AND STRAIN ON THE ACTION OF PHYSICAL FORCES. 
29 
The values of A were determined very carefully, more so perhaps than was necessary. 
The specimens used for this purpose had not been stretched, and when in the water 
were well freed of air bubbles by brushing. The results are certainly correct to the 
third decimal place. 
The ratio of lateral lineal contraction to longitudinal dilatation was calculated from 
the formula cr= 0 — 1* on the assumption of the wires being isotropic. It seems 
evident, however, that the values of cr thus obtained cannot claim to be even approxi¬ 
mately correct when the metal has been rendered very hard by the process of drawing, 
as was the case with copper (1), platinum-silver (1), brass (1), brass (2), and German- 
silver (2) : here we meet with apparently impossible results. 
The mean value of cr for the different substances t employed in the annealed condi¬ 
tion = - 2515, a number closely according with that assigned by Poisson as the value 
of cr for each. 
The metals copper (1), copper (2), platinum, aluminium, silver, and platinum-silver 
were obtained from Messrs. Johnson, Matthey, and Co. as chemically pure, and the 
zinc, lead, and tin wires as being as pure as could be got by the ordinary process of 
distillation. 
Elasticity of Volume. 
If e denote the value of “ Young’s modulus,” and cr the ratio of lateral contraction 
to longitudinal extension, it can easily be proved that the elasticity of volume 
1 €> 
, and as e in Table II. is measured in grammes per square centimetre, it 
follows that the increase of volume per unit resulting from a longitudinal stress of 
1 grm. per square centimetre 
In the following table are given the values of the volume elasticity, which will be 
denoted by v, and of the alteration of volume y produced by the above stress. 
In the same table, in order to complete the information given in Table II., is 
recorded the section of each wire in square centimetres ; the section of the hard- 
drawn metals in Table II. being approximately equal to those given here for the 
annealed wires. 
* Thomson and Tait’s Nat. Pliil., p. 521. 
t Copper (2) is not included in this estimate, as I have reason to believe that it was imperfectly annealed. 
