446 Hon. E. J. Strutt. On the Least Potential Difference 



to excalation of vertebrae between the pelvis and head region of M. 

 Icevis is shown in such facts as the following : — 



(a) The great amount of both excalation and intercalation which 



must be going on in different regions of the animal on such a 

 hypothesis. 



(b) In some cases the girdle-piercing nerve may pass partly over 



and partly through the girdle, not showing that rigidity which 

 on the excalation theory we should be led to expect. 



(c) The serial number of the girdle-piercing nerve may be different 



on the two sides of the same individual. 



On the hypothesis of migration such facts receive an easy ex- 

 planation, which is also in accordance with the existence of a greater 

 caudal extension of the area of innervation of the pelvic fin in the 

 males of M. Icevis than the females, and in the great amount of 

 variability in M. Icevis, which species we suppose to have been derived 

 from a more stable form such as M. vulgaris by a rostral migration of 

 the pelvic girdle. 



Hence migration being rendered very probable on other grounds, the 

 posterior collector must be supposed to be formed as a direct result of 

 that migration, and its undoubted connection with the shifting of the 

 fin along the vertebral column is of great importance in explaining the 

 formation of the anterior nervus collector. 



" On the Least Potential Difference required to produce Discharge 

 through various Gases." By the Hon. E. J. Steutt, B.A., 

 Scholar of Trinity College, Cambridge. Communicated by 

 Loed Eayleigh, F.K.S. Eeceivecl October 17, — Eead Novem- 

 ber 16, 1899. 



(Abstract.) 



The investigation, of which an account is given in this paper, deals 

 with the potential difference required to produce sparks in various 

 gases, between large parallel planes at a fixed distance apart, and at 

 various pressures. 



It was found by Mr. Peace* that the striking potential between two 

 parallel plates in air diminished as the pressure diminished, till a cer- 

 tain point was reached, and then began to rise very rapidly. The 

 pressure at which the striking potential was a minimum, depended on 

 the distance between the plates, and increased as the distance was 

 lessened. The minimum potential itself, however, varied very little 

 with the distance between the plates. 



This minimum potential was of the same order as the cathode fall 

 * < Roy. Soc. Proc.,' vol. 52, p. 99. 



