OF THE NITROGEN OF PLANTS. 71 



pure, it is extremely soluble in water ; it forms 

 soluble compounds with all the acids ; and when in 

 contact with certain other substances, it completely 

 resigns its character as an alkali, and is capable of 

 assuming the most various and opposite forms. 

 Formate of ammonia changes, under the influence 

 of a high temperature, into hydrocyanic acid and 

 water, without the separation of any of its elements. 

 Ammonia forms urea, with cyanic acid, and a 

 series of crystalline compounds, with the volatile 

 oils of mustard and bitter almonds. It changes 

 into splendid blue or red colouring matters, when in 

 contact with the bitter constituent of the bark of 

 the apple-tree (phloridzin), with the sweet prin- 

 ciple of the Variolaria dealbata (ordn), or with the 

 tasteless matter of the Rocella tinctoria (erythrin). 

 All blue colouring matters which are reddened by 

 acids, and all red colouring 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 vege- 

 tables 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 



