398 
MR. W. CROOKES OK THE VISCOSITY 
The successive diminutions of arc are caused by the resistance or the viscosity of the 
air. Fig. 5 shows the damping action at 760 millims., and fig. 7 that at 1 millim. 
Figs. 6 and 8 show the diminution in length of the successive arcs at the same two 
pressures, the line A B joining the extremities being the logarithmic curve. 
In figs. 9 and 10 I give the curves representing the successive diminutions of the 
arc of vibration at high exhaustions due to viscosity. Fig. 9 is copied from a photo¬ 
graph, and shows the damping action at O'02 M. The diminution in length of the 
arcs and the logarithmic curve—now nearly straight—are given in fig. 10. It is 
interesting to compare these figures with figs. 5, 6, 7, and 8, which show similar 
results at low exhaustions. 
A very large number of experiments have been made on the viscosity of air and 
other gases. To reduce the results within convenient limits I will first take the case 
of dry air between the ordinary atmospheric pressure and the i q 0 0 tli of an atmosphere, 
and will then describe the results obtained between the ToVoth and the i,uoo ,o ~ oo th of 
an atmosphere, and higher. I have taken observations at as high an exhaustion as 
0'02 M, but at these high points they are not sufficiently concordant to be trustworthy. 
The pump will exhaust to this point without difficulty if a few precautions are taken, 
but at this low pressure the means of measuring fail in accuracy. 
650. It may be useful if I here give some of the precautions which experience 
shows to be necessary when exhausting to the highest points. In the first place, the 
mercury must be pure. If it contains metals more easily oxidisable than itself, there 
is a tendency, if acids, or even water, come in contact with the mercury, for hydrogen 
to be liberated. This hydrogen is in part absorbed by the mercury, and, carried by it 
through the pump, is slowly liberated at low pressure. The consequence is that the 
highest exhaustions cannot be obtained, whilst the results are due to hydrogen rather 
