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XIX.—Preliminary Note on the Compressibility of Glass. 
By J. Y. BucHanan. 
The following experiments were undertaken with a view to determine by 
actual observation the effect produced on solids by hydraulic pressure. 
The instrument was constructed according to my directions by Mr MILNE 
of Milton House, about two years ago, but it is only now that I have been able 
to devote myself to its application to the purposes for which it was designed. 
It consists of a hydraulic pump, which communicates with a steel receiver cap- 
able of holding instruments of considerable size, and also with a second receiver 
of peculiar form. This receiver consists essentially of a steel tube, terminated 
at each end by thick glass tubes fitted tightly. It is tapped at the centre with 
two holes, the one to establish connection with the pump, and the other to 
admit a pressure gauge or manometer. The steel tube may be of any length, 
being limited only by the extent of laboratory accommodation at disposal. The 
tube which I am using at present has a length of a little over six feet, and an 
internal diameter of about three-tenths of an inch. The solid to be experi- 
mented on must be in the form of a rod or wire, and must, at the ends at 
least, be sufficiently small to be able to enter the terminal glass tubes, which 
have a bore of 0:08” and an external diameter of 0°42”. The length of the 
solid is such that when it rests in the steel tube its ends are visible in the glass 
terminations. 
When the joints have all been made tight, the experiment is conducted as 
follows :—A microscope, with micrometer eye-piece, is brought to bear on each 
end of the rod or wire. These microscopes stand on substantial platforms alto- 
gether independent of the hydraulic apparatus. The pressure is now raised to 
the desired height, as indicated by the manometer, and the ends of the rod are 
observed, and their position with reference to the micrometer noted. The 
pressure is then carefully relieved, and a displacement of both ends is seen to 
take place, and its amplitude noted. The sum of the displacements of the 
ends, regard being had to their signs, gives the absolute expansion, in the 
direction of its length, of the glass rod when the pressure at its surface is 
reduced by the observed amount, and consequently also of the compression 
when the process is reversed. As in the case of non-crystalline bodies like 
glass there is no reason why a given pressure should produce a greater effect 
in one direction than in another, we may put the cubical compression at three 
times the linear contraction for the same pressure. 
VOL, XXIX. PART II 7H 
