s.

Magnificent flint glass is made both in England and France. The
English experimenter will probably prefer to use English glass, and,
if he is wise, will buy a good deal at a time, since it does not
appear to devitrify with age, and uniformity is thereby more likely to
be secured. I have obtained uniformly good results with glass made by
Messrs. Powell of Whitefriars, but I daresay equally good glass may
be obtained elsewhere.

For general purposes flint glass is vastly superior to the soft soda
mentioned above. In the first place, it is very much stronger, and
also less liable to crack when heated--not alone when it is new, but
also, and especially, after it has been partly worked. Apparatus made
of flint glass is less liable to crack and break at places of unequal
thickness than if made of soda glass. This is not of much importance
where small pieces of apparatus only are concerned, because these can
generally be fairly annealed; and if the work is well done, the
thickness will not be uneven. It is a different matter where large
pieces of apparatus, such as connections to Geissler pumps, are
concerned, for the glass has often to be worked partly in situ, and
can only be imperfectly annealed.

Joints made between specimens of different composition are much more
likely to stand than when fashioned in soda glass. Indeed, if it is
necessary to join two bits of soda glass of different kinds, it is
better to separate them by a short length of flint glass; they are
more likely to remain joined to it than to each other. A particular
variety of flint glass, known as white enamel, is particularly
suitable for this purpose, and, indeed, may be used practically as a
cement.

Sec. 7, It is, however, when the necessity of altering or repairing
apparatus complicated by joints arises that the advantage of flint
glass is most apparent. A crack anywhere near to a side, or inserted
joint, can scarcely ever be repaired in the case of soda glass
apparatus, even when the glass is quite thin and the dimensions small.

It should also be mentioned that flint glass has a much more brilliant
appearance than soda glass. Of course, there is a considerable
difference between different kinds of flint glass as to the melting
point, and this may account for the divergency of the statements
usually met with as to its fusibility compared with that of soda
glass. The kind of flint glass made by Messrs. Powell becomes
distinctly s