12 FIRST PRINCIPLES. 



A similar case of the conversion of ' attraction into heat is 

 seen, when water comes in contact with quicklime, and 

 slaked-lime is produced. Some bodies which have a strong 

 chemical affinity for each other, require an impetus to effect 

 their union. Thus, the carbon in the materials (wood, coal, 

 charcoal, or peat) for a fire, will not combine with the oxygen 

 of the air without being set alight. Here we have a case 

 somewhat analogous to that of a stone which, instead of 

 being thrown up in the air and coming down again, sticks 

 in the foliage of a lofty tree during its ascent. It would 

 then require an impetus or shove-off, in order to convert its 

 potential energy into active energy. 



We have seen that by pouring water on quicklime, 

 chemical affinity is converted into heat ; but if we apply heat 

 to slaked-lime, we shall have a portion of the heat changed 

 into chemical affinity by the water being driven off. A 

 similar action takes place in a lime kiln during the conversion 

 of limestone (carbonate of lime) into quicklime. 



When the water in a kettle is boiled over a fire under 

 ordinary conditions, neither the water nor the steam rises 

 above boiling point (212 F.) ; because the surplus heat is 

 occupied in keeping the particles of steam separate from 

 each other. While accomplishing that work, heat is said to 

 be latent (inappreciable by the senses). If, however, we 

 check the expansion of the vapour by means of pressure, as 

 in a digester, the heat which is unutilised for keeping the 

 particles of steam apart, will tend to raise the temperature 

 of both the water and the steam above 212 F. But the 

 moment the artificial pressure is removed, the surplus 

 heat will become converted into latent heat, with the result 

 that the steam will expand, and its temperature and that of 

 the water will fall to boiling point. The cooling effect 

 which the evaporation of perspiration has on the skin, is 

 due to the conversion into latent heat of a portion of the 



