It will be noticed that unlike that of most substances, the solubility of lime in water diminishes as the temperature is raised. It is therefore necessary in employing lime-water as a standard solution to take care that it is saturated at a constant temperature. The results given in the above table are those from pure marble lime. Where the ordinary impure limes from limestone are employed, a somewhat stronger lime-water is often obtained. This is difficult to explain, but possibly some double hydrate of lime and magnesia is formed which is more soluble than either hydrate alone. The results harmonise with the old belief of tanners that chalk-lime is milder in its action on skin than that made from less pure limestones. The solubility of any given lime is easily determined by adding it in excess to water in a stoppered flask, and shaking frequently until a solution of constant strength is obtained. A known volume of this solution (which must be clear and free from undissolved lime) is then titrated with N/10 hydrochloric acid, using phenolphthalein as the indicator. Saturated lime-water may be conveniently used as an alkaline standard solution for many purposes, and if kept on excess of lime is always caustic, and varies very little in strength at ordinary laboratory temperatures. The solution is nearly ¹⁄₂₀ normal, but for accurate work its strength should be exactly determined with N/10 acid. 1 liter of pure lime-water at 15° C. should require 471·4 c.c. of N/10 acid for neutralisation. Lime is much more soluble in sugar solutions than in water. Such solutions have been used as standard solutions, and sugar has been added to limes to increase the action on the hides. The following is the analysis of a lime used in a Leeds tannery, which was made by Mr. G. W. Flower, B.Sc., in the Leather Industries Laboratory of the Yorkshire College[71]:-- Per cent. SiO₂ and insoluble matter 17·70 Fe₂O₃ 6·42 CaO 49·86 CaCO₃ 14·21 CaSO₄ 3·01 CaCl₂ 0·33 MgO 2·09 Organic matter 0·80 Moisture by difference 5·58 ------ 100·00 [71] Journ. Soc. Chem. Ind., 1901, p. 224. The sample only contained 31·02 per cent. of available lime, the remainder being probably combined with the silica. It also contained an appreciable quantity of iron oxide, which might lodge mechanically in the pores of the skin and become dissolved in later processes, darkening the colour of the leather. The lime was also under-burnt, judging from the amount of carbonate it contained. For comparison with this, the analysis of a good specimen of carboniferous-limestone lime from Buxton may be given:--