(b) It is generally made up of selections of different age and different composition, and pieces of different composition, even if the difference is slight, will not fuse together and remain together unless joined in a special manner.
(c) It is generally old, and this kind of glass often devitrifies with age, and is then useless for blowpipe work, though it may be bent sufficiently for assembling chemical apparatus. Devitrified glass looks frosty, or, in the earlier stages, appears to be covered by cobwebs, and is easily picked out and rejected.
§ 5. It might be imagined that the devitrification would disappear when the glass is heated to the fusing point; and so it does to a great extent, but for many operations one only requires to soften the glass, and the devitrification often persists up to this temperature. My experience is that denitrified glass is also more likely to crack in the flame than good new glass, though the difference in this respect is not very strongly marked with narrow tubes.
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.
§ 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 soft soon after it is hot enough to be appreciably luminous in a darkened room, and at a white heat is very fluid. This fluidity, though of advantage to the practised worker, is likely to give a beginner some trouble.
§ 8. As against the advantages enumerated, there are some drawbacks. The one which will first strike the student is the tendency of the glass to become reduced in the flame of the blow-pipe. This can be got over by proper adjustment of the flame, as will be explained later on. A more serious drawback in exact work is the following. In making a joint with lead glass it is quite possible to neglect to fuse the glass completely together at every point; in fact, the joint will stand perfectly well even if it be left with a hole at one side, a thing which is quite impossible with soft soda glass, or is at least exceedingly unusual. An accident of this kind is particularly likely to happen if the glass be at all reduced. Hence, if a joint does not crack when cold, the presumption is, in the case of soda glass, that the joint is perfectly made, and will not allow of any leak; but this is not the case with flint glass, for which reason all joints between flint glass tubes require the most minute examination before they are passed. If there are any air bubbles in the glass, especial care must be exercised.