554 Mr. Richard Threlfall [March 19, 



The apparatus which I have used in making the experiment is 

 based on the transmission of pressure by crystaUine graphite or the 

 softer metals. In order to ascertain how much pressure is lost during 

 transmission I have arranged an apparatus in which the material to 

 be tested is exposed to a known pressure, tending to force it through 

 a cyhndrical space, identical in figure with the space in which the 

 lieating is intended to be carried out. The pressure transmitted is 

 transferred by a simple device to a piston with a hard steel point, and 

 this is forced by the pressure to penetrate a soft steel plate. In a 

 subsequent experiment the same piston is forced by a known pressure 

 into the same steel plate, so as to penetrate to the same depth as in 

 the main experiment. It is then possible to compare the pressure 

 transmitted with the pressure applied. 



Experiments of this kind have been made with lead and with 

 graphite as pressure transmitting substances. 



So far as I know, there is no substance other than graphite 

 combining the property of a certain amount of fluidity with the 

 capacity to resist high temperatures ; and our hope of studying 

 chemistry at really high pressures and temperatures appears at 

 present to depend largely upon it. It is true that some attempts 

 have been made to use compressed gases, but the apparatus is vastly 

 more complicated, and the experiments themselves become really 

 dangerous in view of the immense potential energy possessed by 

 gases at pressures of 100 tons per square inch. As illustrating this 

 I may mention that 100 tons per square inch is about the highest 

 instantaneous pressure noted by Sir Andrew Noble in his well-known 

 experiments on the exploding of cordite in closed vessels. The 

 density of nitrogen at 100 tons per square inch is, taking Boyle's law 

 as a very rough approximation, 15,240 times its density under 

 standard conditions. This works out to rather over 19 — i.e. about 

 the same as gold, and the energy stored is of the same order as that 

 contained in an equal volume of cordite, though its availability is 

 lower. 



The construction of the apparatus I have used can be easily 

 followed from the drawings. It consists essentially of a steel cylinder 

 divided perpendicular to the longitudinal axis by a thin plate of mica : 

 the two halves being clamped tightly together by an insulated ring and 

 clamps at top and bottom. Pressure can be applied by an ordinary 

 hydraulic lifting jack — the one I have used will lift fifty or sixty 

 tons— the bore of the hydraulic cylinder being about 4 J inches. In 

 order to operate at a high temperature it is necessary to line the cylinder 

 with some refractory substance, and I have generally used magnesia for 

 this purpose, though zirconia or thoria might be better. Purified 

 magnesia is first melted in an electric furnace and then ground in an 

 iron mortar till it is very fine. The powder is freed from iron as well 

 as possible by a strong magnet, and after being sifted is pressed into 

 the cylinder little by little by hydraulic pressure so as to form a solid 



