BLOWPIPE REACTIONS. 85 



prevent the assay from falling out. The strength of the 

 draught depends upon the inclination of the tube, and in special 

 cases it should be inclined as much as possible. 



The most common fluxes are borax (sodium bi-borate), salt 

 of phosphorus (sodium and ammonium phosphate), and soda 

 (sodium carbonate, either the carbonate or bi-carbonate of soda 

 of the shops.) These substances, when fused and highly heated, 

 art very powerful solvents for metallic oxides. They should 

 be pure preparations. The borax and soda are much the most 

 important. In using the platinum wire, the loop may be highly 

 heated, and then a portion of the borax or soda may be taken 

 up by it, aDd by successive repetitions of this process the re- 

 quisite amount of the flux may be obtained on the wire. Then, 

 by bringing the melted flux of the loop into contact with one 

 or more grains of the pulverized mineral, the assay is made 

 ready for the trial. With soda and quartz a perfectly clear 

 globule is obtained, cold as well as hot, if the flux is used in 

 the right proportion. Some oxides impart a deep and charac- 

 teristic color to a bead of borax. In other cases the color 

 obtained is more characteristic when salt of phosphorus is em- 

 ployed. The color obtained in the outer flame is often differ- 

 ent from that which is obtained in the inner flame. The beads 

 are sometimes transparent and sometimes opaque. If too much 

 substance is employed the beads will be opaque when it is de- 

 sired that they should be transparent. In such cases the 

 experiment may be repeated with less substance. In many 

 cases pulverized mineral and the flux, a little moistened, are 

 mixed together into a ball upon charcoal, especially in the ex- 

 periments with soda. 



In the examination of sulphides, arsenides, antimonides and 

 related ores, the assay should be roasted before using a flux, in 

 order to convert the substance into an oxide. This is done by 

 spreading the substance out on a piece of charcoal and exposing 

 it to a gentle heat in the oxidizing flame. The sulphur, arsenic, 

 antimony, etc., then pass off as oxides in the form of vapors, 

 leasing the non-volatile metals behind as oxides. The escap- 

 ing sulphurous acid gives the ordinary odor of burning sulphur; 

 irsenous acid, from arsenic present, the odor of garlic, or an 

 alliaceous odor ; selenous acid, from selenium present, the odor 

 of decaying horse-radish ; while antimony fumes are dense white, 

 and have no odor. 



The following is the scale of fusibility which has been adopted, 

 beginning with the most fusible : 



1. Stibnite. — Fusible in large pieces in the candle flame. 



