40 Profs. A. Gray and J. J. Dobbie. Connection between 



meter produced by the current through the glass, the area and thick- 

 ness of part of the specimen in which the current flowed, as 

 described in the former paper. 



In the capacity measurements the plate or flask, as the case might 

 be, was supported as described above. A quadrant electrometer was 

 kept connected to the plates of one of Lord Kelvin's air leydens. 

 This was charged with twelve secondary cells, and therefore to a 

 difference of potential of about 24 volts. After the battery had been 

 removed a reading was taken of the electrometer deflection, and 

 then the specimen was connected for a very short interval of time as 

 a condenser in parallel with the leyden. 



This connection was made by means of a myograph pendulum 

 which, when freed, swung over a considerable arc to a catch which 

 prevented it from returning. At its lowest point a metal piece pro- 

 jecting below the bob touched the top of a tongue projecting upwards 

 from a hinge at its lower end, and leaning against the point of an 

 adjustable screw. The connection between the two condensers thus 

 only endured while the three pieces, the screw, tongue, and bob, 

 were in contact. This was only the interval of time required for a 

 pulse of flexure to travel about a centimetre in a bar of steel about 

 half a centimetre thick, and about a centimetre broad. The interval 

 was reckoned as at most about 1/30,000 of a second. 



The plates were originally rather over a quarter of an inch thick, 

 and after some observations of capacity had been made on some of 

 them, and it had appeared that their resistance was too great to be 

 measurable, they were cut down on a turning- table used for cutting 

 slates, to a thickness of about 3 mm. ; they were then fixed on a bed 

 of pitch, and ground down by hand to a thickness of about 0'24 cm. 

 They were polished and properly cleaned, and then covered on both 

 sides with a dense and thoroughly adherent coating of silver. This 

 was cut away for a space of about half an inch round the edges. 

 Great care was taken to remove every trace of silver, and to make 

 the edge thoroughly clean. 



While being experimented on, the glass plate was laid with one 

 coating of silver resting on a plate of copper at .the bottom of an 

 iron bath. Another plate of copper was laid on the upper coating 

 of silver, and kept down with a weight, and the connections of the 

 battery circuit, described above, were made with the copper plates. 

 The iron bath was placed within a larger bath partly filled with 

 sand, so that the temperature could be raised by heating the outer 

 bath from below. 



The results of the experiments are exhibited in the table which 

 follows. We have there given the density, specific resistance, 

 specific inductive capacity, and chemical composition of each speci- 

 men experimented on. 





