1829.] On the Manufacture of Optical Glass. 279 



time that it has that degree of fusibility, colour, and other 

 properties, which makes it a very promising variety. 



102. The hardness increases with the diminution of the 

 oxide of lead ; but the fusibility diminishes in the same pro- 

 portion ; and this is a property which it is essential to preserve 

 to a certain degree for the removal of striae and bubbles. The 

 borate of lead is so fusible as to soften and lose its form under 

 the surface of boiling oil. The silicated borate, and the glass 

 consisting of the proportions above mentioned, are quite fusible 

 enough to allow of the processes necessary for the removal of 

 striae and bubbles. 



103. The fusibility of these glasses, and of glass generally, 

 must not be confounded with their relative tendency to soften 

 by elevation of temperature. It is not 'that glass which softens 

 first, that becomes most fluid at a certain given high tempera- 

 ture ; for glasses, like other substances, vary in their readiness 

 to pass into the fluid state. Hence it has often occurred 

 amongst the variety of compositions tried for glasses, that 

 when the resulting substances have been placed side by side 

 on platinum foil, and heated, that which first softened did not 

 when heated highly become so fluid as some other specimens 

 that longer resisted the first impression of heat. It has, how- 

 ever, always been found that those glasses which when sub- 

 jected to a rising temperature, most slowly passed from the 

 solid to the fluid state, were also those which when subjected 

 to long annealing processes, were least liable to assume a 

 crystalline structure ; and thus very useful indications of the 

 probable qualities of compounds under investigation were often 

 obtained. 



104. A most important consideration relative to the applica- 

 tion of these glasses to the construction of telescopes, is their 

 liability to change and injury by the action of substances 

 usually occurring in an ordinary atmosphere. When the value 

 of a good object-glass is considered, frequently amounting to 

 many hundred pounds, this point will be thought of no little 

 consequence ; and when it is known that even flint and plate 

 glass are frequently injured in this way, a little anxiety for the 

 capability of resistance in the heavy glasses may readily be 

 allowed, since they contain so much less of the substance 

 (silica) which confers the power of resistance, and so much 



