518 



Mr. H. Tomlinson. The Influence of 



or 0-2505. 



The bulk- modulus can also be determined from "Young's modulus " 

 and this last ratio, and is — 



430-8 x 10 6 

 3(1-2x0-2505) 



= 287*6 X 10 6 grams per square centimetre. 



Now it has been shown in a recent communication* to the Royal 

 Society that the bulk-modulus can be calculated from the thermal 

 capacity per unit volume by the formula- 

 s' 2071 x 10 6 x , 



where e v is the bulk-modulus and C v the mean thermal capacity per 

 unit volume between 0° C. and 100° C. According to Regnault the 

 thermal capacity per unit mass is 0'2499, and according to Kopp it is 

 0'245.f The mean of these two numbers is 0*2480, and the thermal 

 capacity per unit volume is therefore 0*2480 X 1*743, or - 4323. From 

 the above formula, therefore, we obtain — 



e v = 2071 xl0 6 X 0*4*323* 



=292-7 xlO 6 . 



This number does not differ from that obtained by observation by 

 so much as 2 per cent., and considering the difficulties which are in 

 the way of getting correct values of the bulk-modulus, we may regard 

 the agreement of the observed and calculated values of e v as suffi- 

 ciently satisfactory. 



Arrangement of the Magnesium Wire for Observations on the Alteration 

 of Resistance produced by Longitudinal Traction. 



Experiment VIII. 



A pair of magnesium wires were arranged for an examination of the 

 effect of longitudinal traction on the electrical resistance of the metal 

 in the same manner as the nickel wire before mentioned, but, as it was 

 desirable to have the permanent load as light as possible in order to 

 avoid trespassing beyond the limits of elasticity, the scale-pan was 

 dispensed with and the weights were suspended to a small hookj by 

 string so that at the outset the permanent load on the wire only 

 amounted to \ kilo. When a sufficient rest had been allowed to 



* " Proc. Eoy. Soc," No. 232, 1884. 



f Clarke's " Constants of Nature," Smithsonian Miscellaneous Collection, No. 276, 

 p. 15. 



X Loc. cit., p. 58, fig. 10. 



