LECTURE II.] HISTORY OF CHEMISTRY. 27 



ment with the fundamental doctrines of the mechanical theory 

 of heat. His views with respect to the heat disengaged during 

 combustion, although not quite accurate, are also of great 

 importance. He considers 32 that when a solid substance 

 (phosphorus) burns in a gas (oxygen), and the product of the 

 combustion is solid (phosphoric anhydride), the disengagement 

 of heat is due to the condensation which the gas has undergone, 

 in order that it may become solid. If the product is gaseous 

 (carbonic anhydride), he attributes the disengagement of heat 

 to the alteration of the specific heat. He advances the general 

 view that the heat of combustion must be greatest when two 

 gases unite to form a solid substance. How correctly he under- 

 stood the application of these fundamental ideas, is shown by 

 his mode of explaining the lowering of temperature produced 

 by dissolving salts in water. Lavoisier assumes, as we do, that 

 it is the change of state of aggregation which occasions the 

 absorption of heat. 83 He shows, further, that the evolution of 

 heat which occurs on mixing sulphuric acid with water, is 

 accompanied by a decrease in volume, and that both maxima 

 coincide ; so that theory and experiment agree. 



I must not, however, enter too deeply into these matters, 

 which belong, partly at least, to physics ; and, therefore, I 

 return to his purely chemical investigations. 



Lavoisier adheres to Boyle's definition of an element, 34 

 which we retain to-day. With him, an element is any sub- 

 stance which cannot be further decomposed. 35 What the signi- 

 ficance of this definition is, and of what importance this idea 

 of an element has become for the whole of natural science, has 

 been specially pointed out by Helmholtz. 36 



The metals were first regarded as elements by Lavoisier. 37 

 In a long paper he contests the prevailing view, which assumed 

 the existence of phlogiston in the metals. These interesting 



32 Lavoisier, Oeuvres. 2, 647. w Ibid. 2, 654. 34 Kopp, 



Geschichte. 2, 275. 35 Methode de Nomenclature Chimique. Paris 



(1787), 17. 36 Tageblatt der Naturforscherversammlung in Innsbruck. 



37. 37 Lavoisier, Oeuvres. 2, 623. 



