1904] 



On the Compressibility of Solids. 



303 



There is an advantage in having a very slight leak in the apparatus. 

 The routine of an observation is then that the observer in charge of 

 the pump and the manometer gets the pressure up somewhat higher 

 than that desired ; he then settles himself with the relieving lever in 

 his hand and calls out as the mercury in the manometer in falling- 

 passes each division. The observers at the microscopes read their 

 micrometers at the same moment. When the pressure has fallen a 

 little below the desired pressure, the pressure is very carefully relieved, 

 and the readings of the micrometers and of the manometer are taken 

 at atmospheric pressure. The algebraic sum of the movements of the 

 two ends on the micrometers gives the linear expansion of the body 

 which has taken place, and the difference of the two readings of the 

 manometer gives, when interpreted by the help of Piezometer C. No. 1, 

 the difference of pressure which has caused the expansion. The micro- 

 meter measurements are then reduced separately- to their absolute 

 values in terms of the inch. The algebraic sum then gives the linear 

 expansion in terms of the inch. It is then divided by the length of 

 the rod or wire in inches and by the pressure in atmospheres ; the 

 resulting quotient is the linear compressibility of the metal or other 

 substance. Multiplying this by three, we obtain the cubic com- 

 pressibility of the substance, if truly isotropic. 



It will be evident that, to work with this instrument, three observers 

 are necessary, namely, one for each microscope, and one to raise and 

 relieve the pressure and observe the manometer. I was fortunate in 

 being assisted during this investigation by Mr. Andrew King, who 

 was formerly my regular assistant, and is now of the Heriot-TTatt 

 College, Edinburgh, and by Mr. J. Eeid, Demonstrator in the chemical 

 laboratory of that institution. These gentlemen gave up their 

 Christmas vacation for this work, and I owe them a deep debt of 

 gratitude for the willingness and the efficiency of their help. The 

 metals experimented with have been used in the form of wire, and 

 the size chosen was No. 22 of the standard wire gauge (S.W.G.). In 

 the case of aluminium, however, the size was No. 20. The dimensions 

 corresponding to these numbers are given in the following table : — 



!N"o. of wire. 



Diarnetei 



of wire. 



Sectional 

 area of 

 wire. 



Length of 

 1 c.c. 



s.w.a. 



20 ■ 



inch . 

 -036 



mm. 

 -914 



sq. mm. 

 -656 



metre. 

 1 -524 



22 



02S 



0-711 



0-397 



2-519 



The degree in which the actual wires corresponded with the tabular 

 specification was checked by weighing measured lengths of them. 



