38* 



of the President and Managers of the Royal Institution to erect an- 

 other experimental furnace for continuing the investigation on their 

 premises. 



The author being intrusted with the immediate superintendence of 

 the experimental part of the manufacture of the glass, conceives it to 

 be his especial duty, at the present stage of the inquiry, to give an 

 account of what has been done in his department ; for although the 

 investigation is still far from being completed, yet he trusts that a de- 

 cided step has now been made in the manufacture of glass for optical 

 purposes ; and that it is due to the Society, as well as to the Go- 

 vernment, to render an account of the results hitherto obtained. 



The author begins this account by a statement of the usual defects 

 incident to glass, which destroy the regularity of its action on light. 

 These are, on the one hand, streaks, striae, veins, and tails ; and, on 

 the other hand, minute bubbles ; the former arising from the want 

 of homogeneity, the latter from the intermixture of air. Of these, 

 the first class of defects constitute the most serious evil, as they in- 

 terfere with the rectilineal course of the rays of light while traversing 

 the glass, while the latter are injurious merely from the interception 

 of the rays, and their dispersion in all directions. The greater the dif- 

 ference in specific gravity of the ingredients of the glass, the greater 

 is the tendency to form striae when they are fused together ; hence 

 flint glass, which contains a large proportion of lead, is more liable 

 to this defect than either crown- or plate-glass. After numerous 

 trials of materials different from those which enter into the compo- 

 sition of the ordinary kinds of glass, borate of lead and silica were 

 fixed upon as the most eligible, and as near an approximation as 

 possible to a definite chemical union of their elements was aimed at, 

 by taking single proportionals of each, and endeavouring to procure 

 them, previous to combination, in the greatest possible state of purity. 

 The oxide of lead was obtained from the nitrate of the metal previ- 

 ously crystallized. The boracic acid was also selected from the purest 

 crystals afforded by the manufacturer, and carefully tested to ascer- 

 tain its freedom from foreign matters. The silica employed was that 

 of flint glass-maker's sand, obtained from the coast of Norfolk, and 

 well washed and calcined, and freed from iron by nitric acid. It was 

 then combined with protoxide of lead. These materials were then 

 mixed in the proportions of 154' 14 parts of nitrate of lead, 24 of sili- 

 cate of lead, and 42 of crystallized boracic acid, and melted together 

 in a separate furnace adapted expressly for this preliminary operation, 

 and of which a minute description is given. A tray was then pre- 

 pared of a thin lamina of platina, all the apertures of which were care- 

 fully closed by soldering, for containing the pulverized glass, which 

 was to be subjected to the final melting in a furnace of peculiar con- 

 struction, which the author terms the finishing furnace. After nu- 

 merous trials of substances for constructing the chamber in which the 

 fusion of the glass contained in the tray was to be conducted, re- 

 course was had to the materials from which the Cornish crucibles are 

 manufactured, and which were obtained through the kindness of the 



