ATMOS PHE RE,that invisible elastic fluid which surrounds the earth, and encloses it on all sides. It received its name from the Greeks, in consequence of the vapours which are continually mixing with it. The ancients considered it as one of the four elements of which all things were composed, and some of them seem to have thought that it enters as a constituent principle into other bodies, or at least that air and other bodies are mutually convertible into each other. (Lucret. lib. v. 271.) No experiments on its na ture could well be made by the ancients, as they were unprovided with every instrument fitted for such in vestigations, and unacquainted with the principles upon which their construction depended. But it has occupied a great deal of the attention of modern phi losophers, and has given birth to sonic of the most brilliant discoveries that grace the annals of science. Its weight was first ascertained by Galileo, and ap plied by Torricclli to explain the rise of water in pumps, and of mercury in barometrical tubes, and by Paschal to the mensuration of the height of moun tains. Its elasticity was accurately determined by Boyle, who may be considered as in some measure the founder of the science of pneumatics. Halley and Newton explained the effects produced on it by moisture. Hooke, Newton, Boyle, Derharn, point ed out its relation to light, to sound, and to electri city. Its effect upon combustibles and animals was investigated by Boyle, Hooke, Mayow, Hales, Priest ley, Scheele, and Lavoisier. Its constituents were detected and measured by the experiments of Priest ley, Scheele, Lavoisier, and Cavendish. The effect of heat on it was determined by Shuckburgh,Dalton, and Gay Lussac. But it would be an endless task to enumerate all the philosophers who have distin guished themselves by their investigations of the at mosphere, a list which would include almost all the celebrated names of the last century.
From the experiments of Sir George Shuck burgh Evelyn, (Phil. Trans. 1777 and 1798,) made with a degree of precision and patient industry, which perhaps have never been surpassed, it appears, that at the temperature of 60°, when the barometer stands at 30 inches, the specific gravity of atmospherical air is 0.001208, that of water being 1.000, or its weight is to that of water as 1 to 828. Hence 100 cubic inches of it under that pressure and at that temperature weigh 30.5 grains : For a cubic inch of pure Avater at that tem perature weighs 252.506 grains, according to experi ments of Shuckburgh corrected by Mr Fletcher.
(Nicholson'sJournat, iv. 35.) The result of the expe riments of Lefevre Gineau, who was employed by the French government to ascertain the weight of water, in order to fix their standard of weights, was some what different. According to him, a cubic inch of water..at GO° weighs 252.72 grains troy. The dif ference may be partly owing to some small error in the allowance of the expansion of water from 40°, the temperature at which his experiments were made, to GO°. At any rate, the known precision, and the ex cellent apparatus of Sir George Shuckburgh, entitle his result to the preferente. Hawksbee's experiments make air 850 times lighter than water, the barometer being at 29.7, and Dr Halley supposed it about 800. But neither of these numbers is to be put in compe tition with the result of Sir George Shuckburgh given above. The air when weighed, is supposed to be in its usual state of dryness ; when very moist, its specific gravity is diminished. An exact knowledge of the weight of a given bulk of air is of great im portance, because it enables us with much facility to ascertain the weight of all other aerial bodies : for it is easy to determine the relative weight of any elastic fluid to that of air.
When heat is applied to atmospherical air, ite bulk increases ; while cold, on the other hand, dimi nishes its bulk. As this change in bulk is very con siderable, it affects very much the accuracy of all ex periments on it. It has therefore been an object with philosophers to determine the precise amount and rate of the change in bulk produced upon air by heat. M. De Luc, Sir George Shuckburgh Evelyn, Ge neral Roy, Mr Dalton, and Mr Gay Lussac, are the gentlemen to whom we are indebted for the solution of this problem. In examining the dilatability of air by heat, it is necessary that no water be in contact with it. For as heat converts water into vapour, this vapour mixing with the air, would destroy the accu racy of the results, and make the dilatation appear much greater than it really is. According to the ex periments of De Luc, air at the temperature of 55° when heated 1° of Fahrenheit's thermometer, ex pands part ; according to Shuckburgh, the ex pansion is according to General Roy, it is ; according to Dalton, it is ' ; and according to Gay Lussac, -6. As Dalton and Gay Lussac were at pains to exclude moisture, we may consider their ex periments as more accurate than those that preceded them. As to the rate of expansion, General Roy found it a slowly diminishing ratio from 32° to 212°.