454 



Mr. W. Crookes. 



[Feb. 17, 



Up to a pressure of about 350 millims. the presence of aqueous 

 vapour has little or no influence on the viscosity of air. The two 

 curves are, in fact, superimposed. At this point, however, divergence 

 commences, and the curve rapidly bends over, the viscosity falling 

 from 0*0903 to 0*0500, between 50 and 7 millims. pressure. Here it 

 joins the hydrogen curve, and between 7 millims. and 1 millim. they 

 appear to be identical. 



These results are partly to be explained by the peculiar action of 

 water vapour in the apparatus. At the normal pressure the amount of 

 aqueous vapour present in the air, supposing it to be saturated, is only 

 about 13 parts in a million, and the identity of the log decrement 

 with that of dry air shows that this small quantity of water has no 

 appreciable action on the viscosity. When the pump is set to work 

 the air is gradually removed, whilst the aqueous vapour is kept sup- 

 plied from the reservoir of liquid. As the exhaustion approaches the 

 tension of aqueous vapour, evaporation goes on at a greater rate, and 

 the vapour displaces the air with increasing rapidity ; until, after the 

 pressure of 12*7 millims. is passed, the aqueous vapour acts as a gas, 

 and, being constantly supplied from the reservoir of water (as long as 

 it lasts), washes out all the air from the apparatus, the log decrement 

 rapidly sinking to that of pure water gas. 



This explanation requires that the viscosity of pure aqueous vapour 

 should be the same as that of hydrogen, at all events between 

 7 millims. and 1 millim. pressure. The facts can, however, be ex- 

 plained in another way. During the action of the Sprengel pump 

 sufficient electricity is some times generated to render the fall tubes 

 luminous in the dark. It is conceivable that under such electrical in- 

 fluence the falling mercury may be able to decompose aqueous vapour 

 at these high exhaustions, with formation of oxide of mercury and 

 liberation of hydrogen. Of these two theories the latter appears to be 

 the more probable. 



The presence of water vapour shows itself likewise in the very 

 slight amount of repulsion produced by radiation. Repulsion com- 

 mences in air at a pressure of 12 millims., whilst at a higher exhaus- 

 tion the maximum effect rises to over 40 divisions. Here, however, 

 repulsion does not begin till the exhaustion is higher than the baro- 

 meter gauge will indicate, whilst the maximum action after long- 

 continued pumping is only 9 divisions. 



Viscosity of Kerosoline Vapour. 



The rapid diminution of viscosity in the last experiment after reach- 

 ing the pressure of 400 millims. is probably due to the aqueous vapour 

 in the air being near its liquefying point. It was thought advisable 

 to test this hypothesis by employing a somewhat less easily conden- 

 sible vapour, which could be introduced into the apparatus without 



