1894.] On the Electrical Properties of Pure Substances. 35 



exceedingly stiff and well made, supported quite independently of 

 the astatic system, and capable of the finest adjustment. 



6. Stability of zero depends chiefly on. the uniformity of the con- 

 trolling field all over the suspended system ; this is, perhaps, best 

 obtained by using very large and symmetrically disposed magnets 

 Above and below the suspended system. 



7. When this is attended to, there is no advantage in using a 

 " tail " magnet. 



8. The chief remaining difficulty is found to be due to continual 

 small changes taking place in the direction of the earth's horizontal 

 field. This is best overcome by attending to the astaticism of the 

 magnets, and using a fairly strong uniform controlling field opposed 

 to that of the earth. 



9. It is essential that the instrument be entirely surrounded by 

 massive iron screens. 



10. A novel method of illumination has been worked out, ensuring 

 uniformity of brightness of the scale images, without any appre- 

 ciable heating of the galvanometer. The transparent divisions of an 

 opaque scale are caused to give rise to interference fringes, which 

 are then observed in a telescope with a micrometer scale in the eye- 

 piece. Readings of the position of the magnetic system to one 

 second of arc can be easily and certainly made. 



11. The most important improvements we have made relate to the 

 insulation of the instrument, the minute adjustment of the con- 

 trolling field, the recognition of the necessary conditions for high 

 sensitiveness combined with stability, and the method of optical 

 magnification. The instrument can now be used at the sensitiveness 

 mentioned with all the ease and certainty which is generally attained 

 with a millionfold less sensitiveness. 



12. Further improvements can be made by using some material of 

 greater strength than glass for the mirror, and by improved magnetic 

 screening. Our screens were of cast iron, and weighed about 300 

 pounds ; the screening was not nearly sufficiently perfect. 



Section 5 contains an account of a large number of experiments 

 extending over three years on the phenomena of conduction in 

 sulphur of which the following are the chief results. 



Crystalline sulphur, whether monoclinic or " aged " monoclinic 

 {which we have ventured to distinguish as a distinct variety, since it 

 preserves the melting point, but is divested of the crystallographic 

 properties of fresh 'monoclinic sulphur) has a specific resistance of 

 10 38 C.Gr.S. units as a minimum. By exposure to the air of a room 

 the sulphur condenses moisture, which reduces its apparent specific 

 resistance, but not nearly so much as in the parallel case of glass. 

 The total residual charge is either absent, or less than four parts in 

 ten thousand of the original charge, when a film of sulphur about a 



D 2 



