464 
MR. W. CROOKES ON THE SUPPOSED 
But what produces the negative rotation at ordinary atmospheric pressure ? Air- 
currents are the obvious explanation, but there are grave reasons for believing this 
explanation to be inadequate. In the first place, actual air-currents, when tried, did not 
produce the desired result. Secondly, it is most logical to assume that, as the present 
set of experiments are strictly paralleled with those tried in 1875, and as the results 
at high exhaustions are in each case due to molecular bombardment, so also must the 
similar results at low exhaustions be due to the same cause. 
My series of papers on “Repulsion resulting from Radiation”* contain numerous 
observations of attraction in air of ordinary density or at low exhaustions ; and in the 
Fig. 6. 
Bakerian Lecture for 1878 I described an apparatus devised with the object of 
distinguishing between the action of air-currents, and of attraction in air of low 
exhaustion, and the repulsion in air at high exhaustions. In the following description 
I have condensed the experiments tried in 1878, and have added other results 
obtained subsequently with a similar piece of apparatus. 
The apparatus is shown in fig. 6t; it consists of a cylindrical glass vessel sealed at 
the top, drawn off narrow at the other end, and having a stem, d, sealed in to hold a 
needle-point. The vessel is connected with the Sprengel pump by the narrow tube 
* ‘Phil. Trans.,’ 1873, pp. 501-527; 1875, pp. 519-547; 1876, pp. 325-376; 1878, pp. 243-31S; 1S79, 
pp. 87-134. 
t To avoid unnecessary complication, I have omitted from the drawing parts not used in the present 
series of experiments. 
