Prof. W. H. Miller on Two New Forms of Heliotrope. 71 



Young's modulus thus found was, in grammes weight per square cen- 

 timetre, 1 159 X 10 6 for one copper wire, and 1 153 x 10 6 for another 

 which had been very differently treated. 



The highest and lowest rigidities which I have found for copper 

 (extracted from the preceding Table) are as follows : — 



Highest rigidity, 473x 10 6 , being that of a wire which had been 

 softened by heating it to redness and plunging it into water, and 

 which was found to be of density 8*91. Lowest rigidity 393*4 X 10 6 , 

 being that of a wire which had been rendered so brittle by heating 

 it to redness surrounded by powdered charcoal in a crucible and let- 

 ting it cool very slowly, that it could scarcely be touched without 

 breaking it, and which had been found to be reduced in density by 

 this process to as low as 8*6/4. The wires used were all commercial 

 specimens — those of copper being all, or nearly all, cut from hanks 

 supplied by the Gutta Percha Company, having been selected as of 

 high electric conductivity, and of good mechanical quality, for subma- 

 rine cables. 



It ought to be remarked that the change of molecular condition 

 produced by permanently stretching a wire or solid cylinder of metal 

 is certainly a change from a condition which, if originally isotropic, 

 becomes seolotropic* as to some qualities f, and that the changed 

 conditions may therefore be presumed to be aeolotropic as to elasti- 

 city. If so, the rigidities corresponding to the direct and diagonal 

 distortions (indicated by No. 1 and No. 2 in the sketch) must in 

 all probability become different from one 

 another when a wire is permanently stretched, 

 instead of being equal as they must be when 

 its substance is isotropic. It becomes, there- 

 fore, a question of extreme interest to find 

 whether rigidity No. 2 is not increased by 

 this process, which, as is proved by the ex- 

 periments above described, diminishes, to a 

 very remarkable degree, the rigidity No. 1. 

 The most obvious experiment, and indeed 

 the only practicable experiment, adapted to 

 answer this question, will require an accurate 

 determination of the difference produced in 

 the volume of a wire by applying and remo- 

 ving longitudinal traction within its limits of 



elasticity. With the requisite apparatus a most important and inte- 

 resting investigation might thus be made. 



" On Two New Forms of Heliotrope." By W. H. Miller, M.A., 

 For. Sec. R.S., &c. 



A heliotrope is a mirror O provided with some contrivance for ad- 

 justing it so that any given distant point T may receive the light 



* A term introduced to designate a substance which has varieties of property in 

 various directions (Thomson and Tait's ' Natural Philosophy,' § 676). 



t See, for example, a paper by the author, " On Electrodynamic Qualities of 

 Metals," Philosophical Transactions, 1856. 



Vc.l 



Nb.2 



