+----------------------------+-----------------+--------+-------+
| | | Compu- |Analy- |
| | |tation. | sis. |
| | +--------+-------+
|Silicic acid | 5 atoms = 2877· | 59·19 | 59·20 |
|Oxide of lead | 1 = 1394·5| 28·68 | 28·20 |
|Potash | 1 = 590·0| 12·13 | 9·00 |
|Oxides of iron and manganese| -- | -- | 1·40 |
| | -------+--------+-------+
| | 4861·5| 100·00 |100·00 |
+----------------------------+-----------------+--------+-------+
The above analysis by Berthier relates to a specimen of the best English
crystal glass, perfectly colourless and free from air-bubbles. This kind
of glass may however take several different proportions of potash and
silica to the oxide of lead.
The composition of mirror plate, as made on the Continent, is as
follows:--
White quartz-sand 300 pounds
Dry carbonate of soda 100
Lime slaked in the air 43
Cullet, or old glass 300
The manganese should not exceed one half per cent. of the weight of the
soda.
Optical glass requires to be made with very peculiar care. It is of two
different kinds; namely, _crown glass_ and _flint glass_. The latter
contains a considerable proportion of lead, in order to give it an
increased dispersive power upon the rays of light, in proportion to its
mean refractive power.
Optical crown glass should be perfectly limpid, and have so little
colour, that a pretty thick piece of it may give no appreciable tinge to
the rays of light. It should be exempt from striæ or veins as well as
air-bubbles, and have not the slightest degree of milkiness. It should
moreover preserve these qualities when worked in considerable
quantities. Potash is preferable to soda for making optical crown glass,
because the latter alkali is apt to make a glass which devitrifies and
becomes opalescent, by long exposure to heat in the annealing process. A
simple potash silicate would be free from this defect, but it would be
too attractive of moisture, and apt to decompose eventually by the
humidity of the atmosphere. It should therefore contain a small quantity
of lime, and as little potash as suffices for making a perfect glass at
a pretty high temperature. It is probably owing to the high heats used
in the English crown glass works, and the moderate quantity of alkali
(soda) which is employed, that our crown glass has been found to answer
so well for optical purposes.