v THE STUDY OF GASES 83 



volatile spirit remained permanently elastic when collected 

 over mercury, but collapsed immediately when brought into 

 contact with water. Priestley found that alkaline air was 

 only a little less soluble than acid air, one volume of water 

 dissolving 336 volumes of the gas. 1 It also differed from 

 acid air in being lighter instead of heavier than common air. 

 He found that the gas (to which the name of ammonia was 

 given by Bergman in 1782), could be prepared directly by 

 heating crystals of sal-ammoniac with slaked lime in a gun 

 barrel : by passing the product into water he obtained a 

 very strong " spirit " from which he could expel the alkaline 

 ;iir as required {Experiments on Air, 1774, I. 163-169). 



Priestley prepares sal-ammoniac by mixing alkaline 

 air with acid air. Thinking that gases possessed of such 

 opposite properties were likely to combine together to form 

 .1 neutral substance, possibly identical with common air, 

 Priestley brought together vessels containing alkaline air and 

 icid air. To his great astonishment he obtained a solid 

 oroduct which he identified as sal-ammoniac ; this volatile 

 salt was therefore a compound formed by the union of 

 alkaline air with acid air. 



" Having satisfied myself with respect to the relation that 

 alkaline air bears to water, I was impatient to find what 

 would be the consequence of mixing this new air with the 

 other kinds with which I was acquainted before, and 

 especially with acid air ; having a notion that these two airs, 

 being of opposite natures, might compose a neutral air, 

 and perhaps the very same thing with common air. But 

 the moment that these two kinds of air came into contact, 

 a beautiful white cloud was formed, and presently filled the 

 whole vessel in which they were contained. At the same 

 time the quantity of air began to diminish, and, at 

 length, when the cloud was subsided, there appeared to be 

 formed a solid white salt, which was found to be the common 



1 One and a-quarter grains of water dissolved seven-eighths ounce- 

 measures of gas. 



G 2 



