ack away from glass to the same
extent as paraffin or pure shellac.

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.

This anticipation has since been verified.

In many cases one can resort to the device already mentioned of
enclosing a rubber or tape-wrapped joint between two tubes in a bath
of mercury, but in this case the glass must be clean and hot and the
mercury also warm, dry, and pure when the joint is put together,
otherwise an appreciable air film is left against the glass, and this
may creep into the joint.

Perhaps the easiest way of making such a joint is to use an outer tube
of thin clean glass, and bore a narrow hole into it from one side to
admit the mercury; if the mercury is to be heated in vacuo, it is
better to seal on a side joint. It is always better, if pos