26 HISTORY OF CHEMISTRY. [LECTURE II. 



The " matiere du feu " is not possessed of weight. Lavoisier 

 shows this by burning phosphorus in closed vessels, whereby 

 heat is liberated but no loss of weight takes place. 26 Further, 

 he causes water to freeze in closed vessels, and here, likewise, 

 he finds no change of weight. Since he is aware, from his own 

 experiments, that heat is disengaged in the process, he con- 

 siders that he is justified in assuming that heat has no weight. 

 A better notion of what Lavoisier calls " matiere du feu " will be 

 obtained by my stating his views on the constitution of matter, 

 which I take from his " Reflexions sur le Phlogistique." 2T 

 According to him, matter consists of small particles which do 

 not touch one another, since, otherwise, a diminution of volume 

 by lowering of temperature could not be explained : 2S the 

 matter of heat exists between these particles. The hotter a 

 substance is the more of the matter of heat does it contain. 

 In the investigations into the specific heats of various sub- 

 stances, carried out along with Laplace, Lavoisier further proved 

 that, for a like increase in temperature, substances do not take 

 up like quantities of the matter of heat. 29 Into the considera- 

 tion of these experiments, as well as into those on the heat of 

 combustion, 30 I do not enter here. Lavoisier knows that, by 

 the addition of heat, ice is first converted into water, and the 

 water then into steam. Hence gases contain most of the 

 matter of heat. This is what we should understand when he 

 says that his " air pur" consists of the acidifying principle and 

 the matter of heat. During combustion the former unites with 

 the combustible substance, and the matter of heat is liberated. 

 It produces heat and light. 



The following statement is very characteristic of Lavoisier's 

 standpoint : 31 " Heat is the energy which results from the im- 

 perceptible movements of the molecules of a substance ; it is 

 the sum of the products of the mass of each molecule into the 

 square of its velocity." Here we find him in complete agree- 



126 Lavoisier, Oeuvres. 2, 618. ^ Ibid. 2, 623. 28 Compare also 



Lavoisier, Traite de Chimie. I , etc. a Lavoisier, Oeuvres. 2, 289. 



30 Ibid. 2, 318 and 724. 31 Ibid. 2, 286. 



