366 HISTORY OF SCIENCE. 



The conclusions which Lavoisier drew from his experiments were 

 very different from the phlogistic explanations of Priestley. The mer- 

 cury during its calcination, he said, absorbs the salubrious and respir- 

 able part of the air included in the retort c and bell jar F, and the 

 remaining air is a mephitic kind, supporting neither combustion nor 

 respiration. Atmospheric air consequently is composed of two elastic 

 fluids of different and opposite qualities. As a confirmation of this 

 last inference Lavoisier, in another experiment, added to the residual 

 " mephitic air " the vital air which the red substance had given off, 

 and the air thus produced had qualities in every respect agreeing with 

 those of common atmospheric air. He soon changed the terms vital 

 air, mephitic air, etc., for others ; and the reasons for choosing the 

 new name for the respirable air are based on Lavoisier's own discoveries 

 of certain chemical properties of this "air." It will be necessary 

 before proceeding to discuss these to mention his views on heat and 

 the nature of gases. He considers that the phenomena of heat are 

 best explained by attributing them to the effect of a real and material 

 but very subtle fluid, which, insinuating itself between the particles of 

 bodies, and by separating these particles from each other, produces the 

 expansion and the changes from solid and liquid to the vaporous state, 

 etc. In order to prevent any confusion arising from one word being- 

 used to describe the sensation we term heat and the cause of that heat, 

 Lavoisier gave the latter the name of igneous fluid and matter of heat; 

 but soon afterwards, in arranging with some of the most eminent 

 jFrench chemists a new chemical nomenclature, it was agreed that the 

 " exquisitely elastic fluid " that is the cause of heat should be called 

 caloric. But Lavoisier considers that this word need not be appro- 

 priated to any one theory. Now, it was part of Lavoisier's theory to 

 regard every gas as a compound of caloric with some substance which 

 constituted what he termed the base of the gas. Thus he considered 

 the respirable part of the atmosphere was a compound of oxygen and 

 caloric. Lavoisier thus expresses himself on this subject : " We know 

 in general that all bodies in nature are imbued, surrounded, and pene- 

 trated in every way with caloric, which fills up every interval left be- 

 tween their particles ; that, in certain cases, caloric becomes fixed in 

 bodies, so as to constitute a part of even their solid substance, though 

 it more frequently acts upon them with a repulsive force, from which, 

 or from its accumulation in bodies to a greater or lesser degree, the 

 transformation of solids to liquids, and of liquids to the aeriform state 

 of elasticity, is entirely owing. We have employed the generic name 

 gas to indicate the aeriform state of bodies produced by a sufficient 

 accumulation of caloric ; so that \fhen we wish to express the aeriform 

 state of muriatic acid, etc., we do it by adding the word gas to their 

 names; thus, muriatic acid' gas, aqueous gas, etc." It was on account 

 of its power of forming acid compounds that Lavoisier gave to " vital 

 air " the name of oxygen, by which it is now so well known. The name 

 is derived from two Greek roots, meaning d^V/and to produce. 



