200 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1918. 



morphism is to be regarded in its most general aspect as due to regrouping 

 of these units in different arrangements. One of the units is to he thought of 

 as terminated by rigid boundaries; that is, each unit has shape as definite as 

 a brick lias shape. Furthermore at different localities on the surface of the 

 units there are localized centers of force (attractive usually), so that two 

 units, it free, will tend to come together with a definite orientation. A crystal 

 is to be regarded as a system in which a compromise has been affected between 

 the arrangement which the units would take in virtue of the action of the 

 localized centers of force, the arrangement into which the units would be 

 urged by external pressure or the mean internal pressure so as to occupy the 

 smallest possible volume, and the chaotic disarray which temperature agitation 

 tends to produce. 



One implication of the view that regards crystals as built from blocks of 

 definite shape is especially insisted on. Only in exceptional cases will the 

 edifice constructed from the blocks be such that there will be no unfilled crevices 

 around the corners, and in no case where there are two possible structures 

 of different volumes will such empty spaces be absent in at least one of the 

 structures. These empty spaces are to be thought of as playing an essential 

 part in the phenomena of polymorphism. 



THE ELECTRICAL RESISTANCE OF METALS UNDER PRESSURE. 1 



In this paper the effect of pressure combined with temperature on 

 the electrical resistances of 22 metals is investigated. The pressure 

 range is from atmospheric pressure to 12,000 kilograms per centi- 

 meter square and the temperature range from 0° to 100°. The ap- 

 paratus is in all essentials the same as that previously used. It 

 consists of two parts, an upper and lower cylinder connected by a 

 stout tube. In the upper cylinder pressure is produced by the descent 

 of a piston driven by a ram. The upper cylinder is kept at constant 

 temperature and contains the coil of manganin wire which gives the 

 pressure by its change of resistance. The lower cylinders of two dif- 

 ferent lengths were used, according as the wire to be measured was 

 insulated and so could be coiled into a narrow space or was bare and 

 had to be wound in spiral grooves on a core. 



It is essential that the method of winding be such that the pressure 

 is transmitted freely to all parts of the coil without any mechanical 

 hindrance from the frame on which it is wound. This object is ob- 

 viously at once obtained when the wire is wound on itself without 

 a core, but this method is feasible only when the wire can be covered 

 with silk insulation without damage. If the wire is soft like lead 

 it can not be covered without damage and must be wound bare on 

 some sort of a core. Several attempts were made before suitable 

 material for a core was found. At first hard rubber was used, but 

 this is so compressible that at the highest pressures the wire drops 

 out of the grooves and is so expansible that at the highest tempera- 

 tures the wire is stretched. 



1 Abstracted from Proceedings American Academy of Arts and Sciences, Vol. 52, No. 9, 

 Feb., 1917. 



