89 



dissolved is expelled ; this carbonate is then deposited, generally 

 as an incrustation or fur on the sides and bottom of the kettle or 

 boiler. Another way of softening the water is by adding lime 

 water. 



Since lime is, in a sense, the cause of the hardness, some 

 people find it difficult to see how water can be softened by adding 

 more. The matter, however, becomes clear if we bear in mind 



1. That lime in the absence of carbon dioxide is soluble in 



water. 



2. That lime plus a single equivalent of carbon dioxide forms 



the insoluble carbonate. 



3. That lime plus two equivalents of the dioxide forms the 



soluble bicarbonate. 

 If we have a sample of water containing this bicarbonate, 

 to get rid of it we must convert it into insoluble carbonate. This 

 we can do either 



a. By boiling ; out half the carbon dioxide, or 



b. By adding more lime to absorb it. 



The precipitate which is obtained contains both the lime 

 originally present and that which has been added in the form of 

 lime water. 



Our Bournemouth water supply is largely obtained from deep 

 wells in the chalk near Wimborne. The water is very hard as it 

 comes from the wells, but its hardness is materially reduced by the 

 addition of lime. 



We have seen that chalk can be broken up into lime and a 

 gas and so is obviously not a chemical element ; we now have to 

 ask whether lime itself is an element or whether it also can be 

 shown to consist of still simpler substances united together. 



In ancient times lime was considered to be an element ; it was 

 indeed called an earth although it was distinguished from other 

 forms of what was then supposed to be one of the four elements 

 by the name terra alcalina. Suspicions that it might be related to 

 some unknown metal in the same way that k ' calx ' ' of lead 

 (litharge) is related to metallic lead subsequently came to be enter- 

 tained. It was not till 1807 tnat this guess was definitely proved 

 to be well founded. In that year Sir Humphry Davy showed that 

 lime was a compound of oxygen and a metal to which the /name 

 of Calcium was given. Nearly 100 years elapsed, however, before 

 a method of obtaining calcium at a reasonable price was devised. 

 This was at last done, and the price sank in 1905 from £g to 

 eighteen pence per ounce, and, before the outbreak of the war, it 

 could be got for less than 6d. per ounce. 



The method now employed consists in the electrolysis of fused 

 calcium chloride to which a little calcium fluoride is generally 

 added. 



The metal is fairly hard, has a specific gravity of about 1.6 

 and a melting point of 8oo° C, or 1472 0 F., much higher than that 

 of zinc, but considerably below that of copper. It acts slowly 



