COMBUSTION. 



spongy platinum probably combines with the oxygen held in its pores, and the 

 heat developed by the combination renders the platinum red hot.* 



The determination of the quantity of heat produced in the combustion of 

 different substances is a matter not only of great scientific interest, but of con- 

 siderable importance in the useful arts and manufactures. The mutual relation 

 between the quantity of the combustible, and of the oxygen combined with it, 

 and the heat developed, if accurately ascertained for various combustibles, could 

 not fail to throw light, not only on the theory of combustion, but, probably, on 

 the nature of heat in general. In the arts and manufactures, as well as in do- 

 mestic economy, the due selection of combustible matter depends, in a great 

 degree, on the quantity of heat or light developed by a given weight of it in the 

 process of combustion. 



Nevertheless, there is no subject in experimental physics in which more re- 

 mains to be discovered, and in which the process of discovery is more difficult, 

 than in the determination of the quantity of heat developed in the combustion 

 of various substances. Experiments have been made on some combustibles by 

 Lavoisier and Laplace with their calorimeter. A few others have been made 

 by Dalton. Crawfurd and Count Rurnford have also made some experiments 

 on this subject. The method of Lavoisier and Laplace consisted of burning 

 the combustible within the calorimeter, and measuyng the quantity of ice melt- 

 ed by the heat which it developed. Dalton placed a given weight of water, 

 at a known temperature, in a tinned vessel. Having previously ascertained 

 the specific heat of this vessel, that of water being known, he applied the burn- 

 ing matter to the bottom of it, so as to cause it to impart its heat to the water. 

 The quantity of heat developed was measured by the increased temperature of 

 the water, and the vessel which contained it. This process would evidently 

 give results considerably below the truth, because it is impossible that all the 

 heat developed in the combustion could be imparted to the vessel ; some would 

 be necessarily communicated to the surrounding air without reaching the ves- 

 sel, and more would be dispersed by radiation. Dr. Crawfurd contrived to 

 surround the burning matter with water, by the increased temperature of which 

 he measured the heat developed. 



Sir Humphry Davy made experiments to determine the heat developed by 

 some gases in the process of combustion, and adopted a method of experiment- 

 ing differing little from that of Dalton. He caused the flame to act on the bot- 

 tom of a copper vessel, containing a given weight of oil raised to a given 

 temperature, and estimated the heat produced in the combustion by the increas- 

 ed temperature received to the oil. The following are the results obtained by 

 these experiments : 



Thomson on Heat, p. 315. 



