1818,] On the Blow-pipe. 43 



reddish colour like flesh is produced, which, however, is scarcely 

 visible by the light of a lamp. And magnesia may by this 

 process be detected in compound bodies, if they do not contain 

 much metallic matter, or a proportion of alumina exceeding the 

 magnesia. Some inference as to the quantity of the magnesia 

 may be drawn from the intensity of the colour produced. 



All these alkaline earths, when pure, are readily fusible in 

 combination with the fluxes into a clear, colourless glass, without 

 effervescence ; but on adding a further quantity of the earth, the 

 glass becomes opaque. 



Alumina combines more slowly with the fluxes than the pre- 

 ceding earths do, and forms a clear glass, which does not be- 

 come opaque. But the most striking character of alumina is the 

 bright blue colour it acquires from the addition of a drop of 

 nitrate of cobalt, after having been dried and ignited for some 

 time. And its presence may be detected in this manner in com- 

 pound minerals where the metallic substances are not in great 

 proportion, or the quantity of magnesia large. Alumina may 

 be thus detected in the agalmatolite. 



II. THE METALLIC OXIDES. 



Arsenic flies off accompanied by its characteristic smell, re- 

 sembling garlic. When larger pieces of white arsenic are heated 

 on a piece of ignited charcoal, no smell is perceived. To produce 

 this effect the white oxide must be reduced, by being mixed 

 with powdered charcoal. If arsenic is held in solution, it may 

 be discovered by dipping into the solution a piece of pure and 

 well-burned charcoal, which is afterwards to be dried and 

 ignited. 



Chrome. Its green oxide, the form in which it most com- 

 monly occurs, and to which it is reduced by heating in the 

 common air, exhibits the following properties : it is fusible with 

 microsmic salt, in the interior flame, into a glass which at the 

 instant of its removal from the flame is of a violet hue, approach- 

 ing more to the dark blue or red, according to the proportion of 

 chrome. After cooling the glass is bluish green, but less blue 

 than the copper glass. In the exterior flame the colour becomes 

 brighter, and less blue, than the former. With borax it forms a 

 bright yellowish or yellow-red glass in the exterior flame ; and in 

 the interior flame this becomes darker and greener, or bluish- 

 green. The reduction with soda has not been examined. 



Molybdic Avid melts by itself upon the charcoal with ebulli- 

 tion, and is absorbed. In a platina spoon it emits white fumes, 

 and is reduced in the interior flame to molybdous acid, which is 

 blue ; but in the exterior flame it is again oxidated, and becomes 

 white. With microcosmic salt, in the exterior flame, a small 

 proportion of the acid gives a green glass, which by gradual 

 additions of the acid passes through yellow-green to reddish, 



