1921] on Elasticity 379 



When the pressure is sufficiently increased and the temperature 

 reduced, gases first liquefy and finally become solids, and so far as I 

 know there have been no experimental determinations of the volume 

 compressibility of liquid or solidified gases. 



In fact singularly little is known as to the compressibility of any 

 substance, even at the (what may be called) moderate pressure which 

 can be produced in the laboratory. It is very difficult, for various 

 reasons, to use pressure of 100 tons per square inch, which is very 

 small compared to the pressure at the centre of the earth. By a 

 well-known proposition it can be shown that if a certain mass is 

 weighed at the earth's surface, and if the same mass were formed 

 into a rod of uniform section and of the length of the earth's radius, 

 and then lowered into a boring extending from the earth's surface to 

 its centre, the gravitational attraction urging it downwards would be 

 one- half of its surface weight. 



The density of the earth is about 5*7, and the radius about 

 21 million feet. A rod one inch square, and of this density and 

 length, would weigh just under 26 thousand tons at the earth's 

 surface. 



The pressure per square inch at the centre is therefore about 

 13,000 tons. At the centre of the sun the pressure would be more 

 than thirty times as great. 



Compared with this any experimental pressure seems insignificant. 



The direct experiments on volume elasticity are chiefly on glass, 

 water and mercury, in connection with the corrections for deep-sea 

 thermometers, and also on certain other fluids, and the pressures 

 employed range up to a few tons per square inch. (In Mr. C. A. 

 Parsons' experiments on the compressibility of water the pressures 

 were raised as high as 40 tons per square inch.) 



I myself have made many trials of the compressibility of glass, 

 indiarubber, various solid fats, and of minerals, such as asbestos, 

 using pressures up to 24 tons. (These were in connection with the 

 obturating pads used to make a gas-tight joint at the breech-blocks 

 of large guns.) The method adopted in most of these experiments, 

 including my own, depends on the measurement of the descent of 

 a plunger under known loads into a strong vessel filled with the 

 substance whose compressibility is to be found, and then applying 

 corrections for the concomitant variation of the volume of the vessel, 

 etc., induced by the applied pressure. It is iu these corrections that 

 the doubtful element appears in the results. For small pressures up 

 to two tons per square inch I used a different procedure, in which 

 the corrections were more easily dealt with. The matter for com- 

 pression was placed in a glass cylinder, about ten inches high and 

 three in diameter, the open end of which could be closed by a flat 

 glass plate perforated with a small central hole. 



A fine tube, connected with a small cistern of mercury, entered 

 the vessel through this hole, and beneath it a second cup was 

 b.YoL. XXIII. '(No. 115) 2 d 



