of the various glasses, some work on
glass-making may be consulted.

In purchasing glass tubes, it is well to lay in a considerable stock of
tubes made of each of the two first varieties, and, if possible, to
obtain them from the manufacturer, for it frequently happens that pieces
of glass from the same batch may be much more readily welded together
than pieces of slightly different composition. Yet it is not well to lay
in too large a stock, as sometimes it is found that glass deteriorates
by prolonged keeping.

As it is frequently necessary to make additions, alterations, or repairs
to purchased apparatus, it is best to provide supplies both of soft soda
glass and lead glass, for though purchased glass apparatus is frequently
made of lead glass, yet sometimes it is formed from the soda glass, and
as it is a matter of some difficulty to effect a permanent union between
soda glass and lead glass, it is desirable to be provided with tubes of
both kinds.

Many amateurs find that soda glass is in some respects easier to work
with than lead glass. But, on the other hand, it is somewhat more apt to
crack during cooling, which causes much loss of time and disappointment.
Also, perhaps in consequence of its lower conductivity for heat, it very
often breaks under sudden changes of temperature during work. If,
however, a supply of good soda glass is obtained, and the directions
given in this book in regard to annealing it are thoroughly carried out,
these objections to the use of soda glass will, to a great extent, be
removed. I find, however, that when every precaution has been taken,
apparatus made of soda glass will bear variations of temperature less
well than that made of lead glass. Therefore, although the comparatively
inexpensive soda glass may be employed for most purposes without
distrust, yet I should advise those who propose to confine themselves to
one kind of glass, to take the small extra trouble required in learning
to work lead glass.

In order to secure glass of good quality, a few pieces should be
obtained as a sample, and examined by the directions given below. When
the larger supply arrives, a number of pieces, taken at random, should
be examined before the blow-pipe, to compare their behaviour with that
of the sample pieces, and each piece should be separately examined in
all other respects as described subsequently.

Hard glass is used for apparatus that is required to withstand great
heat. It is dif