:i() MR. FAUADAY'S PROCESS FOR PREPARING 



exist in specimens of tried and acknowledged good glass, " as 

 to devise and perfect a process," to use his own words, " bv 

 which the striae period should be passed over before the glass is 

 finished, and the formation of fresh strke be prevented." Now 

 what is called ^flint-glass is at once the most useful for optical 

 purposes, and the most difficult to be obtained in a homoge- 

 neous state. The oxide of lead, which enters largely into its 

 composition, and which is essential to its value for optical 

 purposes, has properties of its own, and also confers proper- 

 ties on the glass, which are very conducive to the formation 

 of striae. Hence, in the manufacture of this glass, it is re- 

 quired that the mixture of the materials shall be far more 

 perfect, than in that of other kinds of glass, such as plate- and 

 crown-glass, which do not contain oxide of lead ; and yet it 

 unfortunately happens that everything tends to make it much 

 less so. The oxide of lead being very heavy, and at the same 

 time very easily melted, when heat is applied to the mixture 

 it melts and sinks to the bottom, leaving the lighter materials 

 to accumulate at the top. Its own powerful action on light, 

 which renders the refractive and dispersive powers of the 

 glass so great, is so different from that of the other materials, 

 that the variation in composition thus caused by its weight, 

 immediately becomes evident, and in becoming evident it be- 

 comes mischievous; the action of the glass upon light ceases 

 to be uniform. The oxide of lead also, uniting qualities at 

 once so useful and so injurious, imparts to the glass the 

 property of dissolving earthy matters from the crucible or 

 melting-pot in which it is made, occasioning also by this means 

 that very irregularity in composition which produces stria?; 

 66 whilst the comparative levity of the matter dissolved at the 

 sides and bottom, and the ascending currents at the hottest 

 parts of the crucible, are constantly mixing this deteriorating 

 portion with the general mass." 



The difficulties which are thus introduced into the manu- 

 facture of flint-glass fit for optical purposes, compelled the 

 Sub- Committee of the Royal Society to consider the possibility 

 of making glasses perfectly distinct from those ordinarily in 

 use, which, at the same time that they possessed the high di- 

 spersive power enabling them to replace flint-glass, might also 

 have such fusibility as would allow of their being perfectly 

 stirred and mixed, and might be retained, without alteration, 

 in such vessels as could be procured of any desired size. 



The borate of lead, formed by the union of the acid of borax 

 with the oxide of lead, and the same metallic salt chemically 

 combined with silica or the pure earth of flints, were the sub- 

 stances which, after some trials, were found to offer such rea- 



