THE YOUNG 
SCIENTIST. 
-who used sealing-wax in the old days of 
seals and twenty cent postage. When the 
seal was used rapidly, so as to become 
heated, the sealing-wax stuck to it with a 
firmness that was annoying, so much so that 
the impression was in general destroyed, 
from the simple fact that the sealing-wax 
would rather part in its own substance than 
at the point of adhesion to the stamp. Seal- 
ing wax or ordinary electrical cement is a 
very good agent for uniting metal to glass 
or stone, provided the masses to be united 
a,re made so hot as to fuse the cement, but 
if the cement be applied to them while they 
are cold it will not stick at all. This fact is 
well known to the itinerant venders of 
cement for uniting earthenware. By heat- 
ing two pieces of delf so that they will fuse 
shellac, they are able to smear them with a 
little of this gum, and join them so that 
they will break at any other part rather tban 
along the line of union. But although 
people constantly see the operation per- 
formed, and buy liberally of the cement, it 
will be found that in nine cases out of ten 
the cement proves worthless in the hands of 
the purchasers, simply because they do not 
know how to use it. They are afraid to 
heat a delicate glass or porcelain vessel to 
a sufficient degree, and they are apt to use 
too much of the material, and the result is 
a failure. 
The great obstacles to the absolute con- 
tact of any two surfaces, are air and dirt. 
The former is universally present, the latter 
is due to accident or carelessness. All sur- 
faces are covered with a thin adhering layer 
of air, which it is difficult to remove, and 
which, although it may at first sight seem 
improbable, bears to the outer surface of 
most bodies a relation difi'erent from that 
maintained by the air a few lines away, and 
until this layer or film of air has been re- 
moved it prevents the absolute contact of 
any other substance. The reality of the 
existence of this adhering layer of air is well 
known to all who are familiar with electro- 
type manipulation, and it is also seen in the 
case of highly polished metals, which may 
be immersed in water without becoming 
wet. Thus the surface of a needle retains 
this film of air so strongly that it will float 
on the surface of water rather than give it 
up. So, too, a drop of melted glue allowed 
to simply fall on a surface of dry, cold 
wood and solidify there, will often fail to 
adhere at all, while if the same drop had 
been rubbed in, it would have attached 
itself to it with wonderful power of adhe- 
sion. What is true of glue is true of almost 
every other cement, and unless this adher- 
ing layer of air is displaced, it will be im- 
possible for any cement to adhere to the 
surface to which it is applied, simply be- 
cause it can not come into contact with it. 
The most efficient agents in displacing 
this air are heat and pressure. Metals 
warmed to a point a little above 200°, be- 
come instantly and completely wet when 
immersed in water. Hence for cements 
that are used in a fused condition, heat is 
the most efficient means of bringing them 
in contact with the surfaces to which they 
are to be applied. In the case of glue, the 
adhesion is best attained by moderate pres- 
sure and friction. When it is intended to 
unite two pieces of earthenware or glass 
together, or a piece of glass or other sub- 
stance to metal, by means of a cement that 
is to be used in a fused state, the surfaces 
that are to be united should always be made 
so hot that the cement will become perfectly 
liquid while in contact with them. 
To he continued. 
College Workshops. 
REFEREING to the fact that the lique- 
faction of oxygen, recently accom- 
plished by MM. Pictet & Cailletet, has been 
within the reach of English scientific work- 
ers for years. Nature makes the following 
pungent remarks : " It is also clear that to 
cope with modern requirements our labor- 
atories must no longer contain merely an 
antiquated air-pump, a Leyden jar, and a 
few bottles, as many of them do. The pro- 
fessor should be in charge of a work-shop, 
instead of an old curiosity shop, and the 
scale of his operations must be large if he 
is to march with the times." 
