SOURCES OF HEAT. 261 



II. SOURCES OF HEAT. 



BEFORE we turn our attention to the special subject of 

 this paper, it will be necessary to consider the means by 

 which light and heat are produced. Heat may be obtained 

 from very different sources. Combustion, fermentation, pu- 

 trefaction, slaking of lime, the decomposition of chloride of 

 nitrogen and of gun-cotton, &c. &c., are all of them sources 

 of heat. The electric spark, the voltaic current, friction, per- 

 cussion, and the vital processes are also accompanied by the 

 evolution of this agent. 



A general law of nature, which knows of no exception, 

 is the following : In order to obtain heat, something must 

 be expended ; this something, however different it may be in 

 other respects, can always be referred to one of two catego- 

 ries : either it consists of some material expended in a chem- 

 ical process, or of some sort of mechanical work. 



"When substances endowed with considerable chemical af- 

 finity for each other combine chemically, much heat is devel- 

 oped during the process. We shall estimate the quantity of 

 heat thus set free by the number of kilogrammes of water 

 which it would heat 1 C. The quantity of heat necessary 

 to raise one kilogramme of, water one degree is called a unit 

 of heat. 



It has been established by numerous experiments that the 

 combustion of one kilogramme of dry charcoal in oxygen, so 

 as to form carbonic acid, yields 7200 units of heat, which fact 

 may be briefly expressed by saying that charcoal furnishes 

 7200 degrees of heat. 



Superior coal yields 6000, perfectly dry wood from 3300 

 to 3900, sulphur 2700, and hydrogen 34,600 of heat. 



According to experience, the number of units of heat only 

 depends on the quantity of matter which is consumed, and 



