88 



remains. If, as is often the case, moisture is present in the lime- 

 stone, the yield of lime would of course be under 56 per cent. 



Lime, or quick lime (where the word quick has the same mean- 

 ing as in the expression " the quick and the dead") is white and 

 usually devoid of crystalline form. Its melting point is exceed- 

 ingly high, but it can be melted and even boiled in the electric 

 furnace (M.P. about i,90o°C. = 3,452° F.). 



When water is added to quick lime chemical combination 

 takes place, slaked or slacked lime is formed and much heat is 

 liberated. A temperature approaching 300° F. is, under favourable 

 circumstances, reached and this often leads to a portion of the 

 water being dissipated as steam. That some of the water has 

 really entered into chemical combination with the lime is readily 

 ■shown. If upon a weighed quantity of quick lime sufficient water 

 is poured to make an obviously damp paste and the product then 

 dried in an oven heated to or a little above 2 12° F. that part of the 

 added water which has not combined with the lime will be expelled 

 as vapour, but that which has entered into chemical combination 

 will not be driven off. The resulting product, slaked lime, or, as 

 It is called by chemists, Calcium Hydroxide, is a dry white powder. 

 From 100 parts by weight of the original quick lime 132 parts of 

 dry slaked lime are obtained, or 1 ton, 6 cwt., 45ms. of slaked 

 lime can be got from 1 ton of quick lime. 



Slaked lime is slightly soluble in water and, unlike the great 

 majority of substances, its solubility decreases with increasing tem- 

 perature. The solution is known as lime water. A mixture of lime 

 water and undissolved lime is called " Milk of Lime. [The 

 •solubility is about 3 parts per 1,000 of water, or more than 3 

 gallons of water are required to dissolve 1 ounce of slaked lime] . 



Lime water is used in medicine and in chemistry. It is alka- 

 line in reaction, changing the colour of litmus from red to blue. 

 It is valuable as a test for carbon dioxide, which causes it to turn 

 milky. 



At this point I would remind you of what I said a little earlier 

 about the practical insolubility of calcium carbonate in pure water 

 and its conversion into the soluble bicarbonate by excess of carbon 

 dioxide. 



A simple experiment can be shown to illustrate this. On 

 passing a current of carbon dioxide from a " Kipp " apparatus 

 into lime water, the clear liquid becomes turbid as the lime in 

 solution becomes converted into carbonate of calcium, but the 

 turbidity disappears and the liquid once more becomes clear as the 

 stream of gas is continued. The liquid now contains calcium bi- 

 carbonate. Such a solution of bicarbonate of calcium is quite 

 common in nature and is yielded by springs or wells in the chalk. 

 The water is hard. On shaking it with a solution of soap it does 

 not yield a creamy lather as in the case of pure water, but forms 

 a curdy precipitate. 



Hardness of this kind is said to be temporary, because, on 

 boiling, the extra carbonic acid which keeps the calcium carbonate 



