 98% sulfuric acid has a <1 mmHg vapor pressure at 40 °C. Pure sulfuric acid is a viscous clear liquid, like oil, this explains the old name of the acid. Commercial sulfuric acid is sold in several different purity grades. Technical grade H2SO4 is impure and colored, but is suitable for making fertilizer. Pure grades, such as United States Pharmacopeia grade, are used for making pharmaceuticals and dyestuffs. Analytical grades are available. Nine hydrates are known, but four of them were confirmed to be tetrahydrate and octahydrate. Anhydrous H2SO4 is a polar liquid, having a dielectric constant of around 100, it has a high electrical conductivity, caused by dissociation through protonating itself, a process known as autoprotolysis. 2 H2SO4 ⇌ H3SO+4 + HSO−4The equilibrium constant for the autoprotolysis is Kap = = 2.7×10−4The comparable equilibrium constant for water, Kw is 10−14, a factor of 1010 smaller.
In spite of the viscosity of the acid, the effective conductivities of the H3SO+4 and HSO−4 ions are high due to an intramolecular proton-switch mechanism, making sulfuric acid a good conductor of electricity. It is an excellent solvent for many reactions; because the hydration reaction of sulfuric acid is exothermic, dilution should always be performed by adding the acid to the water rather than the water to the acid. Because the reaction is in an equilibrium that favors the rapid protonation of water, addition of acid to the water ensures that the acid is the limiting reagent; this reaction is best thought of as the formation of hydronium ions: H2SO4 + H2O → H3O+ + HSO−4 Ka1 = 2.4×106 HSO−4 + H2O → H3O+ + SO2−4 Ka2 = 1.0×10−2 HSO−4 is the bisulfate anion and SO2−4 is the sulfate anion. Ka1 and Ka2 are the acid dissociation constants; because the hydration of sulfuric acid is thermodynamically favorable and the affinity of it for water is sufficiently strong, sulfuric acid is an excellent dehydrating agent.
Concentrated sulfuric acid has a powerful dehydrating property, removing water from other chemical compounds including sugar and other carbohydrates and producing carbon and steam. In the laboratory, this is demonstrated by mixing table sugar into sulfuric acid; the sugar changes from white to dark brown and to black as carbon is formed. A rigid column of black, porous carbon will emerge as well; the carbon will smell of caramel due to the heat generated. C 12 H 22 O 11 ⏞ sucrose → H 2 SO 4 12 C + 11 H 2
Caramel is a medium to dark-orange confectionery product made by heating a variety of sugars. It can be used as a flavoring in puddings and desserts, as a filling in bonbons, or as a topping for ice cream and custard; the process of caramelization consists of heating sugar to around 170 °C. As the sugar heats, the molecules break down and re-form into compounds with a characteristic color and flavor. A variety of candies and confections are made with caramel: brittles, pralines, flan, crème brûlée, crème caramel, caramel apples. Ice creams sometimes contain swirls of caramel; the English word comes from French caramel, borrowed from Spanish caramelo, itself from Portuguese caramel. Most that comes from Late Latin calamellus'sugar cane', a diminutive of calamus'reed, cane', itself from Greek κάλαμος. Less it comes from a Medieval Latin cannamella, from canna'cane' + mella'honey'; some dictionaries connect it to an Arabic kora-moħalláh'ball of sweet'. Caramel sauce is made by mixing caramelized sugar with cream.
Depending on the intended application, additional ingredients such as butter, fruit purees, liquors, or vanilla can be used. Caramel sauce is used in a variety of desserts, though most notably as a topping for ice cream; when it is used for crème caramel or flan, it is known as clear caramel and only contains caramelized sugar and water. Butterscotch sauce is made with dark brown sugar, a splash of whiskey. Traditionally, butterscotch is a hard candy more in line with a toffee, with the suffix "scotch" meaning "to score". Toffee, sometimes called "caramel candy", is a soft, chewy candy made by boiling a mixture of milk or cream, glucose and vanilla; the sugar and glucose are heated separately to reach 130 °C. The mixture is stirred and reheated until it reaches 120 °C. Upon completion of cooking, vanilla or any additional flavorings and salt are added. Adding the vanilla or flavorings earlier would result in them burning off at the high temperatures. Adding salt earlier in the process would result in inverting the sugars as they cooked.
Alternatively, all ingredients may be cooked together. In this procedure, the mixture is not heated above the firm ball stage, so that caramelization of the milk occurs; this temperature is not high enough to caramelize sugar and this type of candy is called milk caramel or cream caramel. Salted caramel is a variety of caramel produced in the same way as regular caramel, but with larger amounts of salt added during preparation. Utilised in desserts, the confection has seen wide use elsewhere, including in hot chocolate and spirits such as vodka. A study conducted in 2017 by the University of Florida suggested that the popularity of salted caramel is due to its chemical composition, as all of its main ingredients have effects on the reward systems of the human brain, resulting in a process described as "hedonic escalation". Caramel colouring, a dark, bitter liquid, is the concentrated product of near total caramelization, used commercially as food and beverage colouring, e.g. in cola. Caramelization is the removal of water from a sugar, proceeding to isomerization and polymerization of the sugars into various high-molecular-weight compounds.
Compounds such as difructose anhydride may be created from the monosaccharides after water loss. Fragmentation reactions result in low-molecular-weight compounds that may be volatile and may contribute to flavor. Polymerization reactions lead to larger-molecular-weight compounds that contribute to the dark-brown color. In modern recipes and in commercial production, glucose or invert sugar is added to prevent crystallization, making up 10%–50% of the sugars by mass. "Wet caramels" made by heating sucrose and water instead of sucrose alone produce their own invert sugar due to thermal reaction, but not enough to prevent crystallization in traditional recipes. Four and six tenths tablespoons of commercially prepared butterscotch or caramel topping contain: Calories: 103 Protein: 0.62 Total lipids: 0.04 Carbohydrates, by difference: 27.02 Fiber, total dietary: 0.4 Cholesterol: 0.0 Media related to Caramel at Wikimedia Commons
Wood is a porous and fibrous structural tissue found in the stems and roots of trees and other woody plants. It is an organic material, a natural composite of cellulose fibers that are strong in tension and embedded in a matrix of lignin that resists compression. Wood is sometimes defined as only the secondary xylem in the stems of trees, or it is defined more broadly to include the same type of tissue elsewhere such as in the roots of trees or shrubs. In a living tree it performs a support function, enabling woody plants to grow large or to stand up by themselves, it conveys water and nutrients between the leaves, other growing tissues, the roots. Wood may refer to other plant materials with comparable properties, to material engineered from wood, or wood chips or fiber. Wood has been used for thousands of years for fuel, as a construction material, for making tools and weapons and paper. More it emerged as a feedstock for the production of purified cellulose and its derivatives, such as cellophane and cellulose acetate.
As of 2005, the growing stock of forests worldwide was about 434 billion cubic meters, 47% of, commercial. As an abundant, carbon-neutral renewable resource, woody materials have been of intense interest as a source of renewable energy. In 1991 3.5 billion cubic meters of wood were harvested. Dominant uses were for building construction. A 2011 discovery in the Canadian province of New Brunswick yielded the earliest known plants to have grown wood 395 to 400 million years ago. Wood can be dated by carbon dating and in some species by dendrochronology to determine when a wooden object was created. People have used wood for thousands of years for many purposes, including as a fuel or as a construction material for making houses, weapons, packaging and paper. Known constructions using wood date back ten thousand years. Buildings like the European Neolithic long house were made of wood. Recent use of wood has been enhanced by the addition of bronze into construction; the year-to-year variation in tree-ring widths and isotopic abundances gives clues to the prevailing climate at the time a tree was cut.
Wood, in the strict sense, is yielded by trees, which increase in diameter by the formation, between the existing wood and the inner bark, of new woody layers which envelop the entire stem, living branches, roots. This process is known as secondary growth; these cells go on to form thickened secondary cell walls, composed of cellulose and lignin. Where the differences between the four seasons are distinct, e.g. New Zealand, growth can occur in a discrete annual or seasonal pattern, leading to growth rings. If the distinctiveness between seasons is annual, these growth rings are referred to as annual rings. Where there is little seasonal difference growth rings are to be indistinct or absent. If the bark of the tree has been removed in a particular area, the rings will be deformed as the plant overgrows the scar. If there are differences within a growth ring the part of a growth ring nearest the center of the tree, formed early in the growing season when growth is rapid, is composed of wider elements.
It is lighter in color than that near the outer portion of the ring, is known as earlywood or springwood. The outer portion formed in the season is known as the latewood or summerwood. However, there are major differences, depending on the kind of wood; as a tree grows, lower branches die, their bases may become overgrown and enclosed by subsequent layers of trunk wood, forming a type of imperfection known as a knot. The dead branch may not be attached to the trunk wood except at its base, can drop out after the tree has been sawn into boards. Knots affect the technical properties of the wood reducing the local strength and increasing the tendency for splitting along the wood grain, but may be exploited for visual effect. In a longitudinally sawn plank, a knot will appear as a circular "solid" piece of wood around which the grain of the rest of the wood "flows". Within a knot, the direction of the wood is up to 90 degrees different from the grain direction of the regular wood. In the tree a knot is either the base of a dormant bud.
A knot is conical in shape with the inner tip at the point in stem diameter at which the plant's vascular cambium was located when the branch formed as a bud. In grading lumber and structural timber, knots are classified according to their form, size and the firmness with which they are held in place; this firmness is affected by, among other factors, the length of time for which the branch was dead while the attaching stem continued to grow. Knots materially affect cracking and warping, ease in working, cleavability of timber, they are defects which weaken timber and lower its value for structural purposes where strength is an important consideration. The weakening effect is much more serious when timber is subjected to forces perpendicular to the grain and/or tension than when under load along the grain and/or compression; the extent to which knots affect the strength of a beam depends upon their position, size and condition. A knot on the upper side is compressed. If there is a season check