JOSEPH BLACK : HIS LIFE AND WORK 79 



Another memorandum, a few pages later, shows that he 

 had solved the mystery. ' When I precipitate lime by a 

 common alkali there is no effervescence. The air quits 

 the alkali for the lime, but it is not lime any longer, but 

 c.c.c. It now effervesces, which good lime will not.' 



But we must trace the chain of reasoning which led him 

 to come to this conclusion. 



Having prepared ' mild ' magnesia by mixing Epsom 

 salt or sulphate of magnesia with carbonate of potash, or 

 ' pearl- ashes/ he found that it is ' quickly dissolved with 

 violent effervescence or explosion of air by the acids of 

 vitriol, nitre, and of common salt, and by distilled vinegar ' ; 

 that the properties of these salts the sulphate, nitrate, 

 chloride, and acetate of magnesium differ greatly from 

 those of the common alkaline earths; that when boiled 

 with 'salt-ammoniac,' or chloride of ammonium, volatile 

 crystals of smelling-salts were deposited on the neck of 

 the retort, which, on mixing with the chloride of mag- 

 nesium remaining in the retort, reproduced the 'mild' 

 magnesia; that a similar effect is produced by boiling 

 ' mild ' magnesia with ' any calcareous substance ' ; while 

 the acid quits the calcareous salt to unite with the mag- 

 nesia, ' mild ' magnesia is again precipitated on addition of 

 a dissolved alkali. 



On igniting ' mild ' magnesia, it changed into a white 

 powder, which dissolved in acids without effervescence. 

 And the process of ignition had deprived it of seven- 

 twelfths of its weight. Black next turned his attention to 

 the volatile part ; he attempted to restore it by dissolving 

 the magnesia in a sufficient quantity of ' spirit of vitriol ' 

 or dilute sulphuric acid, and separated it again by the 

 addition of alkali. The resulting white powder now effer- 

 vesced violently with acids, and 'recovered all those 

 properties which it had lost by calcination. It had 

 acquired besides an addition of weight nearly equal to 



