32 J. 0. Thompson — Law of Elastic Lengthening. 



the least stable one of the " iodine series " and the compound 

 which should contain (C1I), with the least closely related halo- 

 gens, could not be prepared. There is a possible exception in 

 the body Cs— (ClBr) — I, for it has not been noticed that the 

 stability of this varies to any marked extent from Cs— (C1C1) 

 — I. The same view of the effect of the identity of the link- 

 ing halogen atoms will probably apply to all double halides, 

 for it is certain that very few of these containing different 

 halogens are known, although this may be partly due to the 

 fact that mixed double halides have not been sufficiently 

 studied. An investigation of the double halides of caesium 

 and mercury, now in progress, indicates that the generalization 

 will apply in this case. ' Whatever may be the true structure 

 of double halides, this influence of the identity of the halogens 

 in the combining simple salts probably depends upon the same 

 broad chemical law which causes, for example, two oxides or 

 two sulphides to combine more readily than an oxide and a 

 sulphide of the same elements, and which causes sulphates to 

 combine with other sulphates more readily than they combine 

 with nitrates and other dissimilar salts. 



The present communication may be considered as prelimi- 

 nary to a wider study of polyhalides and double halogen salts. 

 It is hoped that other series, studied chemically and crystallo- 

 graphically, may give valuable results. 



Sheffield Scientific School, December, 1891. 



Akt. Y. — The Laio of Elastic Lengthening /* by J. O. 

 Thompson, Haverford College. 



In the theory of elasticity the assumption is usually made 

 that the lengthening of an elastic rod or wire is directly pro- 

 portional to the stretching weight. Moreover experimental 

 results have apparently justified this assumption in all cases 

 where the lengthening was within the so-called limit of elas- 

 ticity. Hookef was the first to formulate this as a law, and 

 his assertion " ut tensio sic vis," although occasionally questioned 

 seems to have stood the test of experiment up to the present 

 time. "WertheimJ in his well-known investigations concerning 

 the elasticity of the metals, and later Edlund§ in his measure- 



* Translation made bv the author for this Journal of a paper which appeared in 

 Wiedemann's Annaleu, vol. xliv, No. 11, 1891. 



f R. Hooke. Lectures and Collections, London, 1679. 



% Wertheim, Pogg. Ann., Erganz, ii, (1848), p. 1. 



§Edlund. Pogg. Ann., cxiv, (1861), p. 1, and cxxvi, (1865), p. 539. 



