Chemistry and Physics. 475 



steel caps ; each tube was observed by a microscope with micro- 

 meter eye-piece. Details in regard to arrangements of the parts, 

 the manometer employed, etc., will be found in the original 

 paper. In regard to the progress of an experiment the author 

 remarks : 



" The effect observed and measured is the lengthening of the 

 rod when the pressure is relieved. As the compressibility of 

 solids is very small the highest pressures have been used which 

 were found to be compatible with the reasonable persistence of 

 the glass terminals ; the usual pressure was in the neighbourhood 

 of 200 atmospheres. Very few of the glass terminals stood over 

 300 atmospheres. The pressures actually chosen were as nearly 

 as possible those at which the manometer had been compared 

 with the ' Challenger ' piezometer. The body under observation 

 is in the form either of a rod or a wire. If it is in the form of a 

 rod then it is fitted with wire ends of sufficiently small calibre to 

 enable them to enter the glass terminals. During an experiment 

 with a rod it contracts while the pressure is being raised, and 

 expands again when the pressure is relieved. The steel tube 

 which holds it, however, acts in the opposite sense: it expands 

 while the pressure rises and contracts while it falls. If the two 

 surfaces were perfectly smooth, one half of the change of length 

 would be measured at the one end and the other half at the other 

 end. As the surfaces are not perfectly smooth, this does not 

 usually occur. Moreover the steel tubes are prolongations of the 

 central steel block which holds them. The block is bored with 

 holes at right angles to each other in the three principal directions. 

 Consequently for a distance of about an inch and a half in pass- 

 ing through the block the rod is not supported at all. With the 

 exception of this small portion, however, the rod is supported 

 throughout the whole of its length by the steel tube. Now, 

 although it is thus nominally supported equally throughout the 

 whole of its length, we know that in reality this is pretty certain 

 not to be the case. At some place, either in the right arm or in the 

 left arm of the apparatus, the rod is sure to bear more heavily than 

 in any other part. The contraction under pressure and the expan- 

 sion under relief of pressure will then apparently take place as from 

 this point as origin. Supposing this point itself to be motionless 

 it is evident that the change of length measured at the two ends 

 will be in the same proportion to each other as would be the arcs 

 which they would describe if the rod were a lever oscillating on 

 the point as a fulcrum. As there is no support at all at the cen- 

 tre this point must lie on one side or on the other of it and the 

 motions of the ends must be unequal. But the fixed point of the 

 tubular receiver is the central block; therefore any point in, let 

 us say, the right-hand tube will, when pressure is being raised, 

 move to the right, and on relief of pressure, retreat by an equal 

 amount to the left. Consequently when we observe and measure 

 the change of position of, for instance, the right-hand extremity 



Am. Jour. Sol— Fourth Series, Vol. XYII, No. 102.— June, 1904. 

 32 



