256 Mr. T. Gray. Electric Conductivity of Glass. [Jan. 12, 



the existence of higher animals of a larger size than the hollow trees 

 were capable of receiving. Nor does it exclude the possibility of 

 higher animals having lived contemporaneously in upland situations 

 remote from the low flats to which our knowledge of the coal forma- 

 tion is for the most part confined. It is to be observed also that 

 as some of the reptilian animals are represented only by single speci- 

 mens, there may have been still rarer forms, which may be disclosed 

 should other productive trees be exposed by the gradual wasting of 

 the cliff and reef." 



II. " On the Variation of the Electric Conductivity of Glass with 

 Temperature, Density, and Chemical Composition." By 

 Thomas Gray, B.Sc, F.R.S.E. Communicated by Pro- 

 fessor Sir William Thomson, F.R.S. Received December 

 28, 1881. 



(Abstract.) 



In this paper the results of the continuation of a series of experi- 

 ments, some preliminary results of which were published in the 

 "Phil. Mag." for October, 1880, are given. The experiments were 

 performed in the Physical Laboratory of the Imperial College of 

 Engineering, Tokio, Japan. 



In the preliminary experiments it was found that the conductivity 

 of glass increased with the temperature, following a similar law to that 

 found to hold for other highly insulating substances. It was also 

 found that the effect of successive heatings and coolings was to 

 diminish the conductivity. Further experiments on this subject show 

 that although the diminution of conductivity here referred to some- 

 times occurs, it does not always occur, and does not seem to do so 

 when the glass is newly manufactured. Reference is made to pre- 

 liminary experiments on the effect of time on the electric conductivity 

 of glass, the results of which indicate an increase in conductivity with 

 time. 



The subject of the main part of the paper is an account of experi- 

 ments on the relation between the electric conductivity of glass and 

 its density and chemical composition. A large number of specimens 

 of lime glass were examined, but, as was to be expected in this case, 

 no marked connexion between electrical quality and density could be 

 observed. It was found, however, on analysing a few specimens, that 

 the composition of those which had a high conductivity differed con- 

 siderably from that required to form an exact chemical compound, 

 while those which had a low conductivity had a composition agreeing 

 more or less closely with that required for a trisilicate of potash and 

 lime, or a mixture of potash, lime, and soda-lime trisilicates. 



