26 Mr. Carl Barus on the 



electrolyte like zinc sulphate in water, or of a liquid metal 

 like mercury, is a decrease of specific resistance. Finally, 

 the present results for insulating-oils contain several examples 

 in which a passage of the positive into the negative pressure- 

 coefficient is directly observed. I conclude that the curve for 

 glass, if prolonged, will eventually intersect the axis of tem- 

 perature somewhere in the region of low red heat, and that the 

 further course will be negative. Hence for all substances, 

 solid or liquid, there is a point or state of electrolytic dissocia- 

 tion at which the effect of pressure on resistance is nil ; or at 

 which the increased molecular stability obtained by pushing 

 the molecules nearer together, and, perhaps, of binding the 

 constituents of the individual molecule more firmly together, 

 counterbalances the decrease of the path over which the ion 

 travels in conveying charge. Taking all the results for glass, 

 oils, the solution and the metal into account, it appears that the 

 complete functionality will be given by a curve which is 

 asymptotic both to the vertical in the region of low tempera- 

 tures and positive pressure-coefficients, and to the horizontal 

 in the region of relatively high temperatures and negative 

 pressure-coefficients. The nature of this curve I have not 

 been able to work out (§ 22), nor will it be the same for all 

 substances. 



25. Having thus found a consistent behaviour in metals and 

 electrolytes in relation to strain, I am prepared to accept the 

 proposition italicized in § 23 as true for solids* generally. 

 For the case of metals Osmond f has partially anticipated me, 

 stating that strain more or less completely converts cold 

 metals from one definite molecular condition into another. I 

 have been unable to find, however, that Osmond has any 

 direct evidence to support this assertion, and I have already 

 pointed out J some of the difficulties which Osmond must sur- 

 mount before Ins view can gain general credence. If me- 

 chanical stress is to convert a-iron into /5-iron, it is not to be 

 overlooked that whereas the hardness called temper changes 

 the electrical resistance of steel enormously (as much as 300 

 to 400 per cent.), the electrical effect of mechanical strain is 

 relatively negligible. Even though much of this discrepancy 



* To my knowledge the only experiments on the effect of pressure on 

 the resistance of solid metals are due to Chwolson (Carl's Repertorium, 

 xiv. p. 26, 1878), who shows that their pressure-coefficient, like that of 

 mercury, is negative. 



■j- Osmond, Annates des Mines, July-August, 1888, p. 6 ; Mem. de 

 Vartilterie de la Marine (Paris, 1888), p. 4. Of course I make no reference 

 against the ingenious experiments from which Osmond infers the spon- 

 taneous conversion of a-iron into /3-iron at red heat. 



% < Nature,' xli. p. 370 (1890). 



