=Sodium Carbonate= (Soda Crystals), Na₂CO₃.10H₂O = 286, is made in enormous quantities in great works and in a very pure state. It forms large transparent crystals, which effloresce in the air, losing a large quantity of water, and so falling to a white powder. Although this property does not interfere with the use of soda, since it is generally used in solution, yet efflorescence should be as far as possible avoided by keeping the salt in well-closed packages, because effloresced soda dissolves more slowly than crystallised, since it has to combine with water before it can enter into solution. In the retail trade a form of soda is found which is adulterated with very large quantities of Glauber’s salt. This is recognised by the different form of the crystals. Manufacturers sell soda stating its strength. The colour maker should only buy with this guarantee. =Sodium Hydroxide= (Sodium Hydrate, Caustic Soda), NaOH = 40, comes into commerce in the form of hard masses, the use of which would be very convenient for the colour maker if it were not often very impure. Thus it is better to prepare a solution oneself, which is accomplished in the manner given above for caustic potash. Caustic soda and caustic potash have similar properties; they have a corrosive action on the skin, readily unite with carbonic acid from the air, and separate the heavy metals from their solutions in the form of hydrated oxides. The strength of caustic soda solutions is given in the following table:— +----------+----------+----------+----------+ | Specific | Caustic | Specific | Caustic | | Gravity. | Soda | Gravity. | Soda | | | per cent.| | per cent.| +----------+----------+----------+----------+ | 2·00 | 77·8 | 1·40 | 29·0 | | 1·85 | 63·6 | 1·36 | 26·0 | | 1·72 | 53·8 | 1·32 | 23·0 | | 1·63 | 46·6 | 1·29 | 19·0 | | 1·56 | 41·2 | 1·23 | 16·0 | | 1·50 | 36·8 | 1·18 | 13·0 | | 1·47 | 34·0 | 1·12 | 9·0 | | 1·44 | 31·0 | 1·06 | 4·7 | +----------+----------+----------+----------+ Besides soda and caustic soda, few soda salts are used in colour making. However, sodium nitrate (Chili saltpetre), NaNO₃, is frequently used instead of ordinary saltpetre, from which it differs in being deliquescent. =Sodium Thiosulphate= (Hyposulphite), Na₂S₂O₃.5H₂O = 248, is a common article of commerce, being much used by photographers; it is used in a few cases in colour making. It forms large crystals with a somewhat bitter taste, permanent in air and readily soluble in water. =Sodium Chloride= (Common Salt), NaCl = 58·5, which has a little application in colour making, is sufficiently pure in the form in which it is generally used for household purposes. SALTS OF THE ALKALINE EARTH METALS.