330 Prof. Thomson, On the electric strength of a gas. [May 1G, 



May 16, 1892. 

 Professor G. H. Darwin, President, in the Chair. 



The following were elected Fellows of the Society : 



W. Robertson Smith, M.A., Fellow of Christ's College, Professor 

 of Arabic. 



J. K. Murphy, B.A., Caius College. 



The following communications were made to the Society : 



(1) Recent advances in Astronomy with Photographic Illustra- 

 tions. By H. F. Newall, M.A., Trinity College. 



A series of photographs was exhibited by the lantern and 

 described, to illustrate recent progress in astronomical photography. 

 The series included some interesting specimens taken with the 

 Newall telescope, in which the object glass is not specially cor- 

 rected for photographic purposes. 



(2) On the pressure at which the electric strength of a gas is a 

 minimum. By J. J. Thomson, M.A., Cavendish Professor. 



The author showed that when no electrodes are present, the 

 discharge passes through air at a pressure somewhat less than 

 that due to 1/250 mm. of mercury ; the discharge passes with 

 greater ease than it does at either a higher or a lower pressure. 

 Mr Peace has lately shown that when electrodes are used, the 

 critical pressure may be as high as that due to 250 mm. of mercury : 

 so that as the spark length is altered the critical pressure may 

 range from 250 mm. to 1/250 of a mm. It was pointed out that 

 this involved the possession by a gas conveying the discharge of a 

 structure much coarser than any recognized by the Kinetic Theory 

 of Gases. The author suggested a theory of such a structure and 

 showed that the theory would account for the influence of spark 

 length and pressure on the potential difference required to produce 

 discharge. 



(3) On a compound magnetometer for testing the magnetic pro- 

 perties of iron and steel. By G. F. C. Searle, M. A., Peterhouse. 



When a bar of iron or steel is subjected to the action of a 

 longitudinal magnetic force, H, it is found that the intensity of 

 magnetisation of the iron thereby produced depends not only 

 upon the value of H at the instant, but also upon the series of 



