and Pyroelectric Properties of Matter. 17 



compression, and less in different proportions for different 

 metals. It is quite certain that india-rubber, jelly of any kind 

 (ever so stiff), and gutta-percha are all of them enormously 

 less rigid in proportion to their resistance to compression than 

 glass or the metals ; and they are all certainly substances 

 which may be prepared so as to be at least as homogeneous as 

 rods, wires, bars, or tubes of metals. From some experiments 

 communicated to me by Mr. Clerk Maxwell, which he has made 

 on iron wire by flexure and torsion, it appears highly probable 

 that iron is more rigid in proportion to its resistance to com- 

 pression than M. Wertheim's experiments on brass and glass 

 show these bodies to be. 



, [17. Since the publication of this paper, the same conclu- 

 sion as to the relative qualities of iron and brass has been 

 arrived at by Everett (Transactions of the Royal Society, 

 1865 and 1866) as a result of fresh experiments made by him- 

 self on these substances — but an opposite conclusion with re- 

 ference to two specimens of flint glass upon which he experi- 

 mented, and which both showed greater rigidity in proportion 

 to compressibility than either his own experiments or those of 

 others had shown for iron or any other substance accurately 

 experimented on. Far beyond these specimens of glass, with 

 respect to greatness of rigidity in proportion to compressibility, 

 is cork ; which though not hitherto accurately experimented 

 on, and though no doubt very variable in its elastic quality, 

 shows obviously a very remarkable property, on which its use 

 for corking bottles depends, viz. that a column of it compressed 

 endwise does not swell out sidewise to any sensible degree, if 

 at all. It is easy to construct a model elastic solid, on the 

 plan suggested above, which shall actually show lateral shrink- 

 ing when compressed longitudinally, and lateral swelling when 

 pulled out longitudinally. The false theory, referred to above 

 as having been first proved to be at fault by Stokes, gives for 

 every kind of solid \ as the ratio of the lateral shrinking to 

 the longitudinal elongation when a rod is pulled out length- 

 wise. The following Table (p. 18) shows how different are the 

 values of that ratio determined by experiment on several real 

 solids.] 



18. The known fact that [many] gelatinous bodies, and the 

 nearly certain fact that most bodies of all kinds, when their tem- 

 peratures are raised, become less rigid to a much more marked 

 extent than that of any effect on their compressibilities, are 

 enough to show that neither the relation first supposed to exist, 

 nor any other constant relation between compressibility and 

 rigidity, can hold even for one body at different tempera- 

 tures. 



Phil Mag. S. 5. Vol. 5. No. 28. Jan. 1878. C 



