ASSIMILATION 79 



oyster-shells, etc. In the true spirit of experiment he began by 

 strongly heating his retorts (one of which was a musket barrel) 

 to make sure that no air arose from them. It is not evident to 

 me why he continued at this subject so long. He had no means 

 of distinguishing one gas from another, and almost the only 

 quality noted is a want of permanence, e.g. when the CO2 

 produced was dissolved by the water over which he collected 

 it. Sir E. Thorpe^ points out that Hales must have prepared 

 hydrogen, carbonic acid, carbonic oxide, sulphur dioxide, marsh 

 gas, etc. It may, I think, be said that Hales deserved the 

 title usually given to Priestley, viz. " the father of pneumatic^ 

 chemistry." 



Perhaps the most interesting experiment made by Hales is 

 the heating of minium (red-lead) with the production of oxygen. 

 It proves that he knew, as Boyle, Hooke and Mayow did before 

 him, that a body gains weight in oxidation. Thus Hales remarks: 

 " That the sulphurous and aereal particles of the fire are lodged 

 in many of those bodies which it acts upon, and thereby con- 

 siderably augments their weight, is very evident in Minium or 

 Red Lead which is observed to increase in weight in under- 

 going the action of the fire. The acquired redness of the 

 Minium indicating the addition of plenty of sulphur in the 

 operation." He also speaks of the gas distilled from minium, 

 and remarks " It was doubtless this quantity of air in the 

 minium which burst the hermetically sealed glasses of the 

 excellent Mr Boyle, when he heated the Minium contained in 

 them by a burning glass " (p. 287). 



This was the method also used by Priestley in his celebrated 

 experiment of heating red-lead in hydrogen ; whereby the 

 metallic lead reappears and the hydrogen disappears by com- 

 bining with the oxygen set free. This was expressed in the 

 language of the day as the reconstruction of metallic lead by 

 the addition of phlogiston (the hydrogen) to the calx of lead 

 (minium). Thorpe points out the magnitude of the discovery 

 that Priestley missed, and it may be said that Hales too was on 



1 History of Chemistry, 1909, i. p. 69. 



^ Hales made use of a rough pneumatic trough, the invention of which is usually 

 ascribed to Priestley (Thorpe's History of Chemistry, i. p. 79). 



