14 
MR. FARADAY ON THE 
referred to the manganese in the flint, glass ; a supposition proved by repeating 
the experiment with other flint glass, and then with flint glass of our own manu- 
facture in which no manganese was used : the latter glass gave no purple colour ; 
the former, a colour as deep as 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 substances far beyond what flint glass pos- 
sesses ; 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 -ffths 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 containing it very liable to tarnish. 
i 
25. Such are the materials from which the heavy optical glass has been 
latterly manufactured. When the composition has been determined upon, the 
proper proportions and quantities of each are weighed out in a clean balance 
and vessels; thus, for the silicated borate of lead glass, consisting of single pro- 
portionals of each substance, 24 parts of the silicate would be taken, for they 
contain 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 equi- 
valent to 104 parts only are required, and this is 154.14 parts: the equivalent 
of dry boracic acid is 24, which being contained in 42 parts of the crystals, that 
quantity is the one required. These proportions when heated and submitted 
to mutual action will leave only 152 parts of glass, or thereabout, for 
154.14 nitrate of lead contain 104 protoxide of lead. 
42.00 crystallized boracic acid contain ... 24 dry boracic acid. 
24.00 silicate of lead contain 
8 ditto. 
16 silica. 
