MIXTURE OF GYPSUM AND OF SALT. 63 



of potassium, such as sea-water, and the Water of most springs, 

 may be viewed as a mixture of an alkaline sulphate with 

 chloride of calcium. From this it must be obvious, that when 

 we furnish to a plant at the same time both gypsum and common 

 salt (chloride of sodium), we actually furnish by such a solution 

 the same materials that we would do if we supplied a mixture 

 of sulphate of soda and chloride of calcium. In order to form 

 the constituents containing sulphur, that element and the alkali 

 must be retained by the plant, while the chlorine and calcium 

 will be expelled by the roots* . 



We know that this process actually does take place in the 

 caso of marine plants. The soda or potash is obtained from 

 common salt or chloride of potassium, which suffers decomposi- 

 tion by the presence of sulphate of lime or sulphate of magnesia. 

 It is necessary to suppose that this process also occurs with the 

 cereal and all other plants, the ashes of which are destitute of 

 lime, and the sulphur of which has been supplied in the form of 

 gypsum. Thus we are enabled to explain the use of comnon 

 salt as a manure ; it enables the plant, for which this manure is 

 useful, to extract its sulphur from the soil in which it existed in 

 the form of sulphate of lime. 



