54 THE CULTURE OF FARM CROPS. 



large quantity of heat is rendered latent in the vapor. 

 Water at 32 degrees absorbs 180 degrees of heat and reaches 

 a temperature of 212 degrees which is the boiling point. 

 But it does boil only slowly and steam is produced very 

 gradually. It is found that if the consumption of one pound 

 of coal will raise a quantity of water from 32 degrees to 212 

 degrees;. 5J Ibs. more will be required to change it all to 

 steam of the same temperature. 5* times 180 or 990 

 units of heat will then have been expended, but have 

 not been lost ; they are stored in the steam and are the 

 latent heat of the vapor of water. And when the vapor 

 of water is condensed into Liquid this heat is given out 

 again. 



And here is another most wonderful instance of the infinite 

 wisdom and beneficent adaptation of the laws of nature to 

 the stability of the universe and the comfort and happiness- 

 of mankind. The expansion of water as it changes into ice 

 has been already mentioned. This is one more effect of 

 heat, that is a reduction of it, upon this liquid, and has an 

 intimate connection with this part of our subject. When 

 water reaches its maximum density which is 39 degrees, un- 

 der the influence of the abstraction of heat, it then begins- 

 to increase in bulk until ice crystals form when the total 

 expansion amounts to one-eleventh of the bulk. Conse- 

 quently the ice floats on the surface and after a time it be- 

 comes thick enough to protect the underlying water from 

 the effects of cold. Were it otherwise, ice would sink to the 

 bottom and as the surface water cooled it would also sink 

 and the whole water would soon be changed into ice. The 

 ocean would then become a vast bed of solid ice, which by 

 the very force of this law would remain permanently and re- 

 sist all the heat of the sun to change it. Then the earth 

 would be uninhabitable. No green blade would appear 011 

 the surface ; no animal would find subsistence ; there would 

 be no clouds, no rain ; everything would be cold and drear 

 and lifeless ; a dead world. 



Again, were it not for the gradual absorption of heat by 

 the melting ice and the evaporating water, the earth would 



