122 Mr. Delaval on the Catije of the 



ambient air, and thereby acquire a confiderablc 

 increafe of weight. When they are again re- 

 duced to their metallic form, a quantity of air, 

 equal to that which was acquired during the 

 calcination, is difengaged, and the mafs lofes 

 the adventitious weight, which it poflefled du- 

 ring its calciform ftate. 



Metals, calcined by heat only, a6l upon light 

 in the fame manner as tiiofe which have been 

 previoufly diffolved, and afterward calcined. 

 They agree, in the difcontinuity of their par- 

 ticles; in their abforption of air, and their con- 

 fequent increafe of weight ; and in their tran- 

 fparency, which is manifefled, when they are 

 mixed with pellucid glafs, or other pellucid 

 media, and when tranfparent metallic glafles 

 are produced by their fufion. 



Metallic precipitates, which are combined 

 with fome of the matters, by which they were 

 diflblved, or precipitated, tranfmit the light 

 which is refleded by thofe matters. When pre- 

 cipitates are difengaged from fuch extraneous 

 parts, by ablution, calcination, or any other 

 means, they are mere calces, fuch as have been 

 already confidered. 



When oil, inftead of air, is interpofed between 

 the particles of paints which are formed of the 

 denfer metals, fuch as vermilion, and minium, 

 their colours are not perceptibly altered : be- 

 caufe, although oil confiderably exceeds air, in 



its 



