1904] 



On the Compressibility of Solids. 



297 



the tube. This end of the tube was then sealed up, and the end of the 

 glass wire was fused into it, so that, while free throughout its whole 

 length, longitudinal motion was prevented. The length of the glass 

 wire was 57 cm., so that there was an empty space in the tube of 

 3 cm. above it. The magnetic index of a broken deep-sea 

 thermometer was re-haired and passed into the tube above the 

 glass wire. The open end of the tube was then sealed up. The 

 result was a piezometer consisting of nothing but glass. In principle 

 it was precisely the same as any of the other piezometers. The indices 

 of these give the difference between the compression produced by the 

 pressure on the contents and on the envelope. In the case of the other 

 piezometers, which contained liquids, the balance was on the side of 

 the contents. In the all-glass piezometers the contents, besides being 

 of the same material as the envelope, were completely protected from 

 pressure, and the whole of the change of length measured fell to the 

 envelope. It has, therefore, a feature which is possessed by no other 

 instrument ; with it the absolute compressibility of a solid is determined 

 by one measurement. 



Before the instrument was attached to the sounding line, the index 

 was brought down by means of a magnet to rest on the end of the 

 internal glass wire, exactly in the same way as if it had been the 

 mercury column in a maximum and minimum thermometer. The 

 instrument was then sent to the bottom, or to whatever depth might 

 be decided on. 



During the descent the temperature of the glass, both inside and 

 outside, fell with that of the water through which it passed, but as the 

 contraction produced was the same on the wire and on the tube, there 

 was no differential effect to be recorded by the index. On the other 

 hand, the increasing pressure, as the instrument descended, affected 

 only the outside tube, which it shortened. In contracting, it was 

 obliged to pass the index, which was kept in its place by the internal 

 wire. When the instrument was being hove up, the reverse process 

 took place ; the tube lengthened, and lifted the index clear of the 

 internal wire by an amount equal to the lengthening of that portion of 

 the tube. As the whole clearance produced by the expansion from the 

 greatest depth did not exceed 1 mm., its amount had to be estimated 

 by the eye with the assistance of a magnifying glass. 



The experiment made on March 29, 1875, was quite successful, and 

 it gave 3 '74 as the cubic compressibility per million per atmosphere, 

 of the glass of which the tube was made. The exact figure found in 

 1880 for glass from the same source was 2-92. A number of observa- 

 tions were made with the instrument, both on the sounding line and in 

 the compression apparatus with which the ship was supplied, and 

 figures from 3 — 5 per million were found. These were sufficient to 

 give the order of the constant which was sought, but it was impossible 



