826 SOURCES OF HEAT. 



units* received by this water only 7 '2 O72 = 6*48 

 units have been actually supplied by the 12 gr of vapour 

 in its passage to the liquid state ; that is, O kgr *012 of 

 vapour at 100 gives up 6*48 units of heat in becoming 

 O kgr '012 of water at 100. The heat given up by 1 

 kilogramme of vapour under the same circumstances 

 follows from the proportion 



0-012 : 1 :: 6'48 ' x 

 x = 540. 



That is, 1 kilogramme of vapour at 100 gives out 540 

 thermal units in becoming 1 kilogramme of water at 

 100; conversely, 540 thermal units become latent if 1 

 kilogramme of water at 100 is converted into steam. 



59. Means of raising and' lowering the temperature of 

 bodies. The heat which becomes latent when solids 

 become liquids, and liquids become vapours, and that 

 which is given out by vapours and liquids when they 

 return to their original state, is very often not only a 

 cause of considerable alterations of temperature, but 

 also a means of artificially producing such alterations. 

 The temperature of bodies may be artificially raised by 

 various means; but for lowering the temperature of a 

 body the disappearance of heat during changes of the 

 state of aggregation presents, in almost all cases, the 

 most ready means. 



The most common means of producing heat is com- 

 bustion, which however is a chemical phenomenon, and 

 the discussion of it, as well as of many other chemical 

 processes in which heat is produced such for example 

 as the slaking of lime belongs to the science of Chem- 

 istry and not to Physics. 



