1829.] On the Manufacture of Optical Glass. 245 



fectly pure materials. The glass when finished and cold was 

 of a deep purple colour : this was immediately referred to the 

 manganese in the flint glass ; a supposition proved by repeat- 

 ing the experiment with other flint glass, and then with flint 

 glass of our own manufacture in which no manganese was used : 

 the latter glass gave no purple colour ; the former, a colour as 

 deep as that produced by the first flint glass. 



23. Thus it appears that this very heavy glass, the silicated 

 borate of lead (and I find it to be the case with other heavy 

 glasses), has the power of developing the colour of mineral sub- 

 stances far beyond what flint glass possesses ; just as flint glass 

 surpasses in the same property plate and crown glass. In the 

 case in question, the manganese, which did not give a sensible 

 tint to the flint glass, produced a strong colour when diluted 

 eight or nine times by the heavy glass, for the proportion of 

 flint glass used was only ^jths of the whole. On making a few 

 experiments with iron, I find that the same strong development 

 of colour is produced with it in these heavy glasses ; so that 

 the utmost care is necessary to preserve all the materials during 

 their preparation, and the glass in every part of the process, 

 from metallic contamination. 



24. The use of flint glass even without manganese was also 

 objectionable, because of the alkali in it, which, as before stated, 

 was found to produce bad effects, and rendered the glass con- 

 taining it very liable to tarnish. 



25. Such are the materials from which the heavy optical glass 

 has been latterly manufactured. When the composition had 

 been determined upon, the proper proportions and quantities 

 of each have been weighed out in a clean balance and vessels. 

 Thus, for the silicated borate of lead glass, consisting of single 

 proportionals of each substance, 24 parts of the silicate were 

 taken, for they contained a proportional of silica equal to 16 parts, 

 and in addition 8 parts of protoxide of lead : the proportional 

 of oxide of lead has been taken as 112 parts ; but there being 

 8 in the silicate, the quantity of nitrate of lead equivalent to 104 

 parts only was required, and this is 154*14 parts: the equiva- 

 lent of dry boracic acid is 24, which being contained in 42 parts 

 of the crystals, that quantity was the one required. These pro- 

 portions when heated and submitted to mutual action leave only 

 152 parts of glass, or thereabout; for 



