jfiine 24, 1880] 



NATURE 



177 



tube in passing through the red himinosity. The effect was 

 quite unexpected, and it was thought at first that it might 

 have arisen from some peculiarity in the development of 

 the dry plate ; it was not therefore until the result had 

 been confirmed by other photographs that they ventured 

 on the explanation above given. 



A few experiments were made with hydrogen in this 

 same tube ; and the appearances observed are shown in 

 Fig. 6, A B C. 



Pressure 22 mm., 28,948 M, 11,000 cells, current o"oi4i 2 

 W. The glow on negative extended to three-eighths of 

 an inch, a spear-hcad luminosity on the positive wire, to 

 which it was attached by a very bright wire-like stem not 

 greater in diameter than the terminal, A (Fig. 6). 



Pressure 15 mm,, I9,737M, 11,000 cells, current 0"O307i 

 W. A spindle-shaped luminosity at the positive about 

 \\ inch long, and the negative ring completely surrounded 

 with a glow which had increased considerably since A. 



After a short time the spindle on the positive lengthened 

 out and nearly reached the negative, hugging the under- 

 side of the tube as in e (Fig. 6). It was not sensitive to 

 the approach of the finger, although close to the glass ; 

 6,300 cells produced the same phenomena. 



Pressure 4 mm., 5,263 M, 6,300 cells, current 0-03459 W. 

 The discharge in the latter case was partially stratified, C. 



The paper closes with the following conclusions : — 



1. For all gases there is a miniimiin pressure which 

 offers /he least resistance to the passage of an electric 

 discharge. After the minimum has been reached, the 

 resistance to a discharge rapidly increases as the pres- 

 sure of the inediiim decreases. With hydrogen the mini- 

 mum is o'64 mm., 842 M ; at o'oo2 mm., 3 M, it is as 

 great as at 35 mm., 46,000 M- 



2. There is neither condensation nor dilatation of a 

 gaseous mediiun in contiguity -with charged terminals. 



3. IVhen the discharge takes place there is a sudden 



dilatation of the medium in addition to, and distinct 

 from, that caused by heat. This dilatation ceases instan- 

 taneously Tuhen the discharge ceases. 



4. The potential necessary to produce a discharge 

 between parallel flat surfaces at a constant distance and 

 various pressures, or at a constant pressure and various 

 distances, may be represented by hyperbolic curves The 

 resistance of the discharge between parallel fat surfaces 

 being as the number of molecules interveninir bet'cueen them. 



5. This law does not hold with regard to points. In 

 Part I. it has been shown that the potential necessary 

 to produce a discharge at the atmospheric pressure and 

 various distances is as the square root of the distances, 

 while with a constant potential and various distances, 

 the pressure has to be diminished in a greater ratio than 

 that of the increase of distance in order to permit a 

 discharge to take place. 



6. The electric arc and the stratified discharge in 



vacuum tubes are modif cations of the same phenomenon. 

 Lastly, the authors say : — 



" We have again pleasure in thanking Prof. Stokes for 

 his much-valued advice during the course of our investi- 

 gations. To our assistant, Mr. Fram, we are indebted 

 for his able co-operation ; and we have to thank Mr. H. 

 Reynolds for his aid and skill in taking photographs." 



THE NEW FRESHWATER fELLY FISH 

 AIT'E have received the following communications ork 

 ^ * this subject : — 



The Freshwater Medusa 

 When I last week sent you an account of the new 

 genus of freshwater Medusa;, to which I gave the name 

 Craspedacustes, I was not aware that Prof. Allman had 

 prepared, or even that he was intending to prepare, an 

 account of the same animal for the Linnean Society's 



