204 BRIDGMAN. 



either plot the above results on a large scale and draw the tangent to 

 the curve at the origin, or we may pass a power series curve through the 

 low pressure points. I have passed a three constant curve through 

 the points at 1000, 2000, and 3000 kg. at 30° and obtain the formula: 



AV 



— = - 1.562 X lO- 5 ^ + 3 X 10- ] V + 1.6 X 10~ n p s . 

 > o 



.- 



This formula is not adapted to represent the change of volume outside 

 its own range; already at 4000 kg. the change of volume given by the 

 formula is 0.0566 against 0.0570 experimental. The above formula 

 would give for the initial compressibility 156.2 X 10~ 7 . Richards' 

 value for the average compressibility over the range of 500 kg. is 

 153 X 10~ 7 . He states that over his range the compressibility did not 

 depart sensibly from constancy, but the above formula would show 

 that the average compressibility over the range of 500 kg. differs from 

 its initial value by 1.5 X 10 -7 . Making this correction, the agree- 

 ment with the result of Richards (now 154.5 X 10~ 7 ) becomes closer, 

 and perhaps is as good as could be expected. 25 



Potassium. This was from the same lot as that for which the effect 

 of pressure on freezing point 15 and electrical conductivity 16 has 

 already been determined. The high value and sharpness of the 

 freezing point are evidence of its high purity. The material was 

 prepared for these measurements by first filtering by forcing in a 

 melted condition through a fine gauze under oil (Nujol); it was then 

 cast under oil into a coherent slug, cleaned by scraping the outside 

 surface, and finally formed by squeezing in a mold into a cylinder 

 0.9 cm. in diameter and 1.7 cm. long. This was mounted as a com- 

 pression specimen in the apparatus for direct measurement without 

 magnification. 



A considerably more elaborate series of measurements was made on 

 this than on other materials. I had already found 12 that the be- 

 havior of electrical resistance under pressure was unusual in that 

 above 6000 kg. the temperature coefficient of resistance, which is 

 usually constant over the entire pressure range, begins to decrease 

 rapidly. At the time of making this observation I ventured the guess 

 on theoretical grounds that at high pressures the thermal expansion 

 would show a large rate of decrease. In order to determine the be- 

 havior of the thermal expansion at high pressures the following 

 measurements were made. It is to be noticed that theoretically a 

 measurement of the compressibility at two different temperatures is 



