1818.] On the Height of the Himalaya Mountains. 47 



glass retains its own colour while hot, but assumes that of the 

 iron as it cools. 



Oxide of Tin in form of hydrate, and in its highest degree of 

 purity, becomes yellow when heated, then red, and when ap- 

 proaching to ignition black. If iron or lead be mixed with it, 

 the colour is dark-brown when heated. These colours become 

 yellowish as the substance cools. Upon charcoal, in the interior 

 flame it becomes and continues white ; and, if originally white 

 and free from water, it undergoes no change of colour by heat- 

 ing. It is very easily reduced without addition, but the reduction 

 is promoted by adding a drop of solution of soda or potash. 



Oxide of Lead melts, and is veiy quickly reduced, either with- 

 out any addition, or when fused with microcosmic salt or borax. 

 The glass not reduced is black. 



Oxide of Copper is not altered by the exterior flame, but be- 

 comes protoxide in the interior. With both microcosmic salt and 

 borax it forms a yellow-green glass while hot, but which becomes 

 blue-green as it cools. When strongly heated in the interior 

 flame it loses its colour, and the metal is reduced. If the quan- 

 tity of oxide is so small that the green colour is not perceptible, 

 its presence may be detected by the addition of a little tin, 

 which occasions a reduction of the oxide to protoxide, and pro- 

 duces an opaque, red glass. If the oxide has been fused with 

 borax, this colour is longer preserved ; but if with microcosmic 

 salt, it soon disappears by a continuance of heat. 



The copper may also be precipitated upon iron, but the glass 

 must be first saturated with iron. Alkalies or lime promote this 

 precipitation. If the glass containing copper be exposed to a 

 smoky flame, the copper is superficially reduced, and the glass 

 covered while hot with an iridescent pellicle, which is not always 

 permanent after cooling. It is very easily reduced by soda. 

 Salts of copper, when heated before the blow-pipe, give a fine 

 green flame. 



Oxide of Mercury before the blow-pipe becomes black, and is 

 entirely volatilized. In this manner its adulteration may be dis- 

 covered. 



The other metals may be reduced by themselves, and may be 

 known by their own peculiar characters. 



Article X. 



On the Height of the Himalaya Mountains. 



I \ the proceedings of the Royal Academy of Sciences at 

 Paris, inserted in the last number of the Annals, is an account 

 of a memoir by Alex. Von Humboldt " On the Height of the 

 Mountains of India." It is there stated that " the 12th volume 



