This cracking away from glass, by the way, is probably an effect of difference in rate of expansion between the glass and cement which probably always exists, and, if the cement be not sufficiently viscous, must, beyond certain temperature limits, either produce cracks or cause separation. Professor Wright of Yale has used a hard mineral pitch as a cement in vacuum work with success.
My attention has been directed to a fusible metal cement containing mercury, and made according to the following receipt, given by Mr. S. G. Rawson, Journal of the Society of Chemical Industry, vol. ix. (1890), P. 150:—
Bismuth 40 per cent
Lead 25 per cent
Tin 10 per cent
Cadmium 10 per cent
Mercury 15 per cent
This is practically one form of Rose's fusible metal with 15 per cent mercury added. It takes nearly an hour to set completely, and the apparatus must be clean and warm before it is applied.
As the result of several trials by myself and friends, I am afraid I must dissent from the claim of the author that such a cement will make a really air-tight joint between glass tubes. Indeed, the appearance of the surface as viewed through the glass is not such as to give any confidence, no matter what care may have been exercised in performing all the operations and cleaning the glass; besides which the cement is rigid when cold, and the expansion difficulty comes in.
On the other hand, if extreme air-tightness is not an object, the cement is strong and easily applied, and has many uses. I have an idea that if the joints were covered with a layer of soft wax, the result would be satisfactory in so far as air-tightness is concerned.