Determination of Young's Modulus for Glass. 413 



viz. : — *50, *I5, "41, *40, '345, '35,' 35 ; showing that, beyond 

 a certain extent, the leak is not further reduced by pumping 

 out the vapour. These experiments indicate that most of the 

 effect is not due to the phosphorus vapour surrounding the 

 wire, but rather to that which has been absorbed by the 

 superficial portions of the platinum. This view appears, 

 without violent straining, to offer a satisfactory explanation 

 of all the effects which have been observed ; but it does not 

 seem advisable to lay much stress on these somewhat refined 

 theoretical points, as it is scarcely possible to test them 

 experimentally as yet. 



It might be thought that this effect was only a case of the 

 ordinary leak produced when phosphorus oxidizes in air, due 

 to traces of air left inside the apparatus. This view seems 

 to be rendered quite untenable by the fact that the hot 

 platinum discharges only positive, and not negative electricity. 

 h\ addition to this, it takes no account of the very marked 

 chemical action between the phosphorus and the hot metal. 



In conclusion the author wishes to thank Prof. Thomson 

 for his valuable advice and criticism during the course of 

 these experiments, which were carried out in the Cavendish 

 Laboratory. 



XL I. Determination of Young's Modulus (Adiabatic) for 

 G lass. By Chichester A. Bell, M.B. With an Appendix 

 by C. Chree, F.R.S* 



ALTHOUGH the elastic constants of glass have been 

 the subject of many investigations, it is not easy to 

 make use of the published results in any experimental 

 inquiry into which these enter. The characteristics of the 

 specimens examined are usually so vaguely given, and the 

 composition of the material is so variable, that it is never safe 

 -nine the identity of any sample purchased in the shops 

 with one of ascertained properties. 



This difficulty was acutely felt by the writer in the course 

 of a series of experiments in which a tolerably exact knowledge 

 of the value of Young's Modulus (adiabatic) for different 

 kind< of glass was all-important. The results of these experi- 

 ment< -bowed variations which it was impossible to account 

 for by any justifiable assumptions, and accordingly it became 

 ssary to determine the required constant by experiments 

 on specimens of the gla<s actually employed, the method 

 selected for this purpose being the simplest of all — the 

 acoustical. 



* Communicated by the Physical Society : read January 27, 190o. 



