76 PHYSIOGRAPHY. [chap. 



tlie air at a high temperature. This is the case, for instance, 

 with quicksilver. The rusting of this particular metal is 

 worth closer study, since it was the means which led, about 

 a century ago, to the discovery of the chemical composition 

 of the atmosphere. 



Quicksilver, or mercury, as seen in the weather-glass, is 

 as brilliant as solid burnished silver, and this brilliancy is 

 retained even after long exposure to air and moisture. But 

 if the liquid metal be kept, for some time, at an elevated 

 temperature in contact with air, small reddish scales slowly 

 appear upon its surface, and ultimately the metal may be 

 entirely converted into this substance. The red rust of 

 mercury thus obtained is identical with a substance long 

 known in pharmacy as " red precipitate," — a substance 

 which is prepared commercially by other processes more 

 convenient and rapid than that of heating mercury. 



It is especially notable that during the rusting of quick- 

 silver, as indeed of all other metals, there is a very appreciable 

 increase of weight in the substance operated on. A pound 

 of metal produces considerably more than a pound of its 

 rust. In point of fact, every loo lbs. of quicksilver will 

 produce not less than io8 lbs. of red rust. This increase 

 of weight shows that, during the operation of nisting, some- 

 thing must be absorbed by the metal ; and as the mercury 

 can be converted into rust when heated in contact with 

 nothing but air, it is obvious that the additional matter 

 must have been absorbed from the atmosphere. The nature 

 of this absorbed matter may be determined by a simple 

 experiment. 



Let a small quantity of red precipitate, or rust of mercury, 

 be strongly heated in a tube of hard glass, represented at a, 

 Fig, 20. If the tube be heated for a sufficient time the 

 red powder may entirely disappear. But by making a bend 



