224: Johnston and Adams — High Pressures on the 



observations recorded by the two men. This inconsistency is 

 easily removed as soon as we consider the forms of apparatus 

 used, and the mode of compression possible in such apparatus. 



Spring's apparatus — the same which was used in nearly all 

 of his work — consisted essentially of a thick-walled steel 

 cylinder, cut in a plane parallel to the axis into two parts 

 which were held together by a screw ; in this cylinder a hole 

 gmm | n di ame ter was bored. The piston was actuated by a 

 lever on which weights were hung* ; from the total weight, the 

 ratio of the arms of the lever, and the cross-section of the pis- 

 ton, the amount of the compression was calculated. But the 

 piston used was not a good fit for the cylinder: Spring in one 

 instance says that the play between piston and cylinder was 

 u kanm einige Zehntelmillimeter,"t and speaks repeatedly of 

 the compressed substance as flowing out of the cracks (fentes) 

 of the apparatus. 



The form of apparatus used by Hallock consisted essentially 

 of a thick-walled steel cylinder with a tight-fitting piston, 

 actuated by an appropriate method. Experiments similar to 

 those of Hallock have since been made by others, using 

 analogous apparatus with a tight-fitting piston ; with, of 

 course, similar results.^ 



The difference between the observations of Spring and those 

 of later investigators is now easily accounted for. In Spring's 

 experiments the compression was very far from being uniform, 

 and part of the material squirted out round the piston ; in 

 other words, Spring made rough, but fairly comparable, deter- 

 minations of the flow pressure of the materials, and was work- 

 ing therefore with " unequal pressure," as we have termed it 

 in this paper. In the work of the other investigators the pres- 

 sure was practically uniform, except perhaps for a few moments 

 until the available space inside the bomb was completely filled 

 with the material; hence, in conformity with the observations, 

 no melting took place since, as we have seen, uniform pressure 

 raises the melting point in general, and in the particular case 

 of paraffin raises it by quite a notable amount. Indeed, one 

 might say that, since uniform pressure tends to increase the 

 rigidity or viscosity of substances, pieces of metal imbedded in 

 paraffin or wax would be the less likely to change their positions 

 the higher the pressure employed. 



The difference between the modes of action of equal and 

 unequal pressure accounts for the difference in the obser- 

 vations. With regard to a large number of substances, the 



* Or, in some experiments, by a screw. 

 + Zs. phys. Chem., ii, 585, 1888. 



% For instance, bv GK Spezia, "Some Presumed Chemical and Physical 

 Effects of Uniform Pressure,-' Atti R. Accad. Sci. Torino, xlv, 1-16, 1910. 



