36 



MINERALS AND GEOLOGY 



other bases, remain unattacked. Manganese compounds form by oxidizing 

 fusion with this reagent a green glass, which becomes blue or bluish-green 

 and opaque on cooling. A very minute amount of manganese may be thus 

 detected. The delicacy of the test is increased by the addition of a small 

 quantity of nitre, as this promotes oxidation ; and if the 'substance contain 

 much lime, magnesia, iron oxides, or other bodies more or less insoluble in 

 carb. soda, it is advisable to add a little borax to the test-mixture. The blue 

 or bluish-green bead thus produced, is technically known as a "turquoise 

 enamel." Chromium compounds produce a somewhat similar reaction ; but if 

 the bead be saturated with silica or boracic acid, it will remain green in the 

 latter case. If the green colour result from the presence of manganese, on the 

 other hand, a violet or amethystine glass will be obtained. Some other appli- 

 cations of carbonate of soda as a blowpipe re-agent will be found under the 

 head of REACTIONS. 



6. Reduction : This term denotes the process by which an oxidized 

 or other compound is converted into the metallic state. Some 'com- 

 pounds become reduced by simple ignition ; others require for their 

 reduction the addition of certain reagents ; and some, again, resist re- 

 duction altogether. The reduced metal is in some cases so highly 

 volatile that it connot be obtained except by a kind of distillatory 

 process. In other cases, one or more fusible globules, or a number 

 of minute infusible grains, are obtained in blowpipe operations. Re- 

 ducible metals may be thus distributed into three groups, as shown 

 (with omission of a few metals of rare occurrence) in the annexed 

 Table : 



A. Yielding metallic globules : Gold, Silver, Copper, Tin, Lead, Bismuth, Anti- 



mony. 



B. Yielding infusible metallic grains : Platinum, Iron, Nickel, Cobalt, Molyb- 



denum, Tungstenum. 



C. Yielding metallic vapours only, when treated on charcoal : Mercury, Arsenic, 



Cadmium, Zinc. 



A metal of the first group may be obtained, unless present in very 

 small quantity, by a simple fusion of the previously roasted test-sub- 

 stance, with some carbonate of soda, on charcoal, in a good reducing 

 flame (Fig. 33 above). In ordinary cases, metallic globules are rapidly 

 produced by this treatment. By a little management the globules 

 may be brought together so as to form a single large globule. This 

 must be tested on the anvil as regards its relative malleability, &c. 

 Gold, silver, copper, tin and lead are malleable ; bismuth and anti- 

 mony, more or less brittle. Gold and silver (if pure) retain a bright 



