140 TlMEHRI. 
phate, should not rightly be claimed, partly or wholly 
for the gypsum contained in it. 
A superphosphate is now supplied by an eminent firm 
of manure manufacturers, from which the gypsum has 
been removed, so that it consists of nearly pure mono- 
calcium phosphate. It would be an interesting experi- 
ment to try the effe6l of this preparation side by side 
with the ordinary kind, on land deficient in sulphates, 
using chloride of ammonium (not sulphate) to supply 
nitrogen in each case. It may also be pointed out here 
that in comparative trials of insoluble phosphates against 
superphosphates an equal weight of gypsum should be 
mixed with the former to equalise the conditions ; other- 
wise, in one case gypsum would be present while absent 
in the other. 
"Dissolved" guano is prepared in a similar way 
to superphosphate by adding to ordinary guano, sul- 
phuric acid, which converts the phosphate of lime 
present into a mixture of monocalcium phosphate and 
gypsum, and the salts of ammonia into sulphate of 
ammonia. Increased efficacy is likewise attributed to 
the more ready solubility of the guano thus treated, but, 
if there be any superiority, it is quite possible it is due 
to the sulphate. 
The treatment with sulphuric acid considerably 
increases the cost of the manures, and, as I have pre- 
viously demonstrated,* it would be practising economy 
to no small degree if natural phosphates and nitrogenous 
matters were used in place of the expensive soluble 
manures now so much employed. As a substitute for 
* Soluble vs. Insoluble Manures, loc. cit. 
