PROF. OSBORNE REYNOLDS (vol. xviii. p. 121) appears to have done himself less than justice in the extracts he has sent you from his earlier papers, as representing his published views on the action of residual gas in radiometers. For the extracts do not suffice to constitute an explanation of this action, whereas the papers from which he makes the extracts contained what, if true, might have been an explanation of the action of residual gas, along with much else that is admittedly erroneous; and although those papers (the only ones published before mine) conclude with Prof. Reynolds's own expression of opinion that residual gas is not the cause of the force observed by Mr. Crookes.