460 Prof. E. J. Chapman on some Blowpipe Reactions. 



the blowing be continued for an instant, a precipitation of 

 silica will immediately take place, the bead becoming milky 

 — or, in the case of many silicates, opaque-white — on cooling. 

 This test may be resorted to for the detection of silica in the 

 case of silicates which dissolve with difficulty in phosphor-salt 

 alone, or which do not give a well-pronounced " skeleton " 

 with that reagent *. 



On the Reactions of Chromium and Manganese with Car- 

 bonate of Soda. — AYhen a mineral substance is suspected to 

 contain manganese, it is commonly tested by fusion with car- 

 bonate of soda. But chromium compounds form with that re- 

 agent a green enamel much resembling that formed by com- 

 pounds of manganese. 



The chromate-of-soda enamel, however, is yellowish-green 

 after exposure to an oxidating name ; and the green colour 

 never exhibits any tinge of blue. 



The manganate-of-soda enamel, on the other hand, is 

 generally greenish-blue when quite cold. 



To avoid, however, any risk of error in the determination, 

 the bead may be saturated with vitrified boracic acid until all 

 the carbonic acid is expelled and a clear glass is obtained. 

 The chrome glass will retain its green colour, whilst the man- 

 ganese glass will become amethystine or violet. In place of 

 boracic acid, silica may be used if more convenient. In this 

 case, the reaction is assisted by the addition of a very small 

 amount of borax. 



On the Detection of Cadmium in the Presence of Zinc in Blow- 

 pipe Experiments. — When cadmiferous zinc-ores, or furnace- 

 products derived from these, are treated in powder with car- 

 bonate of soda on charcoal, the characteristic red-brown deposit 

 of cadmium oxide is generally formed at the commencement of 

 the experiment. If the blowing be continued too long, however, 

 this deposit may be altogether obscured by a thick coating of 



* By whom was the formation of a " silica skeleton " first made 

 known ? There is no reference to it in the early treatise of Von Enge- 

 strom attached to his translation of Cronstedt's Mineralogie (edition 1, 

 1770; ed. 2, by John Hyacinth de Magellan, 1788), although phosphor- 

 salt is mentioned as a reagent under the term of salfusibile microcosmicum, 

 and was indeed used by Cronstedt before 1758, the year in which his 

 Mineraloyie was anonymously published. Bergrnann, who followed as a 

 blowpipe worker, states that " siliceous earth " is very slowly attacked 

 by microcosmic salt; but he does not seem to have remarked the skeleton- 

 formation in the case of any silicate. The reaction appears to have been 

 first definitely pointed out by Berzelius in his standard work on the 

 blowpipe, published in 1821. It was therefore most probably discovered 

 by him ; or perhaps — as he lays no claim to its discovery, whilst claiming 

 to be the originator of other tests — it may have been communicated to 

 him by Gahn ? 



