The sodium bicarbonate employed is generally a fairly pure article. Common grades of the salt contain a little sodium chloride, and in some cases as much as 2 per cent. of the corresponding sulphate; it may also prove to be somewhat deficient in the proportion of carbonic acid present. Cream of tartar (potassium bitartrate), is far more liable to adulteration. A certain quantity of calcium tartrate is often found in the commercial article, originating from its method of manufacture, and amounting, on the average, from 6 to 7 per cent. The salt is, moreover, sometimes intentionally mixed with alum, starch, tartaric acid, gypsum, chalk and terra alba. Occasionally so-called cream of tartar has been found to be wholly composed of starch and calcium diphosphate. In the examination for calcium tartrate and sulphate, a quantitative determination of the total lime and sulphuric acid is made. The quantity of sulphuric acid obtained is calculated to gypsum, any excess of lime left being returned as tartrate. The ash in pure cream of tartar should amount to 36·79 per cent., while that of calcium tartrate is only 21·54 per cent. Naturally, the addition of flour or starch would materially decrease the proportion of ash. The presence of these latter adulterants is recognised by means of the microscope, and by testing the sample with iodine solution. It is generally required that cream of tartar should contain at least 90 per cent. of potassium bitartrate. _Baking powders._--The usual composition of baking powders has already been stated. They all contain sodium bicarbonate, but differ in the acid ingredient present, which may consist of cream of tartar, tartaric acid, calcium diphosphate, or alum. In order to remedy the tendency to deterioration which exists in powders entirely composed of the above salts, it is the practice to add a considerable amount of “filling” (corn-starch, flour, etc.). The quantity of filling employed for this purpose varies from 20 to 60 per cent., but is as a rule, greater than is really necessary. A small proportion of the sodium salt is often replaced by ammonium sesquicarbonate. Alum is a more objectionable constituent of many preparations, and it should be considered an adulteration. The practical value of baking powder is chiefly dependent upon the quantity of carbonic acid it liberates when decomposed, and this is affected by the strength of the acid salt and the amount of “filling” used. The most common varieties of baking powders are:-- (_a_) _Sodium bicarbonate and cream of tartar_, either pure or mixed with starch. In testing this class of powders, it is usual to determine the excessive alkalinity remaining after the decomposition with water, by means of decinormal acid; this is put down as bicarbonate present in excess. The proportions of sodium bicarbonate and cream of tartar are calculated from the alkaline strength of the ash, minus the excessive alkalinity found.