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tive " phosphorescent" is still used in the wider sense. Luminous 

 paint, luminous match boxes, and the like owe their light-giving 

 properties to calcium sulphide. 



When the oxides of such metals as iron, copper, lead, zinc, 

 etc., are heated with carbon the oxygen of the oxide unites with 

 the carbon to form one or both of the gaseous oxides of carbon and 

 the metal is set free. Lime, the oxide of calcium, behaves differ- 

 ently. At the temperatures obtainable in ordinary furnaces no 

 action takes place, but under the fervent heat of the electric fur- 

 nace the case is different. Part of the carbon combines with the 

 oxygen of the lime, giving carbon monoxide, and a further portion 

 of the carbon unites with the calcium, so that the solid product of 

 the reaction is not the metal, but its carbide. This, when quite 

 pure, is colourless, transparent and crystalline, but the com- 

 mercial substance is greyish black owing to the presence of 

 numerous impurities. When it is treated with water acetylene 

 gas is evolved, used, as everyone knows, as an illuminant, on the 

 small scale for bicycle lamps and on the big scale for country 

 houses, churches, etc., which are remote from gas or electric 

 works. 



A further use of calcium carbide is of great and increasing 

 importance. Heated in nitrogen a new compound called Calcium 

 Cyanamide (CN.N Ca) results. This is used in the preparation of 

 ammonia, of the cyanides and other compounds of nitrogen, and 

 as a fertiliser. 



Just as calcite, Iceland spar, marble, chalk and limestone 

 are naturally occurring forms of a single chemical compound, 

 so Selenite, Alabaster, and Gypsum consist of another very 

 important calcium compound, the sulphate. For the benefit of 

 those who have not studied chemistry I may say that whereas 

 Calcium Sulphide is a compound of calcium and sulphur only, 

 Calcium Sulphate contains these elements together with a large 

 proportion of oxygen, the ratio of sulphur to oxygen being the 

 same as in all other sulphates and as' in sulphuric acid. This sub- 

 stance occurs in nature as anhydrite, which has no combined 

 water, and as ordinary gypsum which does contain water. 



The suphate is slightly soluble in water and is often present 

 in natural w T aters. Water containing it is hard, i.e., it does not 

 lather freely with soap, and hardness due to this cause is called 

 permanent since it is not removed or diminished by boiling. For 

 washing and such-like domestic purposes this kind of hardness 

 can, however, be got rid of by the addition of a suitable quantity 

 of washing soda, but water thus softened will contain sodium com- 

 pounds and would not be improved for drinking. 



To return to the water chemically combined with the calcium 

 sulphate in gypsum. The chemical formula CaSO 4 2H 2 0 indicates 

 that 2 molecules of water are present to each molecule of the 

 anhydrous, i.e., waterless, sulphate. This proportion is slightly 

 under 21 per cent, of the whole. By strongly heating the gypsum 

 all the water is expelled and the residue is not only much less 



