LECTURE II.] HISTORY OF CHEMISTRY. 25 



Some time afterwards, in 1777, he advances a complete 

 theory of combustion. 22 He says : 



1. Heat and light are disengaged during every combustion. 



2. Substances burn only in pure air. 



3. This is used up in the combustion, and the increase in 

 weight of the substance burned is equal to the loss in weight of 

 the air. 



4. The combustible substance, by its combination with pure 

 air, is usually converted into an acid ; the metals, on the other 

 hand, are converted into metallic calces. 



In a paper on the composition of nitric acid, Lavoisier tries 

 to prove, in the case of this acid also, the last of the above 

 statements, which is of importance later when we come to deal 

 with theories of acids. 23 He there shows that this acid con- 

 tains oxygen, while he is not aware that it contains nitrogen. 

 The latter fact was discovered a few years afterwards by Caven- 

 dish, on passing electric sparks through mixtures of oxygen and 

 nitrogen, 24 whereby nitric acid was obtained. 



Lavoisier at this time groups together the facts that carbonic 

 acid (carbonic anhydride) consists of carbon and oxygen, sul- 

 phuric acid (sulphuric anhydride) of sulphur and oxygen, phos- 

 phoric acid (phosphoric anhydride) of phosphorus and oxygen, 

 and nitric acid of " air nitreux " (nitrous air, i.e. nitric oxide) 

 and oxygen. He further shows that an acid is obtained by 

 treating sugar with nitric acid (that is, by supplying oxygen), 

 and from this he concludes that Priestley's dephlogisticated air 

 must contain the acidifying principle (Principe acidifiant 

 principe oxygine)^ From this time onwards, he regards all acids 

 as consisting of a basis, or radical, and of \h\s principe oxygine. 

 His " air pur " on the other hand, besides this acidifying prin- 

 ciple, contains also the " matiere de chaleur" (matter of heat). 



It is certainly remarkable to find even Lavoisier speaking of 

 a fire material, which he afterwards designates " calorique" and 

 of which I shall explain the signification. 



22 Lavoisier, Oeuvres. 2, 226. w Ibid. 2, 129. ^ Kopp, 



Geschichte. 3, 231. 25 Lavoisier, Oeuvres. 2, 249. 



