46 
MR. FARADAY ON THE 
quality, and indeed more. The borate of lead is very soft; the bi-borate of 
lead is harder, and the tri-borate equal to flint glass in hardness. The sili- 
eated borate of lead is softer than flint glass; but the glass consisting of 
I proportional oxide of lead, 1 of silica, and 1^ proportional of boracic acid, 
i>< as hard as ordinary flint glass, at the same 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 proportion; 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 con- 
founded with their relative tendency to soften by elevation of temperature. It 
i< not that glass which softens first, that becomes most fluid at a certain given 
high temperature ; 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 platina 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 however always been found that 
those glasses which when subjected 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 application of these 
glasses to the construction of telescopes, is their liability to change and injury 
bv 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, 
