_Test-acids_ may be very simply prepared by gradually diluting concentrated sulphuric acid with water until it is reduced to the proper strength; the dilution being made in a glass vessel containing a ‘hydrostatic bead’ exactly corresponding to the desired specific gravity of the dilute acid. When the proper point is reached, and the mixture has again acquired the normal temperature of 60° Fahr., the bead rises from the bottom of the vessel, and floats about indifferently in the middle of the liquid. The sp. gr. may then he carefully ascertained by means of an hydrometer or a specific gravity bottle; after which the strength must be accurately determined by means of a standard solution of either pure anhydrous carbonate of soda or pure caustic soda. An acid of any given strength or saturating power may also be prepared in the following manner:——49 parts of commercial sulphuric acid (oil of vitriol), sp. gr. 1·825, contain nearly 40 parts or 1 equiv. of anhydrous sulphuric acid; if we, therefore, wish to prepare a dilute acid containing in every 1000 grains weight, or measure, exactly 1 equiv. of hydrated sulphuric acid, we have only to make 49 gr. of such acid up to 100 gr. weight or measure with pure water. After it has recovered the proper temperature, its sp. gr., or rather its saturating power, must be carefully tried, and, if necessary, readjusted. As, however, it very often happens that the oil of vitriol employed is not so strong as that above referred to, it is better first to test its strength with pure anhydrous carbonate of soda, and to calculate the quantity required by the Rule of Proportion. Every 53 gr. of the dry carbonate are equal to 40 gr. of ‘dry sulphuric acid.’ Suppose we find the oil of vitriol to contain only 72% of hydrated acid, then—— 100 : 40 :: 72 : 55·55 or, instead of only 40 gr., fully 55-1/4 gr. will be required, which are to be made up with water to 1000 gr., as before. Finally, the diluted acid must be very carefully re-tested, and if found correct, at once put into a well-stoppered bottle, and labelled, for use. Too much care cannot be taken to ensure the test-liquid, whether for alkalies or acids, being of the proper strength, of which the specific gravity alone is an insufficient proof. In practice, so small a quantity only of test-acid as that referred to above is, of course, seldom made; but as any larger quantities are mere multiples of the smaller one, the necessary proportions to be employed are easily calculated. The common plan is to prepare one or more gallons or quantities of 10 lbs. each, and to preserve the liquid in stoppered green glass ‘Winchester-quart bottles,’ so that it may be always ready for use. Although, as may be inferred from the text, sulphuric acid is generally used as the standard acid, yet oxalic acid in pure crystals is recommended by M. Mohr, and answers admirably, and is prepared and used exactly in the same manner. TABLE II.——_Alkalimetrical Equivalents._