1884.] On the Electrical Qualities of Glass. 497 



It is curious to notice with regard to XX, which is anomalous in 

 most respects, that it gives a chemical formula widely different from 

 that found for X. 



We had hoped to be able to include the results of experiments on 

 the resistances of crystals and minerals. Several sections of different 

 substances have been made, and some of these have been already 

 analysed. Among them are a plate of quartz, cut parallel to the prin- 

 cipal axis, and a plate cut parallel to the secondary axes of the crystals, 

 and by means of these specimens we propose to endeavour to find the 

 resistance and specific inductive capacity in different directions rela- 

 tively to the crystallographic axes. The electrical experiments are 

 not yet, however, completed, but we hope soon to overtake the work. 



As the surface of these sections is small, we propose to use the 

 electrometric method by loss of charge, and to eliminate the capacity 

 of the condenser formed by the substance under examination and the 

 electrometer y by using a sliding cylindrical air-condenser, which will 

 allow us to vary the capacity by a known amount. 



Our proposed experiments on the specific inductivities of different 

 kinds of glass have been delayed principally for want of a standard 

 air-condenser. We propose, however, now to use an air-condenser, 

 the capacity of which can be altered by a known amount, and Sir 

 William Tomson has very kindly placed at our disposal for this 

 purpose his sliding air-condenser. As soon as Sir William Thomson's 

 standard spherical condenser, which is at present on loan, is returned 

 to the laboratory, we shall be able to verify the results to any neces- 

 sary extent by comparison. 



It was at first our intention to employ the method of electric 

 oscillations used by Schiller* in his experiments on the specific induc- 

 tive capacity of different substances, but for various reasons we have 

 decided to employ a different method. In the ordinary theory of the 

 oscillatory discharge of a condenser through a coil of large self- 

 induction, it is assumed that the current at any instant has the same 

 strength throughout the whole length of wire in the coil. Now, 

 although this is no doubt approximately true when the coil connects 

 the plates of a condenser of great capacity, it cannot, on account of the 

 considerable electrostatic capacity of the coil itself (a capacity which, 

 taken per unit length of the wire, must vary along the wire in a manner 

 which, so far as we know, has not yet been worked out) be true when 

 the capacity of the discharging condenser is small in comparison with 

 that of the coil.f On this theoretical ground, and also because of the 

 somewhat elaborate and delicate apparatus for measuring very small 



* Schiller, "Pogg. Ann.," 1874. 



f " On the Trne and False Discharge of a Coiled Electric Cable," by Professors 

 Thomson and Fleeming Jerkin, "Phil. Mag.," September, 1861 ; or "Mathematical 

 and Physical Papers," by Sir William Thomson, Art. LXXXIII. 



