Puberty is the process of physical changes through which a child's body matures into an adult body capable of sexual reproduction. It is initiated by hormonal signals from the brain to the gonads: the ovaries in a girl, the testes in a boy. In response to the signals, the gonads produce hormones that stimulate libido and the growth and transformation of the brain, muscle, skin, hair and sex organs. Physical growth—height and weight—accelerates in the first half of puberty and is completed when an adult body has been developed; until the maturation of their reproductive capabilities, the pre-pubertal physical differences between boys and girls are the external sex organs. On average, girls begin puberty at ages 10–11 and complete puberty at ages 15–17; the major landmark of puberty for females is menarche, the onset of menstruation, which occurs on average between ages 12 and 13. For males, first ejaculation occurs on average at age 13. In the 21st century, the average age at which children girls, reach puberty is lower compared to the 19th century, when it was 15 for girls and 16 for boys.
This can be due to any number of factors, including improved nutrition resulting in rapid body growth, increased weight and fat deposition, or exposure to endocrine disruptors such as xenoestrogens, which can at times be due to food consumption or other environmental factors. Puberty which starts earlier than usual is known as precocious puberty, puberty which starts than usual is known as delayed puberty. Notable among the morphologic changes in size, shape and functioning of the pubertal body, is the development of secondary sex characteristics, the "filling in" of the child's body. Derived from the Latin puberatum, the word puberty describes the physical changes to sexual maturation, not the psychosocial and cultural maturation denoted by the term adolescent development in Western culture, wherein adolescence is the period of mental transition from childhood to adulthood, which overlaps much of the body's period of puberty. Comprehensive sexuality education can contribute to teenagers' better understanding of this process.
Two of the most significant differences between puberty in girls and puberty in boys are the age at which it begins, the major sex steroids involved, the androgens and the estrogens. Although there is a wide range of normal ages, girls begin puberty around ages 10–11 and end puberty around 15–17. Girls attain reproductive maturity about four years after the first physical changes of puberty appear. In contrast, boys accelerate more but continue to grow for about six years after the first visible pubertal changes. Any increase in height beyond the post-pubertal age is uncommon. For boys, the androgen testosterone is the principal sex hormone. A substantial product of testosterone metabolism in males is estradiol; the conversion of testosterone to estradiol depends on the amount of body fat and estradiol levels in boys are much lower than in girls. The male "growth spurt" begins accelerates more and lasts longer before the epiphyses fuse. Although boys are on average 2 centimetres shorter than girls before puberty begins, adult men are on average about 13 centimetres taller than women.
Most of this sex difference in adult heights is attributable to a onset of the growth spurt and a slower progression to completion, a direct result of the rise and lower adult male levels of estradiol. The hormone that dominates female development is an estrogen called estradiol. While estradiol promotes growth of the breasts and uterus, it is the principal hormone driving the pubertal growth spurt and epiphyseal maturation and closure. Estradiol levels reach higher levels in women than in men; the hormonal maturation of females is more complicated than in boys. The main steroid hormones, testosterone and progesterone as well as prolactin play important physiological functions in puberty. Gonadal steroidgenesis in girls starts with production of testosterone, quickly converted to estradiol inside the ovaries; however the rate of conversion from testosterone to estradiol during early puberty is individual, resulting in diverse development patterns of secondary sexual characteristics. Production of progesterone in the ovaries begins with the development of ovulatory cycles in girls, before puberty low levels of progesterone are produced in the adrenal glands of both boys and girls.
Puberty is preceded by adrenarche, marking an increase of adrenal androgen production between ages 6–10. Adrenarche is sometimes accompanied by the early appearance of pubic hair; the first androgenic hair resulting from adrenarche can be transient and disappear before the onset of true puberty. The onset of puberty is associated with high GnRH pulsing, which precedes the rise in sex hormones, LH and FSH. Exogenous GnRH pulses cause the onset of puberty. Brain tumors which increase GnRH output may lead to premature puberty; the cause of the GnRH rise is unknown. Leptin might be the cause of the GnRH rise. Leptin has receptors in the hypothalamus which synthesizes GnRH. Individuals who are deficient in leptin fail to initiate puberty; the levels of leptin increase with the onset of puberty, decline to adult levels when puberty is completed. The rise in GnRH might be caused by genetics. A study discovered that a mutation in genes encodi
The autoignition temperature or kindling point of a substance is the lowest temperature at which it spontaneously ignites in normal atmosphere without an external source of ignition, such as a flame or spark. This temperature is required to supply the activation energy needed for combustion; the temperature at which a chemical ignites decreases as the pressure or oxygen concentration increases. It is applied to a combustible fuel mixture; the ignition temperature of a substance is the least temperature at which the substance starts combustion. Substances which spontaneously ignite in a normal atmosphere at ambient temperatures are termed pyrophoric. Autoignition temperatures of liquid chemicals are measured using a 500-millilitre flask placed in a temperature-controlled oven in accordance with the procedure described in ASTM E659; when measured for plastics, autoignition temperature can be measured under elevated pressure and at 100% oxygen concentration. The resulting value is used as a predictor of viability for high-oxygen service.
The main testing standard for this is ASTM G72. The time t ig it takes for a material to reach its autoignition temperature T ig when exposed to a heat flux q ″ is given by the following equation: t ig = π 4 k ρ c 2, where k = thermal conductivity, ρ = density, c = specific heat capacity of the material of interest, T 0 is the initial temperature of the material. Temperatures vary in the literature and should only be used as estimates. Factors that may cause variation include partial pressure of oxygen, altitude and amount of time required for ignition; the autoignition temperature for hydrocarbon/air mixtures decreases with increasing molecular mass and increasing chain length. The autoignition temperature is higher for branched-chain hydrocarbons than for straight-chain hydrocarbons. Pyrolysis Fire point Flash point Gas burner Spontaneous combustion Analysis of Effective Thermal Properties of Thermally Thick Materials
The German occupation of north-east France refers to the period in which French territory along the Belgian and Luxembourgish border, was held under military occupation by the German Empire during World War I. Owing to the speed of the German invasion of Belgium in 1914, fighting reached French soil early in the war. Though their advance was stopped at the First Battle of the Marne in September 1914, the Germans gained control of a portion of French territory which remained under German occupation behind the stabilized Western Front for much of the rest of the war; the occupied zone represented about 3.7 percent of French landmass and included some of the most industrialized parts of the country. 64 percent of France's pig-iron production, 24 percent of its steel manufacturing and 40 percent of the total coal mining capacity was located in the zone, dealing a major setback to French industry. A number of important towns and cities were situated within it too, notably Lille, Cambrai, Valenciennes and Avesnes.
Because of its proximity to the front, occupied north-east France was ruled by the military, rather than by a civilian occupation administration. Economic exploitation of the occupied zone increased throughout the war. Forced labor became common as the war dragged on. Much of the 1928 novel Schlump by Hans Herbert Grimm is set in German-occupied France where the protagonist works in the occupation administration. Nord-Pas de Calais Mining Basin World War I reparations Wegner, Larissa. "Occupations during the War". International Encyclopedia of the First World War. Retrieved 6 October 2015. Kennedy, Paul; the Rise and Fall of the Great Powers. Vintage Books. ISBN 0-679-72019-7. McPhail, Helen; the Long Silence: The Tragedy of Occupied France in World War I. London: I. B. Tauris. ISBN 978-1784530532. Connolly, James. "Fresh Eyes, Dead Topic? Writing the History of the Occupation of Northern France in the First World War". In Broch, Ludivine. France in an Era of Global War, 1914-1945: Occupation, Politics and Entanglements.
London: Palgrave Macmillan. ISBN 9781137443489. Connolly, James. "Mauvaise Conduite: Complicity and Respectability in the Occupied Nord, 1914-1918". In De Schaepdrijver, Sophie. Military Occupations in First World War Europe. London: Routledge. ISBN 978-1138822368