62 Water, its Properties and Uses. [July, 



of the snow. The portions of snow which fall first through the air, 

 will of course absorb the most of the ammonia, and consequently it is 

 found that those portions also nearest the ground contain the most. 

 The stimulating properties of this gas as food for plants, are well 

 known, and if the idea be true, which is a very common one 

 among farmers, that grain grows under snow, it may be owing to 

 the presence of this gas, carried down to the roots by the first 

 snow that melts, and absorbed by them. Other gases are also 

 absorbed by snow. 



It will not be out of place here, to notice some other effects of 

 ice and snow upon vegetation, and the soil. Snow forms a cov- 

 ering for the grain fields of the farmer, of a kind which is almost 

 if not entirely a non-conductor of heat. Thus the temperature of 

 the surface of the earth is maintained in a uniform condition, and 

 the delicate texture of young plants is not exposed to the sudden 

 and fatal mutations of temperature which destroy them if unpro- 

 tected. This is owing to the non-conducting properties of the 

 snow as well as to the fact that it is light and porous, and con- 

 tains air, which is also a body almost incapable of conducting 

 heat. 



Plants in the temperate zone are not liable to be destroyed by 

 mere intensity of cold. The cause of their being winter killed, is 

 their being exposed to great changes of temperature, by which 

 the fluids of the vessels are suddenly expanded and burst. If a 

 plant is frozen and then thawed out by the application of cold 

 water, there is no danger of destroying its life. In this way 

 plants which lie all winter under a mantle of snow, are safely 

 thawed by the gradual melting of the snow, before they become ex- 

 posed to the warm rays of the sun. But in winters, during which 

 little or no snow falls, vegetation is constantly affected by the 

 warmth of the sun, and again exposed to the intense cold of the 

 winter night. These changes of temperature destroy them. 



Water possesses a peculiar property, during the process of 

 freezing, which is of great use in agriculture. Unlike other 

 bodies, it does not follow the law of contracting by decrease of 

 temperature. It observes the law till it sinks to the temperature of 



