AGRICULTURAL CHEMISTRY. 191 



food, and compelling the soil to yield its ingredients to the support of 

 vegetation. 



Ammonia, when acting on the soil as carbonate, (coming from the 

 decomposition of urea, uric acid, and other nitrogenous bodies,) is not 

 inferior to lime in its solvent effects. 



Gypsum, common salt, carbonate of lime, nitrates of potash and 

 soda, and in fact all the saline compounds which are incorporated with 

 the soil in manures, may exert important physiological effects on the 

 plant in addition to their mere nutritive function. 



We have already intimated that the transpiration of water through 

 the plant is very remarkably hindered when lime, potash, or the salts 

 just named are present in the absorbed liquid. This fact, observed 

 for the first time by Mr. Lawes, in 1850, and recently brought 

 again more strikingly into notice by Dr. Sachs, of Tharand, Saxony, 

 appears to be of great importance in the theory of manures. Dr. 

 Sachs experimented on various plants, viz: beans, squashes, tobacco, 

 and maize, and observed their transpiration in weak solutions (mostly 

 containing one per cent.) of nitre, common salt, gypsum, (one-fifth 

 per cent, solution) and sulphate of ammonia. He also experimented 

 with maize in a mixed solution of phosphate and silicate of potash, 

 sulphates of lime and magnesia, and common salt, and likewise ob- 

 served the effect of free nitric acid and free potash on the squash 

 plant. The young plants were either germinated in the soil, then re- 

 moved from it and set with their rootlets in the solution, or else were 

 kept in the soil and watered with the solution. The glass vessel 

 containing the plant and solution was closed above around the stem 

 of the plant by glass plates and cement, so that no loss of water could 

 occur except through the plant itself, and this loss was ascertained 

 by daily weighings. The result was that all the solutions mentioned, 

 except that of free nitric acid, quite uniformly retarded transpiration 

 to a degree varying from 10 to 90 per cent., while the free acid ac- 

 celerated the transpiration in a corresponding manner. 



As the processes of elaboration — the chemical and structural me- 

 tamorphoses going on within the cells of the plant require time for 

 their performance, we can easily perceive that a too rapid upward 

 current of liquid, by diluting the juices, might measurably interfere 

 with the assimilation of the food, and that the presence of a body 

 may be no less useful by its regulating influence on the circulation of 

 the water than by contributing an ingredient necessary for the forma- 

 tion of the substance ol the plant itself. 



It is also obvious that if a substance added to the soil retard the 

 transpiration of water through vegetation, a given store of hygro- 

 scopic moisture in the soil will serve the needs of vegetation longer — ■ 

 will reach further into time of drought than it otherwise could. Dr. 

 Sachs found that gypsum exerted the greatest effect in preventing 

 loss of water, and this observation gives a scientific ground oi evi- 

 dence to the opinion long maintained among farmers, but rejected by 

 men of science, (and very properly, as no cause could be discovered 

 for such an effect, and the effect' is not capable of measurement in 



