The residue, when emptying the generator after the carbonates are exhausted, is generally considered valueless and thrown away. It depends on the choice of material which kind the residue will be. If acid has not been used in excess neutral combinations of natron (soda), magnesia or lime with sulphuric or muriatic acid will remain, of which but sulphate of magnesia and sulphate of lime are of slight value and their purification may be made a small source of remuneration. The other salts are almost worthless.

Sulphate of magnesia may be there, where magnesia is used in large quantities as a carbonate in the manufacture of carbonated beverages. It can be purified and converted into "bitter salt" (Epsom salt) at a fair remuneration.

Satyhate of lime - the residue of marble dust, whiting or limestone - is only useful as manure and as such advantageous to a great many plants, being of the same nature as gyp-sum. It forms a large proportion of nearly all artificial manure, in some cases 50 per cent., and is very suitable for use on all soils, and at any time of the year. It is a capital absorbent of free ammonia, and would be a useful addition to putrid urine, or the drainage from dung heaps. Either alone or mixed with stable manure, or indeed any manure, it will be found serviceable to the farmer. It should be allowed to drain in a heap, in open air, so that any excess of sulphuric acid might be in great part removed; for some purposes this might be necessary.

Sulphate of lime very quickly hardens in the generator and is thus sometimes a great trouble to the manufacturers; the suggestive remedies we will find in Part III. "Carbonating Apparatus".