732 The Outline of Science 



the nitrogen in the air which the rain brings down into the soil 

 within reach of the rootlets. (See the article on BOTANY.) 



Leaving the exceptional cases aside, we see that plants de- 

 pend on nitrogen compounds in the soil. Where do these come 

 from? Electrical discharges in the atmosphere lead to the for- 

 mation of nitric acid and nitrite of ammonia, which the rain brings 

 into the soil. Various soil-bacteria help to change nitrites into the 

 more useful nitrates, on which plants largely depend ; but it must 

 be admitted that other soil-bacteria (which cannot be credited 

 with philanthropy) work in the opposite direction. But when we 

 pass from extrinsic supplies of available nitrogen such as thunder- 

 storms and Leguminous plants may afford, we face the fact that 

 one cosmic generation lives on the graveyards of its predecessors. 

 The bread we enjoy is a rehabilitation of the guano deposited by 

 the birds of ancient sea-cliffs. 



What we have just begun to do in illustrating the circulation 

 of matter may be readily elaborated, and the idea may be extended 

 to other elements. It should be kept in mind, however, that only 

 a few elements share in the main circulation, for only about a 

 dozen are always represented in living creatures and a not much 

 larger number makes up 99 per cent, of the earth's crust. But in 

 an Outline like this the important thing is to make clear some of 

 the germinal ideas of the science, and this is one, that many of the 

 elements of the chemical world often change their partners in their 

 unending dance. 



Catalysts 



When we turn from a chemical treatise of a generation ago 

 to one of to-day the whole atmosphere seems to be different, and 

 one of the changes is suggested by the word catalysis, still un- 

 familiar to many. What does it mean? 



There exist certain substances [says Professor R. K. 

 Duncan] which may lie in a vessel seemingly inert, and yet 

 by their mere presence may dictate what actions shall or shall 



