1903.] The Elasmometer. 193 



Note on the Specific Heat of Aluminium, added May 27, 1903. A 

 specimen of aluminium from the British Aluminium Company of an 

 approximate purity of at least 99 J per cent., gave in three experiments 

 0-2194, 0-2185, 0-2194, mean 0-2191, as the specific heat between 20 

 and 100. 



" The Elasmometer, a New Interferential Form of Elasticity Ap- 

 paratus." By A. E. H. TQTTON, F.RS. Received May 12, 

 Bead May 14, 1903. 



(Abstract.) 



Being desirous of extending the investigation of the physical 

 characters of the crystals of isomorphous salts to the subject of their 

 elasticity, the question arose as to the best form of apparatus to employ 

 for the determination of the coefficient of elasticity. The most accurate 

 form hitherto devised is that of Koch*. The amount of flexure of a 

 thin plate of the crystal was determined by the interference method, 

 sodium light being employed. The great convenience and high 

 accuracy of the optical part of the interference dilatometer which the 

 author has described to the Koyal Society!, suggested the advantage 

 of utilising it as interferometer for the measurement of the amount of 

 the flexure of the plate, and many other possible improvements on the 

 apparatus of Koch also suggested themselves. Eventually the instru- 

 ment now described was devised. It has been constructed by Messrs. 

 Troughton and Simms. 



The observing telescope, with its auto-collimator and attached 

 hydrogen Geissler tube, is exactly as used for the dilatometer, and its 

 pedestal is mounted on a detachable plinth in front of the elasmometer. 

 The rigid iron base on which the latter is mounted also accommodates, 

 on its back portion, the pedestal of the vertical tube which carries the 

 train of prisms to select the monochromatic G or F hydrogen light 

 employed, and direct it on the interference apparatus. The interference 

 chamber of the dilatometer is detached from the lower end of this tube, 

 as the elasmometer carries its own interference tripod. 



The elasmometer proper consists of the following seven essential 

 parts : 



(1.) A pair of platinum-iridium wedges, arranged parallel to each 

 other and with the knife-edges downwards, up against which the plate 

 of the substance (not necessarily a crystal) is to be bent by a weight 

 applied under its centre. They are carried by a pair of gunmetal 



* ' Ann. der Phys.,' N.F., 1878, vol. 5, p. 251. 

 t ' Phil. Trans.,' A, vol. 191, p. 313. 



