428 
MR. W. CROOKES OK THE VISCOSITY 
701. I was unable to take pressure after l - 0 millim., as the McLeod measuring 
apparatus does not give trustworthy indications when aqueous vapour is present. 
As soon as the pressure in the measuring tube rises above the tension of aqueous 
vapour, water condenses in it, and measurements can no longer be taken. 
These results plotted as a diagram give the curve shown in diagram A, marked 
“ Moist Air.” 
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 
log. dec. 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. 
702. 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 m illion, 
and the identity of the logarithmic 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 supplied 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 logarithmic 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 explained in another way. During the action of the 
Sprexgel pump sufficient electricity is sometimes generated to render the fell tubes 
luminous in the dark (52). It is conceivable that under such electrical influence 
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, but I have not sufficient data to enable me 
to decide between them. 
703. The presence of water vapour shows itself likewise in the very slight amount 
of repulsion produced by radiation. Expulsion commences in air at a pressure of 
12 millims., whilst at a higher exhaustion the maximum effect rises to over 40 
divisions. Here, however, repulsion does not begin till the exhaustion is higher than 
the barometer gauge will indicate, whilst the maximum action after long-continued 
pumping is only 9 divisions. This confirms the results frequently met with in my 
researches on “ Repulsion Resulting from Radiation,” where the presence of even a trace 
