Prof. P. E. Cliaso on Momentum and Vis viva. 297 



both my own view of the kinetic energies of perfect gases, and 

 Hicks's view of the importance of temperature-relations in 

 condensable gases. 



The variation of the nucleal radius as the | power of the 

 atmospheric radius (Phil. Mag. Sept. 1876, k) may furnish a 

 mechanical explanation of results which seem to have been 

 obtained independentlv, and nearly simultaneously, by Silas 

 W. Holman (Phil. Mag. Feb. 1877) and E.Warburg (Pogg. 

 Ann. clix. p. 415). Holman concludes, from the results of a 

 number of careful experiments, that the '^ viscosity of air 

 increases proportionally to the 0'77 power nearly of the ab- 

 solute temperature between 0° and 100° C." The extreme 

 range of his results is '738 to -799. Warburg, from experi- 

 ments both with hydrogen and with air, deduces the expo- 

 nents between 20° and 100°, '76 for air (the extremes being 

 •74 and -76), and " about |- " for hydrogen (the extremes 

 being -57 and -Qb). The closeness, the narrow range, and the 

 mutual confirmation of these independent results, as well as 

 the new analogy between molar and molecular forces which 

 seems to be indicated by the atmospheric experiments, are all 

 interesting. The viscous particles, so far as they are affected 

 by the same movements, may be compared to the rotating 

 particles of a solid nucleus ; the thermal undulations, in a sup- 

 posed asthereal medium, present a like analogy to the motions 

 of an elastic atmosphere. The well-known anomalies in the 

 elasticity of hydrogen are in accordance with its lov*^ viscosity. 

 Warburg's extremes (hydrogen '57, air "76) seem to indicate 

 secondary nucleal and atmospheric relatioxis between air and 

 hydrogen. 



In my identification of the velocity of solar dissociation with 

 the velocity of light (Phil. Mag. I. c, f to t inclusive), although 

 the conception of successive wave-impulses seems most natural, 

 it is by no means essential ; if the pressure of the ultimate 

 force is constant, the result is the same. The ratio of the ve- 

 locity of dissociation to the velocity of perfect fluidity (ib.,77, 6) 

 is approximately illustrated by Draper's estimate of the ratio 

 between the temperature of glow (977° F., or 1436° from ab- 

 solute 0°), and the temperature of fluidity (32° F., or 491° 

 from absolute 0°), 1436-;-491 = 2-9. Here complete fluidity 

 is compared w^ith incipient glow. The ratio ir : 1 would require 

 an additional allowance of 107°, or about 7*5 per cent, for the 

 difference between the temperatures of complete and incipient 

 glow, and for other considerations. 



The vis viva of terrestrial dissociation being equivalent to 

 half the vis viva of incipient planetary dissociation at the sun 

 (ib., X, jjbj v), the temperature-ratio of water-vaporization to 



