These are made by diluting sulphuric acid to a sp. gr. of near 1·071 and 1·086 respectively; 1000 grains, by measure, of the first neutralising exactly 100 grains of pure potassa, or 113 of pure anhydrous soda carbonate, and the latter neutralising exactly 100 grains of pure soda, or 171 gr. of pure anhydrous sodium carbonate. There is another system of preparing standard acids by means of a Faraday’s alkalimeter. A strong acid is prepared by diluting sulphuric acid to a sp. gr. of 1·1268 at 60°, and 455·7 grains exactly neutralise 100 of anhydrous carbonate of soda. The glass tube here referred to, and known as Faraday’s ALKALIMETER, is graduated centesimally, in the usual manner; but opposite the numbers 22·1, 48·62, 54·43, and 65, are cut the words ‘soda,’ ‘potassa,’ ‘carbonate of soda,’ and ‘carbonate of potassa,’ to indicate the quantity of the test-acid to be employed for each of these substances. (See _engr._) It is used by pouring the test-liquor into it until it reaches the line marked against the alkali, or carbonate, under examination, the remaining divisions being filled up with pure water, and the whole well mixed by placing the thumb on the orifice of the tube and shaking it well. The measure of the resulting dilute acid must then be very carefully observed, and more water added, if required, to bring it up to the zero (0) or 1000 gr. on the scale; careful agitation being again employed as before. The test-acid thus prepared is then added, with the usual precautions, to the sample until exact neutralisation is effected. The quantity consumed for this purpose, read off from the graduated scale, expresses the exact per-centage of the pure ALKALI, or of its CARBONATE, as the case may be, contained in the sample examined, provided 100 gr. have been taken for the assay. [Illustration] Another method sometimes used is that of M. Mohr, and practised as follows:——The alkaline solution, slightly coloured blue with litmus, is strongly super-saturated with a standard acid (sulphuric or oxalic) of known strength, supplied from an alkalimeter in the usual manner; the last traces of carbonic anhydride being removed by boiling, shaking, blowing into the flask, and, finally, sucking out the air. A standard solution of caustic soda (of a strength exactly corresponding to that of the test-acid already used) is now cautiously added, drop by drop, until the colour, rendered yellowish-red by the acid, just appears of a light blue. The difference between the quantity of the solution of the test-alkali and of the test-acid consumed, expresses the exact quantity of acid neutralised by the alkali, and hence also its strength.