284 Messrs. J. H. Gray and J. B. Henderson. 



print of Mathematical and Physical Papers ' ; of Dr. A. Matthiessen, 

 F.R.S., published in the 'British Association Reports' for 1862, 

 1863, 1864, and 1865; and those of Mr. Herbert Tomlinson, F.R.S., 

 given in several papers communicated to the Royal Society in 1877 and 

 subsequent years. The paper of the last, dealing most particularly 

 with the present investigation, is contained in the ' Phil. Trans.,' 

 1883, pp. 1 72, " On the Influence of Stress and Strain on the 

 Action of Physical Forces." 



As this work was done at two different periods of time, it has been 

 found convenient to divide the paper into two parts. Part I, which 

 contains the results of change of density due to the different kinds of 

 treatment, was done previous to last July. Part II contains the 

 results of alteration of resistance due to stretching, the preliminary 

 work and trial methods, which occupied a very considerable time, 

 having been done in conjunction with the work of Part I. 



PART I. 



By JAMES H. GRAY. 

 Density. 



As, in every case, the alterations were expected to be small, great 

 care had to be taken to have all the apparatus as sensitive as possible. 

 A very delicate Oertling balance was used, capable of weighing 

 accurately to within -^ milligramme, and all the usual precautions 

 observed. 



Effect of Stretching. 



A well-annealed wire of practically pure copper, electrical con- 

 ductivity 98 per cent., diameter 2 mm., was stretched by weights till 

 it broke. The wire was securely fixed to a strong hook near the 

 ceiling of the laboratory, and two ink marks made, one near the top 

 end, the other near the bottom end. Behind these marks were fixed 

 l-mm. scales, which enabled the stretching to be recorded. After 

 the wire had been stretched by a weight, a length of about 8 inches 

 was cut off. In this manner successive weights were applied, pieces 

 of the wire being cut off each time, till it broke. Even when so great 

 lengths as 8 inches were taken however, it was found very difficult 

 to weigh very accurately in water, and this, along with possible 

 differences, due to the manner in which the several tests were made, 

 may account for the fact that the intermediate values of the densities 

 varied in different series. The original and final densities of the 

 wire could always be determined very accurately, as much greater 

 lengths of the wire could be used. The numbers below give the 

 values, which were found to be very constant in all the specimens 

 tested. 



