The Romance of Chemistry 731 



3 ** 

 Circulation of Matter 



One of the epoch-making ideas in science is that of the cir- 

 culation of matter a corollary of the idea of conservation. It 

 is an old idea, for one of the Greek philosophers, Heraclitus, is 

 responsible for the epigram "All things flow"; but its demonstra- 

 tion belongs to the nineteenth century. Just as we associate the 

 name of William Harvey with the circulation of the blood, so we 

 should link the name of Justus Liebig (1803-1873) with vivifying 

 the fact of the circulation of matter. Many people know the 

 name of this great chemist in connection with beef -tea, but he has 

 greater claims for commemoration. 



Everyone knows how the clouds condense on the cold rocks 

 of the mountains, how the drops of water flow together into run- 

 lets, how they fill up the bog-moss sponges and overflow into 

 streamlets, how these make rivers which join the sea, how the 

 sun-drawn mist rises from the ocean and forms the clouds which 

 break on the mountain-side. So the world goes round. But this 

 is far too simple a case, for we have been dealing with water from 

 beginning to end. Let us take a better case. 



Circulation of Nitrogen 



All living matter includes the nitrogenous carbon compounds 

 called proteins, such as white-of-egg and the gluten of bread. In 

 other words, all living creatures require nitrogenous supplies. 

 Animals obtain these by eating other animals or by eating plants ; 

 so we may leave them, for the moment, out of account as far as the 

 problem of nitrogen-supply is concerned. The question is where 

 plants obtain nitrogen, and the generally valid answer is that the 

 roots absorb nitrates and the like from the soil. There is abun- 

 dance of nitrogen in the air indeed, four-fifths of the atmosphere 

 is nitrogen but this treasure-house is closed to plants, except in 

 the case of sundry types which, with the help of partner-bacteria 

 (see Sir Ray Lankester's article BACTERIA), are able to capture 



