ON TIIK INDUCTIVE CAVACITV Ob' A OIUOD .>lMtENUKL VACUUM, ETC. 1 97 



with yellow, over the shoulder especially, so that when flying forth from 

 his retreat I thought it was a specimen of Noctilio leporimis. In this re- 

 spect this individual contrasted remarkably with all the cave-haunting 

 specimens I had examined, for in them the colour of the fur appeared to 

 be almost quite uniform, namely, dai-k brown in the terminal third, the 

 extreme tips of the hairs greyish, the basal three-fourths pale greyish 

 bi'own ; beneath the greater part of the hairs uniooloured greyish brown, 

 paler towards the extremities. The facial streaks were more or less 

 defined in all the individuals captured by me. 



Avtiheus quadririttaftis, Ptrs. 

 Add Popayan to the localities of this species. 



Ghirodertna salvini, Dobson. 



An adult male specimen, also from Popayan, in the collection of the 

 Gcittingen Museum, has the facial streaks faintly marked, thus showing 

 that the development of these marks are probably as variable as I have 

 already noticed in the case of Artiheus pJanirostris. There is also a very 

 faint white line along the spine, which is absent in the type. The 

 peculiar form of the first lower premolar is, however, as well marked as 

 in the type. 



Preliminary Report of the Committee, consisting of Professor W. E. 

 Ayrton {Secretary), Dr. 0. J. Lodge, Mr. J. E. H. Gordon, and 

 Mr. J. Perry, appointed for the purpose of accurately measuring 

 the specific inductive capacity of a good Spjrengel Vacuuvi, and 

 the specific resistance of gases at diferent pressures. 



In 1870 two of the members of your present Committee concluded, from 

 theoretical reasons, based on the analogy between the viscous yielding of 

 bodies to mechanical stress and the absorption of the electric charge in a 

 Leyden jar, that some connection of an inverse order would be found to 

 exist between the specific inductive capacities and the specific resistances 

 of dielectrics. As at that time it was only for gutta percha and india- 

 rubber that the specific resistances had been measured, it was necessary, 

 in order to put the theory to the test of experiment, to carefully measure 

 the specific resistances of several other dielectrics of which the specific 

 inductive capacities were known. The substances selected were paraffin- 

 wax, shell lac, mica, ebonite, &c., and it was found that, in a general way, 

 if bodies were arranged in increasing order of specific inductive capacity, 

 they would be found arranged in decreasing order of specific resistance.' 



Again, since different gases had different indices of refraction for light, 

 it was felt that Faraday's not having succeeded in finding experimentally 

 different specific inductive capacities for the various gases, must have 

 arisen from the comparative roughness of his apparatus ; and very delicate 

 experiments undertaken in consequence, showed that hydrogen had a 

 decidedly less specific inductive capacity than air, and that carbonic 



' ' The Viscosity of Dielectrics,' bv W. E. -iyrton a.v<\ .Tolin Perry, Proe. Boy. Soc. 

 No. 180,1878. 



