CHEMISTRY. 367 



rises, it becomes cooler ; and the aqueous vapor which it contains, is 

 partly or wholly condensed, thus forming a cloud, or mist ; which, 

 by the aggregation of small drops, causes rain, or, if it be sufficiently 

 cold, hail, or snow. If the surface of the earth be cool enough to 

 condense the aqueous vapor in the contiguous air, it causes the depo- 

 sition of dew. The dew point, is that temperature at which the 

 condensation commences : and as it varies with the quantity of vapor 

 in the air, being the highest when there is the most vapor, it furnishes 

 the best hygrometer, or measure of the humidity of the atmosphere. 

 We regret the want of room to explain this subject more fully. 



CHAPTER V. 



CHEMISTRY. 



CHEMISTRY, is that branch of Acrophysics, which treats of the 

 composition of all ponderable substances ; their sensible properties 

 and mutual relations ; and the effects produced upon them by cohe- 

 sion, affinity, light, heat, and electricity. The name is derived from 

 the Arabic or Egyptian word kimia ; originally applied to Alchemy, 

 and signifying the occult science. The general properties of pon- 

 derable substances, or those which can be collected and weighed, 

 including solids, liquids, and gases, have already been referred to, in 

 the introduction to this department ; and the imponderables, as they 

 are often termed, have formed the subject of the two preceding 

 branches. Of course the study of Chemistry is aided by that of light, 

 heat, and electricity ; while it not only reflects light upon them in 

 return, but becomes itself subsidiary to the natural and medical 

 sciences which are to follow. The applications of Chemistry, extend 

 throughout the wide" range of the physical arts, wherever it is desira- 

 ble to change the state or composition of the materials employed : 

 and hence it ranks among the most useful of the sciences. 



The Greeks ascribed the invention of Chemical science to Hermes, 

 or Mercurius Trismegistus, called by the Egyptians, Thoyt ; and, 

 in honor of him, they gave to this branch the name of the Hermetic 

 art. It was brought from Egypt to Greece, about 460 B. C., by 

 Democritns ; who had learned to soften ivory, and to make glass, 

 If we except the working of the metals, little was known of this 

 science by the ancients ; and that little was obscured, rather than 

 enlarged, by the reveries of the earlier alchemists. Alchemy, appears 

 to have been first mentioned in the writings of Maternus, about 

 A. D. 330 ; and it was fully introduced into Europe by Rhazes, and 

 Geber, Arabian chemists of the ninth century. The object of Alche- 

 my, was to discover an imaginary substance, called the philosopher's 

 stone, elixir vitse, universal solvent, or grand catholicon ; which 

 would transmute all the other metals into gold, and prevent or cure 

 all diseases. This research, though of course a failure, led to several 

 important discoveries; as those of gunpowder, of sulphuric, nitric, 

 and muriatic acids, and of phosphorus, antimony, and zinc. 



