46 Notes on Barometer Gonstruction. 



oxidation of the metal forming the impurity ; and this oxi- 

 dation will be attended with the fouling and breaking up of 

 the mirror surface by the formation of minute globules of 

 mercury — a grey mass which the adventitious oxide pre- 

 vents aggregating once again into the mirror form. But I 

 think it may be correctly stated of pure mercury that, 

 although it may be converted into red oxide at a compara- 

 tively high temperature, at ordinary temperatures of the 

 atmosphere it undergoes no perceptible oxidation of any 

 kind. Henry Watts* reiterates Gmelin's statement that 

 " mercury remains unaltered when agitated for any length 

 of time with oxygen gas, common air, hydrogen, nitrogen, 

 nitrous oxide, nitric oxide, carbonic acid gas, or alcohol ;" 

 and I believe that statement is strictly true as applied to pure 

 mercury and the ordinary constituents of atmospheric air. 



If the mercury is found to be impure by the tests already 

 given, or if it leaves the slightest residue — say of gold or 

 silver — after evaporation of a small sample, it may be dis- 

 tilled. A cast-iron retort, with wrought iron exit-tube, 

 is used for the purpose. It is furnished with a lid or cover 

 with turned joint, and fastened with screw-bolts or key- 

 wedges ; a lute of moist clay secures the joint. The lid of 

 the retort may be furnished with a stopper, which permits 

 renewal from time to time of the charge of mercury without 

 breaking the luted joint. The temperature at which the 

 metal " boils," or is said to boil, is rather high, say 662° 

 Fahr. or 350° C; but the capacity for heat of the vapour of 

 mercury, as compared with that of aqueous vapour for 

 example, is so low that a small quantity of fuel will do a 

 large amount of distillatory work, and the distillation is 

 therefore rapid. Among the papers of the Royal Society of 

 London, in the Proceedings "of that body, and probably also 

 in its Transactions, is a valuable contribution by W. R. 

 Grove on the ''Phenomena of Ebullition," in which it is shown 

 how great an influence the gases dissolved in water exert 

 upon the phenomenon. Water deprived of air can be con- 

 verted into vapour, but in a manner which it would be 

 incorrect to call boiling. As we apply heat, its temperature 

 gradually increases, and eventually mounts beyond the ordi- 

 nary boiling temperature ; finally the super-heated water is 

 in part converted into vapour by a sudden explosive act, 



* Dictionary of Chemistry, article " Mercury." 



