HOMOGENEOUS HEAVY OPTICAL CLASS. 



sonable hopes of success as to justify a persevering series of 

 experiments. The metal platinum, on account of its infusi- 

 bility and inalterability by heat, its resistance to solvent action, 

 and the readiness with which thin plates of it might be m:uli- 

 to assume any desired form, was looked to as the material 

 from which the vessels intended to be used might be fabri- 

 cated. It was soon ascertained that a range of propoi t 

 between the three ingredients boracic acid, silica, and prot- 

 oxide of lead was 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. Platinum, also, was ultimately found to 

 answer perfectly the purpose of retaining the fluid glass; for 

 though at first it was continually liable to failure, yet it was 

 eventually ascertained that neither the glass itself, nor any of 

 the substances entering into its composition, separate or mixed, 

 had the slightest action upon it. " Finally," observes Mr. 

 Faraday, " it was found that several kinds of glass formed of 

 these materials, were in their physical properties fitted to re- 

 place flint-glass in the construction of telescopes, in some 

 cases apparently even with advantage ; since which time the 

 experiments have been unremittingly pursued." To detail, or 

 even to enumerate, the various expedients, characterized 

 equally by science and by ingenuity, which were adopted by 

 Mr. Faraday in the manufacture of this new species of glass, 

 would far exceed the prescribed limits of the present section of 

 this memoir. It is requisite to state, however, that he has 

 principally worked with a silicated borate of lead, consisting 

 of silica, the acid of borax, and protoxide of lead or litharge. 

 The materials are first purified, then mixed, fused, and made 

 into a rough glass, which is afterwards finished, and annealed, 

 (or allowed to acquire a regular and uniform tension of its 

 parts, by slow cooling,) in a tray of platinum *. 



* The composition of this " heavy optical glass," as it is called by Mr. 

 Faraday, is accurately as follows, by weight : 



1 atom or proportional of protoxide of lead .112 or per cent. 73.7 



1 atom or proportional of silica 16 10.5 



1 atom or proportional of boracic acid ... 24 1 5.8 



152 100.0 



For the purpose of giving a familiar explanation of the subject, litharge 

 has been mentioned above, as the form in which the oxide of lead is employed 

 in the preparation of this glass. But in order to avoid the possible result of 

 misleading any one who might be induced to make any provisional experi- 

 ments on the subject, from the perusal of these remarks, it may be useful 

 to state, that Mr. Faraday was obliged to abandon the use of litharge, as well 

 as that of red-lead, on account of the particles of metallic lead contained 

 in the former, and of the impurities existing in both. He found it necessary 



