iv CHALK, LIME, AND THE ALKALIS 53 



before and after burning it to lime. In the first experi- 

 ment Black saturated 120 grains of chalk with diluted spirit 

 of salt in a long-necked Florentine flask, and found a loss 

 in weight of 48 grains, as compared with 52 grains when 

 an equal weight of chalk was burnt to lime. In the second 

 experiment he found that 120 grains of unburnt chalk were 

 dissolved by 421 grains of diluted spirit of salt, whilst the 

 same quantity of chalk, burnt to quick-lime and slaked with 

 an ounce of water, required 414 grains of the acid, but 

 dissolved "without any sensible effervescence or loss of 

 weight" (A. C. R. I. 28). 



Fixed air is present in mild alkalis such as soda and 

 potash. It had been known from early times that the mild 

 alkalis resembled chalk in that they effervesced when acted 

 on by acids. This property was regarded as a chief 

 characteristic of the alkalis, an idea that is preserved in 

 the use of the word " kali " to describe a mixture of sugar 

 with a mild alkali and a solid acid, which gives an 

 effervescent drink when added to water. As in the case of 

 the action of acids on chalk, Black did not test the gas by 

 passing it into lime-water, but he obtained an even more 

 satisfactory proof of the presence of fixed air in the alkalis 

 by showing that they could reconvert lime into chalk 

 quantitatively. This process had the advantage that it 

 rendered the use of an acid quite unnecessary in proving 

 the composition of the alkalis. 



" A piece of perfect quick-lime made from two drams of 

 chalk, and which weighed one dram and eight grains, was 

 reduced to a very fine powder, and thrown into a filtrated 

 mixture of an ounce of a fixed alkaline salt, and two ounces 

 of water. After a slight digestion, the powder being well 

 washed and dried, weighed one dram fifty-eight grains. 1 

 It was similar in every trial to a fine powder of ordinary 



1 i.e. One hundred and twenty grains of chalk gave 68 grains of lime 

 from which 118 grains of chalk were recovered by the action of an 

 alkali. 



