CH. iv CHALK, LIME, AND THE ALKALIS 49 



described as QUICKLIME because it becomes hot and steamy 

 when water is poured upon it ; the soft powder formed when 

 the hard lumps of quicklime are wetted with water, or are 

 left exposed to the air, is known as SLAKED LIME. Slaked 

 lime dissolves easily in acids and to a slight extent in water ; 

 the solution in water is known as LIME-WATER. 



At the time when Black began his investigations, it was 

 generally believed that the caustic properties of lime were 

 due to the absorption of igneous particles from the fire in 

 which it had been burnt ; this view, like the phlogiston 

 theory of calcination, was found to be untenable as soon as 

 exact quantitative measurements were made. Black's experi- 

 ments were described in a paper " Experiments upon 

 Magnesia Alba, Quick-lime and some other Alkaline 

 Substances" (A. C. R. No. I.) published in 1755. The y 

 were of importance on account of his success in solving the 

 problem of the relationship between chalk and lime, and also 

 because of the stimulus that they gave to exact quantitative 

 work. The principal points of this investigation are set out 

 in the following paragraphs. 



Chalk loses in weight when burnt to lime owing to 

 the escape of fixed air. Black found that "a piece of 

 perfect quick-lime made from two drams of chalk . . . 

 weighed one dram and eight grains " (A. C. R. I. 28), i.e., 

 120 grains of chalk gave 68 grains of lime, a loss in weight 

 of 52 grains, or nearly 44%. This large loss in weight 

 could only be due to the escape of a gas, since nothing but 

 a little water could be condensed, when chalk was burnt to 

 lime in a retort. 



" That the calcareous earths really lose a large quantity of 

 air when they are burnt to quick-lime, seems sufficiently 

 proved by an experiment of Mr. Margraaf. . . He subjected 

 eight ounces, of [chalk] to distillation in an earthen retort, 

 finishing his processs with the most violent fire of a 

 reverberatory, and caught in the receiver only two drams of 



