52 HISTORICAL INTRODUCTION TO CHEMISTRY CHAP. 



a crust when exposed in open and shallow vessels, may be 

 preserved, for any time, in bottles which are but slightly 

 corked, or closed in such a manner as would allow free 

 access to elastic air, were a vacuum formed in the bottle," 

 and (2) under an exhausted receiver the same quantity of 

 air escapes from common water, and from water from 

 which the fixed air has been removed by the addition of 

 lime (A. C. R. I. 30). 



Fixed air is liberated from chalk by the action of 

 acids, Although Black was able to show that the loss of 

 weight in burning chalk to lime was due to the escape of 

 an invisible gas, it did not occur to him to attempt to 

 collect the gas, or even to render it visible by causing it to 

 bubble through water. But no special methods were 

 needed to render obvious the " violent breaking out of air " 

 which takes place when chalk is acted on by acids, nor the 

 absence of this effervescence when well-burnt quick-lime is 

 substituted for chalk. Remembering that 

 Chalk = lime + fixed air, 



the contrast between the action of acids in the two cases 

 was sufficient to suggest that the gas so easily set free by 

 acids must be the same as the fixed air which escapes 

 when chalk is burnt to lime. 



It is remarkable that Black did not attempt to test the 

 gas by passing it into lime-water, 1 but apparatus such as is 

 now used for this purpose had not yet been devised, and 

 the systematic study of gases did not begin until nearly 

 twenty years later. 



A very satisfactory proof of the identity of the two gases 

 was, however, obtained by showing .(i) that the weight of 

 gas which escapes is the same, whether the chalk is decom- 

 posed by acids or by heating it in a furnace ; and (2) that 

 the same weight of acid is required to dissolve the chalk 



1 See footnote, p. 385, for Black's use of lime-water -as a test for 

 fixed air. 



