2 4 o THE POPULAR SCIENCE MONTHLY 



the biologist, we shall, however, be dependent on nitrogenous fertil- 

 izers; and one of the great tasks of the chemist is to cheapen such 

 fertilizers by obtaining the nitrogen contained in them directly from 

 the air. During the last ten years great progress has been made in 

 this direction; and it remains to describe briefly, without entering into 

 technical details, the general lines along which this problem has been 

 successfully attacked. 



Two kinds of processes have been developed. One of these has the 

 object of producing nitric acid, a compound of water with one of the 

 oxides of nitrogen. The other kind of process has for its object the 

 production of ammonia, a compound of nitrogen and hydrogen. For 

 use in a fertilizer the nitric acid, which is a liquid, or the ammonia, 

 which is a gas, must of course be converted into a solid salt. This is 

 most cheaply done by neutralizing the nitric acid with lime or the 

 ammonia with sulfuric acid, yielding calcium nitrate or ammonium 

 sulfate, respectively. Whether the nitrate or the ammonium salt is 

 made the constituent of the fertilizer makes little difference ; for, though 

 plants directly assimilate the nitrogen only in the form of nitrate, yet 

 there are always present in soils the so-called nitrifying bacteria, whose 

 function it is to convert ammonium compounds into nitrates. 



Xitric acid is a compound whose constituents, nitrogen, oxygen and 

 water, are present in unlimited quantities in the air. The raw ma- 

 terials are available free of cost. The problem is therefore only to 

 make them combine under economic conditions. The difficulty arises 

 from the fact that nitrogen is an extremely stable substance; so that, 

 instead of tending to form compounds with oxygen, the nitrogen oxides 

 tend rather to break down into their elements, nitrogen and oxygen. 

 Thus, scientific investigations have shown that if a mixture of these 

 two gases in the best proportions is exposed to a temperature of 1500° 

 centigrade, that is, to a white heat, only one third of one per cent, 

 unites to form nitric oxide, however long the mixture be heated. But 

 these investigations have also shown that while most compounds decom- 

 pose with rise of temperature, this one, nitric oxide, becomes more 

 stable, the higher the temperature. Thus at 3000° five per cent, of 

 the mixture of nitrogen and oxygen will unite to form nitric oxide. 

 To get a fair yield of our product we must therefore expose air to an 

 enormously high temperature. But this isn't all ; for we must cool off 

 the gas without causing the nitric oxide which has been formed to 

 break up again into nitrogen and oxygen. To do this, we must call 

 to our aid another chemical principle, which is this : although the 

 quantity of a product finally formed in a chemical piocess sometimes 

 increases and sometimes (as in this case) decreases with falling tem- 

 perature, yet the rate at which that product forms or decomposes 

 always decreases very rapidly as the temperature is lowered. We must, 



