ELECTRICAL RESISTA^XE UNDER PRESSURE. 127 



diameter. For the resistance measurements a square prism was cut 

 from the center of this about 0.2 inches on a side. The resistance 

 was measured by the potentiometer method, with the three terminal 

 plug. The terminals were attached to the phosphorus by means of 

 helical coils of very fine wire snapped oxer the prism in grooves filed 

 on its surface. The distance between potential terminals was about 

 1.5 cm. 



One run was made with this sample, at 0°. The points with increas- 

 ing pressure ran smoothly, and on decreasing pressure the points with 

 increasing pressure were repeated, except the final zero, where there 

 developed a parasitic e.m.f. so large that further readings were impos- 

 sible. The general character of the pressure effect was an enormous 

 decrease of resistance under pressure. The results before the para- 

 sitic e.m.f. appeared were very nearly the same as those found later 

 with the second sample. The parasitic e.m.f. Avas ascribed to the 

 imperfect removal of the phosphoric acid, and the specimen was 

 again treated for a number of hours with boiling water, but without 

 success. It was e\ident that the acid permeated the material too 

 deeply to be removed by surface treatment in this way. It was 

 accordingly necessary to prepare a fresh specimen of phosphorus. 



In preparing this fresh sample, advantage was taken of an observa- 

 tion made by Dr. A. Smits ^^ in preparing the phosphorus for measure- 

 ments of the vapor pressure. He found that the kerosene by which 

 pressure had been transmitted to the phosphorus during formation 

 was exceedingly difficult to remove. A chemical analysis by Pro- 

 fessor Baxter had also shown some carbon as an impurity of the 

 phosphorus; it is possible that some of this might also have been 

 introduced by the kerosene. It was therefore indicated that the black 

 phosphorus should be formed if possible without contact Avith kero- 

 sene. This was simply done by surrounding the yellow phos- 

 phorus with water in the lower cylinder, transmitting pressure to the 

 water with kerosene as usual, but so choosing the dimensions that 

 the kerosene should never come in contact with the phosphorus. 

 This was entirely successful; the transition went essentially as before, 

 when kerosene was used. In particular, occasion was taken to again 

 measure the time rate of transition, and the same resvilts found which 

 have already been published, ^^ and which make the explanation of 

 the transition from yellow to black phosphorus so puzzling. That 

 the phosphorus formed under water was purer than the phosphorus 

 previously formed under kerosene was suggested by the absence of 

 the peculiar odor, w^hich had permeated the earlier product, char- 



