250 On the Manufacture of Optical Glass. [1829. 



the gold freely and cause perfect adhesion. In such cases, a 

 single or double piece of platinum foil loosely laid over the part, 

 prevents loss of heat from the upper surface, and frequently 

 causes such increased elevation of temperature as to render the 

 soldering perfect and effectual. In the few cases where this 

 expedient has not succeeded, I have resorted to the oxyalcohol 

 blowpipe, vising a small bladder of oxygen with a little attached 

 jet for the purpose. This has never failed to produce an 

 effectual heat, and 15 or 20 cubical inches of oxygen are suf- 

 ficient for many operations. 



34. This application of patches and soldering is only secure 

 for small holes, i. e. such as a pin might pass through, and 

 smaller. The patches are always to be applied on that surface 

 of the plate which is to constitute the outside of the tray ; and 

 therefore, before the soldering begins, the two surfaces should 

 be examined, and the most polished and perfect selected as 

 that intended for the inside. The patches are valuable in 

 their use far beyond what the mere application of gold to the 

 hole would be ; for the heat afterwards applied to the tray, 

 when charged with glass, is abundantly sufficient to melt gold ; 

 in which case, if unsupported by the platinum patch, the weight 

 of glass and the action of stirring would probably force the 

 gold out of the hole and cause the tray to run ; whereas the 

 patch of platinum, although the gold holding it to the plate is 

 liquid, still adheres by so strong a capillary action as to be 

 sufficient to retain its place, and being outside is not disturbed 

 by the motion of the stirrer. Besides, after a long application 

 of heat, the gold and platinum combine so perfectly as to become 

 one piece of white alloy, infusible at the heat applied. 



35. The plate is now to be folded into a tray, preparatory 

 to which, a piece of thin board is to be provided as a gauge, 

 which in the present instance must be 7 inches square. 

 This laid upon the plate and held tightly down, directs the 

 foldings of the sides, and would, if placed in the middle, leave 

 sufficient for edges one inch and a half high all round ; but as 

 the plate should serve for use several times, it is advantageous 

 to apply the gauge a little eccentric ; for then, when used for 

 a second and third operation, its place may be shifted, and 

 the folds not occurring where they did before, there is less 

 chance of holes being broken through the platinum. The folds 



