234 PK0GRE8S IN CHEMISTRY, 



the senses of sig-ht, smell, or taste, and though it can })e felt, yet it 

 eludes our grasp. The word gas, moreover, was not invented until 

 Van Helmont devised it to desig-nate various kinds of ''airs'" which he 

 had observed. The important part which g-ases play in the constitu- 

 tion of many chemical compounds was accordingly overlooked, and, 

 indeed, it appeared to be almost as striking a feat of necromancy to 

 produce a quantity of gas of great volume from a small pinch of solid 

 powder as for a "Jinn" of enormous stature but of delicate texture 

 to issue from a brass pot, as related in the Arabian Nights Entertain- 

 ments. Gradualh', however, it came to be recognized, not mereh^ that 

 gases have corporeal existence, but that they even possess weight. 

 This, though foreshadowed by Torricelli, Jean Rey, and others, was 

 first clearly proved by Black, professor of chemistry in Edin>)urgh, in 

 1762, through his masterly researches, as carbonic acid. 



The ignorance of the material nature of gases and of their weight 

 lies at the bottom of the phlogistic theory, a theory devised hy Stahl 

 about the 3'ear KJJH), to accoimt for the phenomena of combustion and 

 respiration and the recovery or ''reduction" of metals from their 

 ""earths" by heating Avith charcoal or allied bodies. According to 

 this inverted theory, a substance capable of burning was imagined to 

 contain more or less phlogiston, a principle which it parted with on 

 burning, leaving an earth deprived of phlogiston, or " dephlogisti- 

 cated" behind if a metal. This earth when heated with substances 

 rich in phlogiston, such as coal, wood, flour, and similar bodies, recov- 

 ered the phlogiston, which it had lost on burning, and with the added 

 phlogiston its metallic character. Other substances, such as phos- 

 phorus and sulphur, gave solids or acid liquids to which phlogiston 

 was not so easy to add; but even they could be rephlogisticated. On 

 this hypothesis, it was the earths, and such acid liquids as sulphuric or 

 phosphoric acids which were the elements; the metals and sulphur and 

 phosphorus were their compounds with phlogiston. 



DISCOVERY OF OXYGEN. 



The discovery of oxygen by Priestlej^ and by Scheele in 1774, and 

 the explanation of its functions by Lavoisier during the following ten 

 years gave their true meaning to these phenomena. It was then rec- 

 ognized that combustion was union with oxygen; that an ''earth" or 

 "calx" was to be regarded as the compound of a metal with oxygen; 

 that when a metal becomes tarnished and converted into such an 

 earthy powder it is being oxidized; that this oxide, on ignition with 

 charcoal or car) )on, or with compounds such as coal, flour, or wood, 

 of which carbon is a constituent, gives up its oxygen to the carbon, 

 forming an oxide of carbon, carbonic oxide, on the one hand, or car- 

 bonic "acid" on the other, while the metal is reproduced in its "reg- 

 uline" or metallic condition, and that the true elements are metals, 



