50 THE CULTURE OF FARM CROPS. 



exercised in igniting the gas, and the first which is set free 

 should be permitted to escape until the air has been all car- 

 ried off. 



Water undergoes continual decomposition and recombi- 

 nation in the interior of plants and animals. As a fluid it 

 finds its way into every cell and pore and passes out by 

 transpiration after it has given up to the tissues the matter 

 which is extracted from it. And so slight is the hold which 

 its elements have upon each other, and so strong is their 

 affinity for other elements, that they are ready to separate 

 upon very slight impulses ; the oxygen forming compounds 

 with one and the/ hydrogen with others, as the production 

 of the various substances of which the plants form them- 

 selves, require^and demand. And when the nature of chem- 

 ical combinations begins to be understood, there is no more 

 wonderful fact in the study of vegetable physiology than 

 the great variety of changes which are continually going 

 on through the agency of the elements of water and others 

 which it conveys into the tissues of plants and animals. 



In the state of vapor too, water exerts a very potent in- 

 fluence upon the life and growth of farm crops. Vapor 

 escapes from water into the air, or is absorbed by the air, 

 not only at boiling heat, but at all temperatures. Even at 

 a zero temperature the air takes up water, as is known by 

 the housewife whose linen freezes dry in the cold, crisp, 

 wintry, air. A piece of ice exposed to the air in the coldest 

 weather gradually evaporates and disappears. It is how- 

 ever in the summer that the evaporation of water is most active; 

 and it is then that the effects of the condensation of the at- 

 mospheric moisture is most perceptible and useful. Dew is the 

 product of this condensation. The air charged with the vapor 

 which has been gathered during the heat of the day, is 

 cooled at night by contact with the soil, from which the 

 heat is rapidly lost by radiation. The cooling of the air 

 causes the moisture to condense, forming sometimes visible 

 vapor, seen in the evening and night fogs which prevail in 

 some localities ; but always a burden of moisture which is 

 too heavy to be suspended in the air. This moisture then 



