iv CHALK, LIME, AND THE ALKALIS 51 



the water and join itself to the air, for which it has a 

 superior attraction, and is therefore restored to its first state 

 of mildness and insolubility in water " (A. C. R. I. 24). 



The conversion of soluble lime into insoluble chalk was 

 thus a test for the presence of fixed air. The test was most 

 sensitive when lime-water was used, because the chalk could 

 then be seen immediately. But in all important cases 

 Black preferred to rely on quantitative experiments in which 

 the chalk, after burning to lime, was recovered in such a 

 way as to show that the original weight of chalk had been 

 reproduced. 



Fixed air is present in common air and in air 

 dissolved by water. By noticing its action upon lime, 

 Black was able to show that a small quantity of fixed air 

 was dissolved in ordinary water, since : 



'' When slaked lime is mixed with water, the fixed air in 

 the water is attracted by the lime, and saturates a small 

 portion of it, which then becomes again incapable of dis- 

 solution, but part of the remaining slaked lime is dissolved 

 and composes lime-water " (A. C. R. I. 24). 



The presence of fixed air in common air was also shown 

 by its action on lime-water, for : 



" If this fluid be exposed to the open air, the particles of 

 quick-lime which are nearest the surface gradually attract 

 the particles of fixed air which float in the atmosphere. 

 But at the same time that a particle of lime is thus saturated 

 with air, it is also restored to its native state of mildness 

 and insolubility ; and as the whole of this change must 

 happen at the surface, the whole of the lime is successively 

 collected there under^ its original form of an insipid 

 calcareous earth, called* the cream or crusts of lime-water" 

 (A. C. R. I. 24). 



The fixed air forms, however, only a small proportion 

 both of ordinary air and of the air which is dissolved in 

 water, for(i) "lime-water, which soon attracts air, and forms 



E 2 



