400 



Dr. A. Macfarlane on the Disruptive 



qpper one charged by being pressed firmly against a pro- 

 jecting conductor of the Holtz machine. The heating was 

 effected by means of a powerful Fig. 3. 



Bunsen burner placed below the 

 lower plate, and was carried on till 

 the plate became red-hot. The 

 thermometer indicated a range from 

 25° C. to 245° C; and sparks were 

 taken at each interval of 10° during 

 both heating and cooling. The de- 

 flections are represented on Plate 

 XI. diag. 4. The ordinates of the 

 cooling-curve are less than the cor- 

 responding ordinates of the heating- 

 curve — a result confirmed by two 

 other series of observations. The 

 falling off cannot have been en- 

 tirely due to a leaking of the charge 

 of the electrometer during the time occupied by the observa- 

 tions ; for readings taken after the lapse of twenty-four hours 

 showed that not more than one third of the difference could 

 be so accounted for. The bulb of the thermometer being 

 in the position indicated by the figure, gives the tempera- 

 ture of the air between the disks : hence it is probable that 

 the effect was mainly due to the temperature or other state 

 of the disks. 



These curves also throw some light upon the question 

 whether the difference of potential required to effect a dis- 

 charge depends upon the temperature of the gas only if the 

 change of temperature cause change of density. I have cal- 

 culated the density of the air between the disks at the different 

 temperatures, and plotted the difference of potential as ordi- 

 nate to the density as abscissa. The curve obtained should 

 coincide approximately with a portion of a hyperbola if the 

 above question is to be answered in the affirmative. I find that 

 it is convex towards the axis of abscissae, and that it does not 

 look like passing through the origin. 



The Discharge through Liquid Dielectrics, 



On proceeding to investigate the discharge through insu- 

 lating liquids, we first took up oil of turpentine. The liquid 

 was placed in a glass vessel of 7 inches diameter and 5 inches 

 height. A screw passing through the bottom of the vessel 

 served to fix the lower electrode, and also to connect it with 

 the earth. The upper electrode was held immersed in the 

 liquid by means of the rod of the receiver. We observed four 



