1886.] A General Theorem in Electrostatic Induction. 427 



The electrical condition of the boiler was the thing tested in all 

 the experiments : a gold leaf electroscope connected to it served as 

 indicator. 



The steam was led from the boiler through a straight brass tube 

 about 12 cm. diameter and 120 cm. long, to a steam-globe of copper 

 10 cm. in diameter. This steam-globe was always kept well supplied 

 with distilled water, as Faraday points out how essential it is to have 

 the steam wet. A " feeder- tube " was used to contain the substance 

 to be experimented upon : it was of glass 1"5 cm. internal diameter, 

 and about 15 cm. long. A short length of narrow tubing, furnished 

 with a glass stopcock, was fused to the main tube at the centre and at 

 right angles to the axis. It was used to renew the supply of material 

 in the wider tube below along which the current of steam was 

 passed to sweep the material away. This arrangement was found 

 convenient in working ; and it possessed the great merit of allowing 

 the pieces to be easily cleaned. 



For the friction-piece that was rubbed by the current of fluid, I 

 worked principally with a boxwood tube of as nearly as was con- 

 venient the dimensions of the tube described by Faraday as an 

 " excellent exciter."* 



By a fortunate chance, this tube was found to be very nearly at 

 the neutral line where Dr. Kerr's "positive" and "negative" sub- 

 stances meet. 



In order to find out if possible what was the meaning of some 

 anomalous results that appeared in the experiments, a number of 

 observations were made with, amongst others, tabes of pine, haw- 

 thorn, birch, sulphur, plaster- of- paris, and a tube formed out of a 

 piece of carbon rod 1*2 cm. diameter, such as is used in electric 

 lighting. The results obtained with the sulphur and plaster-of-paris 

 tubes were interesting, but in no way decisive, as the tubes were 

 found to have become very much disintegrated by the action of the 

 current of steam ; they are, therefore, not recorded in what follows. 

 The results with the hawthorn tube will illustrate the effect of 

 friction-tubes of materials whose place is on the negative side of the 

 neutral line ; carbon stands on the positive side. 



Between the two lies the boxwood tube. This is well shown by 

 the results obtained with methylic alcohol and amylic alcohol as 

 shown in the list given below. 



The wooden tubes used were always kept well soaked with distilled 

 water. 



After each day's work with the apparatus, it was taken to pieces, 

 and the copper steam-globe and the feeder-tube were left to steep in a 

 strong solution of carbonate of soda ; then they were well rinsed out 

 with distilled water before being used again. As occasion required, 

 * " Experimental Researches," footnote, § 2102. 



