il|4 GTPSUM. 



Let us now remember that salts can osly act on ve^evables in the 

 Btate of solution, and we shall understand how those only which are 

 but sparingly soluble, can ever be advantageously employed in agri- 

 culture. Water, in fact, having the power to dissolve only a very 

 limited quantity of the mineral manure, will present it to the grow- 

 ing plant nearly in a constant quantity, so long as the soil contains 

 any fair proportion of the substance. It is in this way precisely 

 that gypsum appears to gain its superiority over the generality of 

 mineral or saline manures ; water does not take up more than j^T^th 

 part of its weight before it becomes saturated ; a certain proportion 

 of the moisture of the earth being dissipated by evaporation, there 

 is forthwith a precipitation of sulphate of lime ; but the moisture 

 that remains is nevertheless charged as before, neither more nor less, 

 and in the fittest state, as it seems, to administer to the wants of the 

 growing plant. If instead of sulphate of lime we suppose some salt 

 that is much more soluble, sulphate of soda for example, we have 

 nothing of the same state of equilibrium between the quantity of 

 moisture and its charge of saline ingredients maintained. Suppos- 

 ing the moisture of the ground to hold ^^^^h of sulphate of soda in 

 solution, and this quantity calculated to produce good effects upon 

 growing vegetables : suppose now that a drought sets in, which by 

 dissipatmg one-half of the moisture, increases the charge of saline 

 matter to 3 jo^b of its bulk, it may very well happen that this pro- 

 portion, instead of proving beneficial, will be felt as injurious to vege- 

 tation. 



The hypothesis of Davy, supported by these ingenious views of 

 M. Chaptal, would therefore lead us to regard gypsum as behaving 

 to plants in the same general way as the insoluble salts which usual- 

 ly form an element of the soil or of manures, the phosphate and car- 

 bonate of lime, in particular, salts which are made apt to enter the 

 tissues of plants by the carbonic acid which is found in all the water 

 that falls from the clouds and that moistens the soil, and which has 

 the property of dissolving small quantities of them. But while the 

 str .ngth of these solutions, weak at all times, is liable through at- 

 mospherical vicissitudes to vary, when the mere traces of saline 

 matter which at best they offer at any time are inadequate to meet 

 the demands of a crop disposed to grow rapidly and luxuriantly, 

 such as clover, sainfoin, andlucern, the solution of sulphate of lime, 

 of the same strength at all times and under all circumstances, is 

 ready to supply the plants with the mineral substance they require, 

 however rapid and vigorous their growth. 



The theory of the action of gypsum proposed by Professor Liebig 

 is extremely ingenious. He admits, with M. de Saussure, the pre- 

 sence of carbonate of ammonia in the atmosphere, and consequently 

 in rain-water. This fact established, and it appears undeniable, the 

 infiuence of gypsum would consist in its faculty of fixing the infinite- 

 ly small quantity of carbonate of ammonia which is brought down 

 by the rain and the dew, and so preventing its dissipation on the 

 return of drought and sunshine. Carbonate of ammonia, in fact, as 

 •"•^ hare alreadyspen, when speaking- of manures, in contact witb 



