8 
MR. FARADAY ON THE 
6. With plate glass the same circumstances hold nearly in an equal degree. 
This substance is composed of silica and alkali essentially, other elements being 
only in small quantities. Its action upon the crucible is greater than crown 
glass, but then it has a second application of heat in such circumstances as are 
calculated to give a very uniform temperature to the contents of a whole pot, 
and it is delivered into its final form in the manner least likely to cause mix- 
ture of the different parts. 
7. With flint glass many circumstances are altogether different. Oxide of 
lead enters into its composition to the amount of one third of its weight, or 
more, and by its presence gives that proportion of refractive and dispersive 
power, which makes the glass valuable in conjunction with crown or plate: 
this it does in consequence of its own powerful action on light ; and it makes 
the glass heavy also, because of its own great specific gravity. A third property 
belonging to it, namely its high fluxing or dissolvent powers, it also confers 
upon the glass. Now these three properties are unfortunately very conducive 
to the formation of striae. If the least difference in composition exists between 
one part and another it becomes evident, because of the great difference 
between the qualities of the oxide of lead and the other ingredients ; and 
a variation in proportions which in crown or plate glass would produce 
no sensible effect to the naked eye, would in flint glass form strong striae. 
Hence it is required that the mixture be in this case far more perfect than 
in the other glasses; and yet it unfortunately happens that every thing tends 
to make it much less so. The oxide of lead is so heavy a material, and at 
the same time so fusible, that it melts and sinks to the bottom, leaving the 
lighter materials to accumulate at the top : and so imperfect are the means of 
mixture, under ordinary circumstances, that glass of very different specific 
gravity is procured from the bottom and top of the same crucible. The fol- 
lowing are some cases of this kind, from pots containing glass not more than 
six inches in depth, made from the usual materials, and retained at a full heat 
for twenty-four hours : — 
Top 3.38 3.30 3.28 3.21 3.15 3-73 3.85 3.81 3.31 3.30 
Bottom ... 4.04 3.7/ 3.85 3.52 3.80 4.63 4.74 4.75 3.99 3.74 
These differences are great, and selected for illustration ; but from appear- 
ances there is little reason to doubt that the same state of things, though not 
