COMPRESSIBILITY OF METALS. 205 



capable of giving information as to the variation of thermal expansion 

 with pressure, but the information so given is not very accurate. In 

 the first place, the compressibility of potassium was measured in the 

 ordinary way by two runs at 30° and 60°. The maximum pressure of 

 these measurements was 10000 kg. instead of the usual 12000; the 

 compressibility of potassium was so high that at 12000 kg. the relative 

 deformation was beyond the range of the apparatus. At 60° the 

 pressure was not allowed to fall below 500 kg., because of the prox- 

 imity of the melting point. After these two series of runs over the 

 pressure range, direct measurements of the thermal expansion were 

 made by changing the temperature from 30° to 60° at constant posi- 

 tion of the piston (constant mean pressure) at a number of different 

 pressures distributed over the range. Two series of these tempera- 

 ture observations were made. The scheme of the method and the 

 manner of computing the results is precisely the same as that which I 

 have previously described in detail in connection with the measure- 

 ment of the properties of liquids under pressure. 17 The only difference 

 is that here the dimensions of the specimen are obtained by an electri- 

 cal measurement instead of by measurement of the position of the 

 piston. Is it not worth while again going into the details of the 

 computations, which involve nothing not sufficiently obvious. 



The average arithmetical departure from a smooth curve of the 

 points of the two pressure runs (no discards) was 0.41% of the maxi- 

 mum pressure effect, and the average change corresponding to the 

 thermal expansion under a 30° change of temperature was about ten 

 times this. The measurements of thermal expansion, being much 

 smaller, were not relatively so consistent among themselves. The 

 first run for the determination of expansion gave scattering points at 

 the high pressures, which, however, were not inconsistent with the later 

 more accurate results. On the second run a better technique was used, 

 and all the points except one lay on a smooth curve, this one lying off 

 by some 25% of the observed effect. It is a question whether weight 

 is to be attached to this discordant point. It has seemed to me that a 

 certain amount of weight is to be attached to this point, as thus a 

 curve is obtained without sudden changes of curvature, and the results 

 tabulated have been thus computed. If no weight whatever is 

 attached to this point, then the thermal expansion in the extreme case 

 remains constant over the range from 6000 to 12000 kg., instead of 

 dropping from 83 to 50, as shown. 



The curvature of the graph of volume against pressure is so great 

 that, as also in the case of sodium, it was necessary to compute the 



