JOSEPH BLACK 95 



alkaline salts contain a large quantity of fixed air, which they emit 

 in great abundance when joined to a pure acid. In the present case, 

 the alkali is really joined to an acid, but without any visible emission 

 of air ; and yet the air is not retained in it ; for the neutral salt, into 

 which it is converted, is the same in quantity, and in every other 

 respect, as if the acid employed had not been previously saturated 

 with magnesia, but offered to the alkali in its pure state, and had driven 

 the air out of it in their conflict. It seems therefore evident, that 

 the air was forced from the alkali by the acid, and lodged itself in the 

 magnesia. 



These considerations led me to try a few experiments, whereby I 

 might know what quantity of air is expelled from an alkali, or from 

 magnesia, by acids. 



Two drams of a pure fixed alkaline salt, and an ounce of water, 

 were put into a Florentine flask, which, together with its contents, 

 weighed two ounces and two drams. Some oil of vitriol diluted with 

 water was dropped in, until the salt was exactly saturated ; which it was 

 found to be, when two drams, two scruples and three grains of this 

 acid had been added. The phial with its contents now weighed two 

 ounces, four drams and fifteen grains. One scruple, therefore, and 

 eight grains, were lost during the ebullition ; of which a trifling portion 

 may be water, or something of the same kind ; the rest is air. 



