348 SALT, NITEATE OF SODA, SALTS OP AMMONIA, ETC. 



usually sought for in the composition of the plant, but I 

 do not think that this is always to be relied upon. The 

 composition of the seed of plants of wheat, for instance, 

 is so constant, or varies so little, that it is quite impossible 

 to infer from the results of the analysis of the seeds 

 whether the soil on wliich they grow abounded or was 

 deficient in phosphoric acid, nitrogen, potash, &c. The 

 abundance or deficiency of food in a field exercises an 

 influence upon the number and weight of the seeds, but 

 not upon the relative proportion of their component 

 elements. Thus, for instance, Pincus found a somewhat 

 larger percentage of magnesia in the unmanured clover 

 than in the plants manured with the sulphates ; but 

 taking the magnesia of tlie whole crop, the quantity of 

 this substance was much larger in the latter than in the 

 former. 



Amount of magnesia in — 



Variations in the percentage proportions of potash, 

 lime, and magnesia, may be often observed in all those 

 plants in which, as in the case of tobacco, the vine, and 

 the clover plant, potash may be substituted for hme, and 

 vice versa. But in such cases the decrease of one body 

 is invariably attended by a corresponding increase of the 

 other. 



Now if gypsum has the property of effecting a distri- 

 bution of the potash in the ground, and this is wanting 

 in magnesia, more potash should be contained in the 

 clover manured with gypsum than with sulphate of 

 magnesia. According to the analysis made by Pincus, 

 the ash of the clover-hay contained : — 



