JOSEPH BLACK: HIS LIFE AND WORK 81 



of quick-lime in causticising alkali is similarly explained 

 by its removing the fixed air from the alkali, thus render- 

 ing the latter caustic, while itself becoming mild. 



Reasoning further, Black foresaw that caustic alkali, 

 added to Epsom salt or vitriolated magnesia, should give 

 a precipitate of magnesia which should not effervesce with 

 acids, for here fixed air is excluded ; and, also, that caustic 

 alkali should separate from acids lime in the quick state, 

 only united with water. 



Similar experiments of treating chalk with acids and 

 heating it, which had been performed with magnesia, 

 showed similar results. 



But it had yet to be demonstrated that fixed air did not 

 share the properties of ordinary atmospheric air. So 

 Black placed four fluid ounces of lime-water, as well as 

 four ounces of common water, under the receiver of an 

 air-pump, and exhausted the air; air rose from each in 

 about the same quantity ; it therefore appeared that the 

 air which quick-lime attracts is of a different kind from 

 that which is mixed with water. Quick-lime does not 

 attract air when in its most ordinary form, but is capable 

 of being joined to one particular species only, 'which is 

 dispersed through the atmosphere, either in the state of a 

 very subtle powder, or, more probably, in that of an elastic 

 fluid. To this I have given the name of fixed air, and 

 perhaps very improperly ; but I thought it better to use a 

 word already familiar in philosophy than to invent a new 

 name, before we be more fully acquainted with the nature 

 and properties of this substance.' 



The next step was to examine the nature of caustic 

 alkali, and to prove whether it gained weight on being 

 made ' mild.' This was achieved indirectly, by finding the 

 amount of acid required to neutralise the same weight of 

 caustic alkali, and ' salt of tartar ' what we know as 

 potassium carbonate. Six measures of acid were required 



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