1865.] Prof. W. H. Miller on Two New Forms of Heliotrope. 297 



The highest and lowest rigidities which I have found for copper (extracted 

 from the preceding Table) are as follows : 



Highest rigidity, 473X10 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 letting 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*674. 

 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 submarine cables. 



It ought to be remarked that the change of molecular condition pro- 

 duced by permanently stretching a wire or solid cylinder of metal is cer- 

 tainly 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 seolotropic as to elasticity. 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, therefore, a question of extreme interest 

 to find whether rigidity No. 2 is not increased by 

 this process, which, as is proved by the experiments 

 above described, diminishes, to a very remarkable 

 degree, the rigidity No. 1 . The most obvious ex- 

 periment, and indeed the only practicable experi- 

 ment, adapted to answer this question, will require 

 an accurate determination of the difference produced 

 in the volume of a wire by applying and removing 

 longitudinal traction within its limits of elasticity. With the requisite ap- 

 paratus a most important and interesting investigation might thus be made. 



V. " On Two New Forms of Heliotrope." By W. H. MILLER, 

 M.A., For. Sec. R.S., and Professor of Mineralogy in the Uni- 

 versity of Cambridge. Received May 17, 1865. 



A heliotrope is a mirror O provided with some contrivance for adjusting 

 it so that any given distant point T may receive the light of the sun S 



* 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. 



l&Jt 



