dioxid, but also that it is the stimulus which em- 

 powers vegetation to make chlorophyll, the con- 

 stituent to which plants owe their various hues of 

 green. It is well known that a period of rainy 

 weather, with the absence of sunlight, diminishes 

 very much the quantity of chlorophyll in plants; 

 also that in dark cellars they lose it entirely, and, 

 as a consequence, their lives, though an abundance 

 of carbon dioxid be present. 



From the soil plants get most of their water, and 

 nearly all of their nitrogen (so very important to 

 their well-being), besides the various salts of pot- 

 ash, lime, etc., which they require. Most of these 

 substances exist in the soil in forms which can be 

 absorbed readily by the roots; this is not the case, 

 however, with nitrogen, so we offer a few words 

 concerning it. 



Nitrogen is an inert gas, colorless, odorless, and 

 tasteless; it will neither burn nor support combus- 

 tion, and yet it is indispensable to life not only in 

 plants but in human beings. Although it is so 

 abundant in the air (simply mixed with oxygen), 

 neither plants nor human beings are able to appro- 

 priate it directly from the atmosphere. It seems nec- 

 essary that it first should be made into compounds 

 before plants can utilize it. These compounds 

 are generally formed by oxygen and hydrogen unit- 

 ing with it under certain conditions: nitrous acid 

 (HNO 2 ) and nitric acid (HNO 3 ) are examples. 

 Usually compounds of nitrogen are produced by 

 the decomposition of organic matter by ferments; 

 thus prepared they are taken up readily by plants 



