On the Viscosity of Gases as affected by Temperature. l..~ 



Latenci/. The effects and after-effects occur very sharply at begin- 

 ning and end of strong illumination of moderate duration. The latent 

 period is between 3 and 10 sec. 



Fatigue and Eecovery. The effects of successive illuminations (of F> 

 minutes' duration) progressively diminish if repeated at " short " in- 

 tervals (10 minutes). At intervals of about 1 hour, successive illumina- 

 tions of 5 minutes produce approximately equal effects. 



With the leaf of Mathiola, I have used periods of illumination of 

 2 minutes at intervals of 15 minutes without provoking any obvious 

 sign of fatigue. 



Conclusions. The leaves of certain plants under favourable conditions 

 of life exhibit electromotive effects and after-effects, amounting to 

 0'02 volt in response to illumination. 



As in the case of animal tissue, it is possible that the negative 

 (zincative) effect may be significant of dissimilation, and the opposite 

 effect or after-effect significant of assimilation. 



The absence of distinct response in petals indicates that chloroplasts 

 are essential to the reaction. 



The absence of distinct response in the green leaves of trees and 

 shrubs is possibly due to a lower aA r erage metabolism in such leaves, as 

 compared with the activity of leaves of small young plants, in which 

 leaf-functions are presumably concentrated within a smaller area. 



" On the Viscosity of Gases as affected by Temperature." By 

 LORD EAYLEIGH, F.RS. Received June 20, Read June 21, 

 1900. 



A former paper* describes the apparatus by which I examined the 

 influence of temperature upon the viscosity of argon and other gases. 

 I* have recently had the opportunity of testing, in the same way, an 

 interesting sample of gas prepared by Professor Dewar, being the 

 residue, uncondensed by liquid hydrogen, from a large quantity collected 

 at the Bath springs. As was to be expected,! it consists mainly of 

 helium, as is evidenced by its spectrum when rendered luminous in a 

 vacuum tube. A line, not visible from another helium tube, approxi- 

 mately in the position of D 5 (Neon) is also apparent. J 



The result of the comparison of viscosities at about 100 C. and at 



* 'Roy. Soc. Proc.,' vol. 66 (1900), p. 68. 



t ' Roy. Soc. Proc.,' vol. 59 (1896), p. 207; vol. 60 (1896), p. 56. 



J I gpeak doubtfully, because to my eye the interval from T) l to D 3 (helium) 

 appeared about equal to that between D 3 and the line in question, whereas, accord- 

 ing to the measurements of Ramsay and Travers ('Roy. Soc. Proc.,' vol. 63 (1898), 

 p. 438), the wave-lengths are - 



