DIFFERENTIAL PRESSURE ON MINERALS AND ROCKS 495 



if the copper tube be sawn open the sulphur within is found to be to 

 all appearances as hard and solid as any mass of sulphur could be, 

 although in the mass, here and there, little surfaces can be detected 

 which have a slight shimmer and which are evidently planes of 

 slipping. The cohesion of the mineral along them, however, is to all 

 appearances as great as elsewhere in the mass. 



In connection with Kick's process there is one point of some 

 importance to which attention does not seem to have been paid by 

 those who have employed the process. This is the question as to 

 whether, in carrying out the experiment, as the deformation of the 

 cylinder goes forward, the pressure exerted on the specimen is con- 

 veyed to it entirely through the embedding material, or whether, in 

 the latter stage of the compression, the specimen is actually nipped 

 between the top and bottom of the tube or box which incloses it, 

 that is to say, between the plates of the press, and is pressed upon 

 directly by these without the intervention of any of the embedding 

 material, or of a mere film which remains and which on account of 

 its thinness exerts no influence. 



Any experiment may be arranged so as to have the pressure 

 exerted in either of the above w^ays, but no distinction seems to have 

 been made between the two cases by former workers. As a matter 

 of fact, however, very different results are obtained as one or other 

 method of experimentation is adopted. If the specimen submitted to 

 pressure continues throughout the experiment to lie entirely sur- 

 rounded by the embedding material and is not pressed upon by the 

 plates forming the ends of the tube, the value of the differential 

 pressure to which it is subjected depends on the "stiffness" or vis- 

 cosity of the embedding material, that is to say, on its internal friction. 

 In the present state of our knowledge of the mathematics of plastic 

 flow, it is impossible to calculate accurately the stresses set up in the 

 inclosed specimen; although if movement is taking place in the 

 erhbedding material the stresses are differential. When, however, 

 the substance experimented upon offers great resistance to deforma- 

 tion, as for instance glass or porcelain, the differential stresses set up 

 in any of the embedding materials hitherto employed are not, under 

 the conditions of experimentation adopted, sufficiently powerful to 

 bring about a deformation of the material. The alum, or whatever 



