78 13. HAANBL ON BLOWPIPE 



so very uncertain, that for analytical purposes the coating is useless. I mention the coat- 

 ing, only because it frequently happens that violet spots make their appearance on the 

 tablet when minerals are examined with hydriodic acid before the blowpipe. This 

 happens either because the mineral or the plaster of Paris, of which the tablet is made, 

 contains iron. 



III. — Distinction of Selenium from Mercury. 



When a selenide of mercury is treated on a tablet with hydriodic acid before the 

 blowpipe, the coatings due to the selenium and mercury overlap throughout their whole 

 extent. It is difficult to determine from the resulting combination-colour by mere inspec- 

 tion the presentee of either of the constituents. In this case a differentiation may be effected 

 by touching the coating with ammonium sulphide, and gently heating the moistened 

 spot before the blowpipe ; the mercury will then be driven from the moistened spot and 

 appear above it as a black coating, while the selenium remains near the spot as an orange- 

 yellow coating. 



IV. — Coatings on Tablets, per se. 



Plaster of Paris tablets, as supports in blowpipe operations, are preferable to any 

 others hitherto in use, not alone because tablets permit of the use of such reagents as 

 hydriodic acid, hydrobromic acid and ammonium sulphide in the detection of the 

 substances entering into the composition of the mineral under examination, but because 

 the range of coatings per se is very much greater for plaster tablets than for any other 

 support. These per se coatings are, on account of the readiness with which they are 

 obtained, especially valuable for purposes of analysis. I have, therefore, undertaken their 

 study, the results of which I beg to offer in this paper. 



Description of Coatings. 



Selenium — Eed brown volatile coating, far away from assay. 



Tiemannite (Se. Hg.) — Black coating, near assay, surrounded by reddish coating farther 



away from assay. 

 Arsenic — White coating over brownish-black. Coating volatile. 

 Silver — Brownish-yellow coating, close to assay. Coating produced by flaming tablet 



beyond assay after the assay has been heated for a time with the 0. F. 

 Alloys of 



Bismuth and Silver. ^ Purplish coating, near assay. Coating produced by flaming 



Lead and Silver. > tablet beyond assay, after the assay has been heated for a time 



Antimony and Silver 3 with the 0. F. 

 Galena — Grreyish-black feathery coating, near assay. This coating is more strikingly 



exhibited, when obtained from precipitated lead sulphide. 

 Orpiment — Orange-brown coating, near assay, white over faint brownish, farther away. 

 Realgar — Coating similar to preceding, the brown is less orange and more dingy. 



