90 



upon water, setting free hydrogen gas and being itself converted 

 into slaked lime. 



There are a great many compounds of calcium besides those 

 already mentioned, and I purpose giving a few particulars con- 

 cerning a small selection of these. 



Lime is not the only known compound of calcium and oxygen. 

 When hydrogen peroxide is added in excess to lime water a 

 precipitate is obtained which consists of a hydrate of Calcium 

 Dioxide. By collecting, drying, and gently heating this precipi- 

 tate the combined water is expelled and the pure dioxide results. 

 On the large scale it is prepared by other methods. It contains 

 44.4 per cent, of oxygen as against the 28.5 per cent .present in 

 lime. 



Calcium can combine with hvdrogen ; the compound known 

 as Calcium Hydride, or commercially as hydrolith, is formed by 

 heating the metal in a current of hydrogen gas. It burns brilli- 

 antly, especially in oxygen, and is readily attacked by water, an 

 ounce of the solid compound yielding about one cubic foot of 

 hydrogen, half of which comes from the water. 



An interesting natural compound of calcium is the fluoride, 

 long known as fluor spar, the Blue John of the Derbyshire Caves. 

 The name fluor is from the Latin fluo, I flow, the mineral having 

 long been used in the treatment of metallic ores to increase the 

 fluidity under the action of heat of stony substances that could not 

 otherwise be easily melted. The vocabulary of optics owes the 

 word " fluorescence " to fluor spar, its property of becoming 

 luminous w T hen heated having been observed as far back as 1677. 



Calcium Chloride, which must not be confounded 

 with the very different substance known in commerce 

 as Chloride of Lime, is a very useful substance 

 to the chemist. It is exceedingly soluble in water and 

 readily absorbs water vapour, hence it is largely used in drying 

 gases. It has been known since the 14th century, and an experi- 

 ment which consists in mixing a concentrated solution of this 

 substance with a strong solution of caustic potash or soda was 

 described in 1686 under the name of " the chemical miracle." 



The valuable disinfectant and bleaching agent known as 

 Bleaching Powder or chloride of lime is made on a very large 

 scale by the action of chlorine on dry lime. A simple experiment 

 suffices to illustrate the right of this substance to the first of these 

 names. 



If a solution of bleaching powder is added to an infusion of 

 logwood the deep red colour at once disappears. 



Calcium Sulphide can be so prepared as to be luminous in the 

 dark. Made by heating calcined oyster shells with sulphur it was 

 formerly known as Canton's Phosphorus. The word phosphorus, 

 or light-bearer, was formerly applied to any substance that glowed 

 in the dark instead of being limited to a particular member 1 of the 

 family of chemical elements as is at present the case. The adjec- 



