The most convenient test-acid, or normal solution, both for commercial and chemical assays, is perhaps dilute sulphuric acid, which, when intended to be used VOLUMETRICALLY, has the sp. gr. 1·032 at 60° Fahr., and contains in 100 alkalimetrical divisions 1000 water-grains measure, or 1 litre, exactly 49 gr. (or grammes) of sulphuric acid; and when intended to be used GRAVIMETRICALLY, or by weight, has the sp. gr. 1·033, and contains in 1000 gr. (or grammes) weight exactly 49 gr. (grammes) of sulphuric acid; and, in both cases, consequently corresponds to 1 equiv. of every other base. These dilute acids are easily prepared by mixing 1 part of the concentrated acid with 11 or 12 parts of distilled water; the precise quantity depending on the strength of the acid employed, and must be so arranged that 1000 grains shall exactly neutralise 1000 grains of water containing 53 grains of pure anhydrous sodium carbonate. This acid (as well as all those hereafter mentioned) may be kept faintly tinged with litmus, which is often more convenient than tinging the alkaline solution at the time of making the assay. It will at once be seen that every alkalimeter division of the first of the above acids, and every 10 gr. of the second, represent the 1/100th part, or 1% of alkali whenever the equivalent weight[18] of the latter is taken for the assay. Every 1-10th part of an alkalimeter-division (or every drop), and every grain weight (when a Schüster’s alkalimeter is employed) then respectively represents the 1/10 of 1%; and the result sought is obtained without the necessity of any calculation. [Footnote 18: See Table II, at the end of this article.] This is obvious——for if the equivalent of a pure alkali or of its carbonate (_i. e._ one of 100%) requires an equiv. (100 alkalimeter-divisions, or 1000 gr.) of test-acid to saturate it, an alkali or alkaline carbonate of 75%, 50%, or 25%, will respectively require only 75, 50, or 25 divisions, or 750, 500, or 250 gr.; and so of other strengths in proportion. The only precaution necessary is always to take the standard weight for the assay answering to the equiv. of the denomination of the per-centage result sought. Thus, in testing a carbonate of potash, we may either wish to determine its per-centage richness in ‘dry carbonate,’ or in ‘pure potassa,’ the latter being usually the case. To obtain the first, we must take 69 gr. for the assay; and to obtain the second, 47 gr. With _CAUSTIC ALKALIES_, or mixtures containing them, the weight, in grains, taken for the assay, must always correspond to the equiv. of the pure base. See Table II, at the end of this article.