Gypsum as a Cane Fertiliser. 139 
obtain their supplies as best they can. It so happens) 
however, that the manures almost exclusively used in 
the colony, viz., sulphate of ammonia, superphosphate 
of lime, and dissolved guano, supply not only the nitrogen 
and phosphoric acid they are intended to do, but also 
sulphuric acid, which they contain as a kind of minor 
constituent. It is indeed probable, as I pointed out in 
my paper on British Guiana Cane Soils and Artificial 
Manures (Timehri, i. 288J, that the preference shown for 
sulphate of ammonia over other nitrogenous salts — chlo- 
ride of ammonium, nitrate of soda, etc., reputed equally 
efficacious in supplying nitrogen- may rest on some more 
solid basis than mere fashion or caprice, and that it is ac- 
tually found to be a better manure because it supplies 
sulphuric acid as well as nitrogen. A similar possibility 
also attaches to superphosphate of lime and dissolved 
guano, in the preparation of both of which, sulphuric acid 
is largely used. 
To prepare superphosphate, ordinary phosphate of 
lime (or tricalcium phosphate) is simply mixed with a 
proper proportion of sulphuric acid, and kept for a time. 
The sulphuric acid gradually combines with two thirds 
of the lime, forming sulphate of lime or gypsum, leaving 
only one third in combination with the phosphoric acid 
in the form of a strongly acid salt called by chemists 
monocalcium phosphate. This mixture of gypsum and 
monocalcium phosphate with more or less of the original 
phosphate, and various adventitious impurities, is known 
commercially as superphosphate of lime. It will be seen 
that in applying superphosphate to the soil, gypsum is 
also applied, and it is a question whether any superiority 
exhibited by the superphosphate over insoluble phos- 
S 2 
