202 



Mr. T. Gray. 



the temperature requires to be raised until the acid evaporates, it 

 becomes extremely disagreeable. The acid has the advantage that it 

 keeps the vessel dry, and hence is to be preferred for low temperature 

 measurements. 



Abridged tables of results for a few characteristic specimens are 

 annexed, and serve to illustrate the very wide range of resistance 

 which may be obtained by using different specimens of glass. The 

 variation with temperature of several specimens is illustrated by 

 means of curves. These curves only show the variation with tempera- 

 ture through a small range, as it was found almost impossible to 

 include both a number of curves and long range of temperature 

 in the same diagram. 



It will be observed on examining these curves that the rate of 

 variation with temperature is very nearly the same, not only for 

 different specimens of the same kind of glass, but for all the kinds of 

 glass there figured. Other specimens, not included in this diagram, 

 gave a very similar variation. On an average it may be said that the 

 specific resistance of glass is halved for every 8° '5 C. rise of tem- 

 perature.* 



In the tables of results the density of each specimen is recorded, 

 and in some cases the chemical composition also. The chemical 

 analyses were performed in the Chemical Laboratory of the Imperial 

 College of Engineering, Tokio, by Messrs. Fujii and Shimidzu, under 

 the superintendence of Dr. Edward Divers, to whom the author is 

 much indebted for the great interest he has taken, and assistance he 

 has given, in the carrying out of these experiments. 



It is very interesting to notice how very closely a change of density 

 in flint glass agrees with a change of electrical resistance, and also 

 that the electrical resistance of this kind of glass increased regularly 

 until the density reached that point at which the composition of the 

 glass was almost exactly that required for a trisilicate of lead, potash, 

 and silica. The very high density of lead oxide causes the density of 

 the glass to be an indication of the quantity of lead present, and 



* Note added April 26, 1882. — Although the fall of resistance with rise of tem- 

 perature generally follows very nearly the logarithmic law, the results show varia- 

 tions from that law which I am not yet able to explain. The resistance at high 

 temperatures is generally higher than would be inferred from the resistance and 

 rate of variation at low temperatures. It is remarkable that specimens which had a 

 high resistance gave results more nearly in agreement with the logarithmic law than 

 specimens of comparatively low resistance. 



The resistances quoted in the tables are those calculated from observations after 

 one minute's electrification, the direction of the current being alternately in opposite 

 directions, and only allowed to flow for about one minute at each observation. The 

 method of observation was thus similar to that described as " the first method " in 

 my paper in the " Philosophical Magazine " above referred to. (See " Phil. Mag.," 

 October, 1880, page 227.) 



