AGENCY OF WATER. 93 



frozen shoot, it droops and turns black ; but if it be thawed 

 gradually, it will not be injured. This is supposed to be due 

 to the fact, that as the fluids in the vegetable organs freeze, 

 they expand, but the air contained in adjacent vessels suf- 

 fers a corresponding contraction, so that the organs are rare- 

 ly burst by frost. When sudden heat is applied to the plant, 

 in this state, the air expands more rapidly than the ice con- 

 tracts, arid the vessels are burst ; but when the plant is thawed 

 more slowly no such effect is produced. Hence the reason 

 for putting frozen vegetables into water, where they will thaw 

 gradually, in order to preserve them. Potatoes may thus be 

 preserved frozen during the winter, and by thawing them 

 slowly and drying rapidly they are not injured. Liebig has 

 shown that snow contains ammonia, and this fact will serve 

 to explain its influence upon winter wheat. It also absorbs 

 oxygen and nitrogen, in proportions quite different from those 

 in the atmosphere. The oxygen is only about seventeen in- 

 stead of twenty-one per cent. Dana has shown that an acre 

 receives annually fifty pounds of geine and salts in the snow. 



II. The agency of water, in the liquid form, may be con- 

 veniently studied under the following heads. 1. Its solvent 

 properties ; 2. Its chemical agency ; 3. Its mechanical agen- 

 cy ; and, 4. As affording food. 



1. Solvent properties of water. Water has the power of 

 dissolving, and holding in solution a great variety of sub- 

 stances, animal, vegetable and mineral. It is the great sol- 

 vent in all the operations of nature. 



(1) When water passes through the soil, it dissolves out 

 its soluble salts, such as common salt, potash, lime, nitre, etc., 

 and conveys these substances to the roots, and thence into 

 the organs of vegetables. 



(2) Water dissolves out the soluble parts of vegetable mould 

 and of compost manures, as fast as the chemical changes in 

 the soil render them soluble, and fitted for the nourishment of 



8* 



