42 MINERALS AND GEOLOGY 



the evolved acid reddens moistened litmus-paper. (See Operation 4, 

 above.) The natural sulphates do not possess a metallic aspect, and 

 the streak is either colourless or pale green or blue. They do not 

 omit the smell of brimstone when heated, 



Other results, if exhibited, may be noted down for after reference. 



Remarks : Reactions a and b are sometimes produced by the same 

 mineral, from the simultaneous presence of sulphur and arsenic, (Arse- 

 nical Pyrites, Realgar, Orpiment, &c.) Reaction b is also produced by 

 Selenides and Seleniates, but these are of exceedingly rare occurrence , 

 and they evolve at the same time a strong odour of cabbage-water or 

 decomposing vegetable matter. The rare tellurides also exhibit the 

 reaction. 



Experiment 2. Fuse a solid particle of the test-mineral with phos- 

 phor-salt on platinum wire. 



This experiment serves directly for the detection of Carbonates and 

 Silicates. 



a. The substance dissolves rapidly and with marked effervescence : 

 Carbonates (essentially).* 



Note : Sulphates, Phosphates, and various other compounds, also 

 dissolve readily by fusion with phosphor-salt, but produce no effer- 

 vescence. 



b. The substance dissolves in part only, the undissolved portion 

 retaining the original form of the test-fragment but becoming more 

 or less translucent. (On cooling, the glass often becomes opales- 

 cent) : Silicates (see under " Phosphor-salt," page 35, above). Free 

 silica, or quartz, melts into a clear glass with carb. soda, in expelling, 

 with effervescence, the carbonic acid from the latter. Some silicates 

 produce the same reaction. The test-substance should be added little 

 by little. If the soda be in excess, the glass remains opaque, and 

 with too much silica it becomes infusible. 



Note : Other reactions that may ensue from this experiment, such 

 as the coloration of the glass, <fcc., may serve to detect the base or bases 

 in combination with the carbonic or silicic acid. These reactions, 

 therefore, should be noted down for after reference. 



*Nitrates and certain bodies (Pyrolusite or Black Manganese Ore, &c.) which evolve oxygen 

 on ignition, also dissolve in phosphor-salt with effervescence before the blowpipe ; but these 

 bodies are of comparatively exceptional occurrence. To avoid risk of error, however, the sub- 

 stance may be warmed in a test tube with a few drops of hydrochloric acid. Thus treated, all 

 arbonates dissolve with marked effervesence, and evolve a colorless, inodorous gas. 



