10 
MR. FARADAY ON THE 
retained, without alteration, in such vessels as could be procured of any desired 
size. 
1 1 . The borate of lead, and the borate of lead with silica, were the substances 
which, after some trials, were found to offer such reasonable hopes of success 
as to justify a persevering series of experiments ; and the metal platina was 
looked to as the material out of which to form the vessels intended to be used. 
It was soon ascertained that the borate of lead could be readily formed from 
dry materials, and that silica might be added with great advantage to the 
resulting glass ; a range of proportions between the three ingredients being 
permissible, which gave much command over the properties of hardness, colour, 
weight, refractive and dispersive power, &c., and yet remained within the required 
range of fusibility. Platina also was ultimately found to answer perfectly the 
purpose of retaining the glass : .for though at first it was continually liable to 
failure, yet it was ultimately ascertained that neither the glass nor any of the 
substances entering into its composition, separate or mixed, had the slightest 
action upon it. Finally, it was found that several kinds of glass formed of 
these materials, were in their physical properties fitted to replace flint glass in 
the construction of telescopes, in some cases apparently even with advantage ; 
since which time the experiments have been unremittingly pursued. 
12. The great proportion of oxide of lead in these glasses rendered attention 
to vciy minute points essential ; for otherwise strise were inevitably formed, 
and even the destruction of the apparatus involved. For this reason, after 
a certain number of trials upon composition had been made, one unvarying 
set of proportions were adopted, and the attention given altogether to the dis- 
covery and establishment of a process which should yield constantly good 
results. This, as far as it has been carried into effect and proved, it is now my 
object to describe. 
13. The glass with which I have principally worked is a silicated borate of 
lead, consisting of single proportionals of silica, boracicacid, and oxide of lead. 
The materials are first purified, then mixed, fused, and made into a rough 
glass, which is afterwards finished and annealed in a platina tray. 
14. Purification of materials. Oxide of Lead. — The oxide of lead at first 
used was litharge; hut this source occasioned frequent destruction of the platina 
trays, in consequence of the existence of particles of metallic lead, which 
