Oxalic acid is the easiest to make of any. A sufficient quantity of pure crystallised oxalic acid is powdered and pressed between filter paper, so as to absorb the moisture which occasionally is retained in cavities of the crystals. 6·3 _grm._ is then weighed out and dissolved in water, exactly as was done with sodic carbonate, forming a decinormal solution. It is used in Löwenthal's tannin estimation process and may also be employed to determine alkalies, but forms insoluble calcium oxalate with lime salts, and does not give a sharp reaction with methyl orange indicator. Hence litmus must be used, or a few drops of a neutral solution of calcium chloride added to the methyl orange, when hydrochloric acid will be liberated as soon as there is excess of the acid, and the indicator will be promptly reddened. Sulphuric acid is the most permanent of any acid solution, and may be generally employed. It forms insoluble sulphates with lime, baryta, and strontia. To make a normal solution, 35 _c.c._ of the pure concentrated acid are poured into at least 3 or 4 times as much distilled water, and allowed to cool, and are then made up to about 1 _litre_ and well shaken. The burette is filled with the mixture, 10 _c.c._ of the standard sodic carbonate are measured into a beaker, 2 or 3 drops of methyl orange solution are added, and the acid is run in with constant stirring till the indicator is just beginning to redden. This must be repeated, and the two titrations should exactly agree. Suppose that 9.5 _c.c._ are required, then 950 _c.c._ of the trial acid are equal to 1 _litre_ of the soda. If therefore 950 _c.c._ be measured into a test mixer, and made up to 1 _litre_, the solution should be accurately decinormal. Of course great care must be used in the whole process. If a gauged flask only is at hand it will be easier to measure into it the water required to make up the _litre_, and then fill to the mark with the trial acid. Normal hydrochloric acid may be made exactly as described for sulphuric acid, but using about 100 _c.c._ of the strongest acid. Decinormal solutions of both these acids may be made by the same methods; using 1 tenth the quantities, or by dilution of the normal solution. Beside comparison with sodic carbonate solution, hydrochloric acid may also be checked by determining the amount of chlorine present, with silver nitrate (see p. 98) 10 _c.c._ of decinormal acid should of course be equal to 10 _c.c._ of decinormal silver nitrate. Table giving the Quantity of the Following Substances contained in or equivalent to 1 _litre_ of Normal or 10 _litres_ of Decinormal Standard Solution.