THE PHILOSOPHY OF FLUXES. 485 
compound of boracic acid and soda, resembling water glass, which is a compound 
of silicic acid and soda. Both of these when heated with metallic oxides form 
glassy compounds. Blow-pipe analysis largely consists of fusing metallic oxides 
with borax or boracic acid, thus obtaining a glass bead in which the oxide is dis- 
solved, the color and other appearances of such bead indicating the presence of 
the particular oxide. When powdered borax is laid over the metal to be soldered, 
and the metal is heated, the borax fuses, forms a glassy film upon the surface of 
the metal, which film takes up all the oxide and combines with it, forming a 
glassy borate of the metal. But this is not sufficient, for if that glassy film re- 
mained it would be as bad as the oxide in preventing the adhesion or amalgama- 
tion of the solder. Here then comes to our aid another property of the borates, 
viz., their volatility at high temperatures. The borax, with the oxide it has 
taken up flies off at a bright red heat, just at the temperature at which the granules 
of the silver or the spelter brass melt. Thus they fuse upon a virgin surface of 
clean metal and unite with it. 
We have thus gone a little aside from our direct subject on account of the 
confusion that the use of this term " flux " may induce. Borax is also used as a 
flux in metallurgical operations where a metal, such as gold, mixed with impuri- 
ties is melted. These impunities are taken up by the borax, made into a fusible 
glass, and thus fluxed or melted away. It is not the gold which is made more 
fluid, but the oxides of the baser metals that have to.be removed. 
The fluxing in the blast furnace is similar to this. Its action is not the lique- 
faction of the iron itself, but the gathering together of the impurities of the ore, 
converting them into a glass that shall fuse and run down with the melted metal, 
for we must always remember that the fundamental problem of the modern blast 
furnace is that everything pitched into its throat must come out either as a gas 
or a liquid. The retention or formation of any infusible solid would choke or 
"gobi" the furnace and put an end to its profitable operation. 
Silicic and boracic acid are the two chief glass-making agents, the boracic 
glasses fusing at lower temperatures than the corresponding silicic glasses, and 
also being more volatile. But we may now dismiss these, and confine our atten- 
tion to the silicates, and they do the fluxing in the blast furnace. This statement 
may appear paradoxical for the moment, seeing that not silica, but lime, is the so- 
called flux of the blast furnace charge. This is easily explained by reference to 
the analysis of the clay ironstone ores already given. They all contain silica ; in 
some cases partly free as sandy matter, or combined with alumina, or combined 
with the iron itself. 
Silicate of alumina alone is objectionable, on account of its high fusing point. 
Silicate of iron fuses readily enough, but it is a thief, as it carries away as useless 
glass the material we are seeking to obtain as reguline metal. We, therefore, 
require to add something that is more powerfully basic than the iron or its oxides, 
which will therefore combine more readily with the silica, and this should form a 
glass that is readily fusible and which will somehow separate itself from the melted 
iron. Cheapness is another primary desideratum. 
