SILICON AND BORON. 193 



a yellowish-red color, oxide of cobalt blue, oxide of manga- 

 nese purplo and violet, oxide of copper a ruby red, etc. 



264. Annealing. Glass, like steel, must be annealed to de- 

 prive it of its brittleness. For this purpose the articles that 

 are made are placed in the annealing furnace, which is a very 

 long gallery containing iron trays that are moved very slowly 

 through it by means of an endless chain. The heat at the 

 end where the articles are put in is very great, and gradually 

 lessens toward the other end. Every article is from twenty- 

 four to forty-eight hours in passing through the gallery, and 

 the particles of the glass have time, in this slow cooling, to 

 assume such an arrangement as to give them 



their highest degree of firmness. We see the 

 opposite result in what are called "Prince 

 Rupert's Drops," which are prepared by 

 taking up on an iron rod some melted glass 

 and allowing the drops of it to fall into cold 

 water. They assume the shape given in Fig. 

 83. The particles in this case, solidifying 

 hastily, have an exceedingly unstable ar- 

 rangement, which can be wholly destroyed 

 by a very slight disturbance. If, therefore, 

 you scratch the surface or break off the lit- 

 tle end, the whole flies into powder so quick- 

 ly as to cause a considerable report. 



265. Slag. The slag which is so often seen in reducing 

 metallic ores is composed of silicates, and is a kind of glass. 

 In the process of reducing iron ore, described in Chapter 

 XVIIL, the lime is used, because it makes, with the silica 

 that is mixed with the ore, a glass that is very fusible, and 

 is therefore easily removed. It is for this reason that oys- 

 ter-shells, introduced among the anthracite coal in a stove, 

 remove the clinker. The lime unites with the silica, and 

 the silicate formed, melting easily, runs down and min- 



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