Berzelius on the Use of the Blow-pipe. 327 



oxidating effect of charcoal en the parts of the assay in contact 

 •with if, might be injurious. 



" Glass Tubes. — When it is necessary to roast a substance 

 to ascertain what it is combined with, I use a glass tube, 

 two inches long at least, and about one-eighih of an inch 

 in diameter, and open at both ends; I introduce the assay 

 into it at a little distance from one of the ends, and, ac- 

 cording to the temperature required, I heat the tube either 

 •with the spirit lamp or with the common lamp by means of 

 the blow-pipe, inclining it more or less (the part containing 

 the assay being held downwards) as it is necessary to pass a 

 more or less rapid current of air over the assay. The volatile 

 substances, not permanently gaseous, sublime, and condense, 

 in the upper part of the tube, where their nature may be ascer- 

 tained." When charcoal is used as the support, the author 

 directs the assay to be laid with its flux on " one of the ends 

 perpendicular to the layers of the wood ; if placed on the section 

 parallel to the layers, it would spread over the surface. Al- 

 though the space between the woody layers may consume 

 faster than the layers themselves, there is this advantage at- 

 tending it, that the assay rests in that case merely on their 

 summits, and thus often has only two or three points of contact 

 with the charcoal." 



The re-agents are carbonate of soda, borax, salt of phospho- 

 rus, (double phosphate of soda and ammonia), saltpetre, vitri- 

 fied boracic acid, gypsum and fluor spar, nitrate of cobalt, tin, 

 iron, lead, bone-ashes, silica, and oxide of copper. The three 

 first are the principal, the others are only wanted occasionally. 

 It is absolutely necessary that the re-agents should be quite 

 pure, for obtaining them in which state, and the method of ap- 

 plying them, ample directions are given. Those respecting the 

 use of soda are very minute and valuable. This re-agent is of 

 great service in the fusion of bodies, and the reduction of 

 metallic oxides. Gahn's method of discovering very minute 

 portions of metal, reduced ou charcoal by means of soda, de- 

 serves particular attention. 



Borax is employed to effect the solution or fusion of a great 

 number of substances. Its use " is founded on the tendency of 

 its component parts to form compounds that are all fusible, 

 though in different degrees. On one hand, it dissolves bases, and 

 forms with them a fusible double salt with excess of base ; on 

 the other, it dissolves acids, amongst which I place silica, and 

 even to a certain extent alumina, and forms with them acid and 

 fusible double salts. As all these salts commonly preserve 

 their transparency on cooling, we can hence judge the more 

 certainly of the colour which the compound acquires from the 

 substance dissolved." 



Salt of phosphorus " acts as a re-agcnt, principally by 



