1818.] On the Blow-pipe. 41 



knife : the moisture eauses the particles to cohere, and prevents 

 their being blown away when placed on the charcoal. The flux 

 must then be melted to a clear bead, and the substance to be 

 examined placed upon it. It is then to be submitted to the 

 action, first of the exterior, and afterwards of the interior flame, 

 and the following circumstances to be carefully observed : 



1 . Whether the substance is dissolved ; and, if so, 



2. Whether with or without effervescence, which would be 

 occasioned bv the liberation of carbonic acid, sulphurous acid, 

 oxvgen, gaseous oxide of carbon, &c. 



3. The transparency and colour of the glass while coohng. 



4. The same circumstances after cooling. 



5. The nature of the glass formed by the exterior flame, and 



6. By the interior flame. 



7. Tiie various relations to each of the fluxes. 



It must be observed that soda will not form a bead on char- 

 coal, but with a certain degree of heat will be absorbed. When, 

 therefore, a substance is to be fused with soda, this flux must be 

 added in very small quantities, and a veiy moderate heat used at 

 first, by which means a combination will take place, and the 

 soda mil not be absorbed. If too large a quantity of soda has 

 been added at first, and it has consequently been absorbed, a 

 more intense heat will cause it to return to the surface of the 

 charcoal, and it will then enter into combination. 



Some minerals combine readily with only very small portions 

 of soda, but melt with difficulty if more be added, and are ab- 

 solutely infusible with a larger quantity : and when the substance 

 has no affinity for this flux, it is absorbed by the charcoal, and 

 no combination ensues. 



When the mineral or the soda contains sulphur or sulphuric 

 acid, the glass acquires a deep yellow colour, which by the 

 light of a lamp appears red, and as if produced by copper. 



If the glass bead becomes opaque as it cools, so as to render 

 the colour indistinct, it should be broken, and a part of it mixed 

 with more of the flux, until the colour becomes more pure and 

 distinct. To render the colour more perceptible, the bead may 

 be either compressed before it cools, or drawn out to a thread. 



When it is intended to oxidate more highly a metallic oxide 

 contained in a vitrified compound with any of the fluxes, the 

 glass is first heated by a strong flame, and when melted is to be 

 gradually withdrawn from the point of the blue flame. This 

 operation may be repeated several times, permitting the glass 

 sometimes to cool, and using a jet of large aperture with the 

 blow-pipe. 



The redaction of metals is effected in the following manner : 

 The glass bead, formed after the manner already pointed out, is 

 to be kept in a state of fusion on the charcoal as long as it re- 

 mains on the surface, and is not absorbed, that the metallic 

 particles may collect themselves into a globide. It is then to be 



