SOURCE AND ASSIMILATION OF NITROGEN. 87 



Ammonia forms urea,* with cyanic acid,t and a 

 series of crystalline compounds, with the volatile 

 oils of mustard and bitter almonds. It changes 

 into splendid blue or red coloring matters, when in 

 contact with the bitter constituent of the bark of 

 the apple-tree (^phloridzin)^ with the sweet principle 

 of the Variolaria dealbata {orcin)^ or with the taste- 

 less matter of the Rocella tinctoria (^erythrin). All 

 blue coloring matters which are reddened by acids, 

 and all red coloring substances which are rendered 

 blue by alkalies, contain nitrogen, but not in the 

 form of a base. 



These facts are not sufficient to establish the 

 opinion that it is ammonia which affords all vegeta- 

 bles, without exception, the nitrogen which enters 

 into the composition of their constituent substances. 

 Considerations of another kind, however, give to 

 this opinion a degree of certainty which completely 

 excludes all other views of the matter. 



Let us picture to ourselves the condition of a 

 well-cultured farm, so large as to be independent of 

 assistance from other quarters. On this extent of 

 land there is a certain quantity of nitrogen contained 

 both in corn and fruit which it produces, and in the 

 men and animals which feed upon them, and also in 

 their excrements. We shall suppose this quantity 

 to be known. The land is cultivated without the 

 importation of any foreign substance containing 

 nitrogen. Now, the products of this farm must be 

 exchanged every year for money, and other necessa- 

 ries of life — for bodies, therefore, which contain no 

 nitrogen. A certain proportion of nitrogen is ex- 

 ported with corn and cattle ; and this exportation 

 takes place every year, without the smallest com- 

 pensation ; yet after a given number of years, the 

 quantity of nitrogen will be found to have increased. 



* Urea was discovered in urine, being a constituent of uric acid. It 

 contains the elements of cyanate of ammonia (NH4 04-^4 NO). 



t This acid consists of 1 cyanogen and 1 oxygen. See Webster's 

 Chemistry y p. 398. 



