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XXVII. On the Nature of the Force 'producing the Motion of a Body exposed to Rays of 
Heat and Light. By Arthur Schuster, Ph.D., Demonstrator in the Physical 
Laboratory of Owens College. Communicated by Balfour Stewart, F.B.S. 
Received February 24, — Read March 23, 1876. 
Mr. Crookes has lately drawn attention to the mechanical action of a source of light 
on delicately suspended bodies in vacuo. I have made a few experiments which will, I 
think, throw some light on the cause of these phenomena, and assist us in the explanation 
of the manifold and striking experiments made by Mr. Crookes. 
Whenever we observe a force tending to drive a body in a certain direction we are 
sure to find a force equal in amount acting in the opposite direction on the body or on 
the bodies from which the force emanates. It was with the view of finding the seat of 
this reaction that I have made the experiments described in these pages. 
If the force is due directly to radiation, the reaction will be on the radiating body ; if, 
on the other hand, it is due to any interior action, such as the one suggested by Professor 
Reynolds, the reaction will be on the exhausted vessel enclosing the bodies on which 
the force acts. I have been able to test this by experiment, and have found that the 
action and reaction are entirely between the light bodies suspended in vacuo and the 
exhausted vessel. The strength of the reaction is a measurable quantity, and hence we 
are able to calculate the absolute force acting on the bodies. 
Description of Experiments. 
The instrument best fitted for an experimental investigation of this kind is the one 
which has been called “ Radiometer ” by Mr. Crookes. These instruments have been 
made in great perfection by Mr. Geissler, of Bonn, under the name of “ Light-mills.” 
It is needless to describe the instrument in detail, as it does not materially differ from 
Mr. Crookes’s radiometer. Light bodies are driven round continuously by the differential 
action of a source of light and heat on their faces, one of which is covered with lamp- 
black. The motion is in such a direction that the faces covered with lampblack recede 
from the light when they are turned towards it. The vessel containing the “light-mill” 
ends above and below in a vertical tube of about L5 centimetre diameter. A wire was 
laid round the tube near the top, and then brought over the top of the vessel in such a way 
that it formed a hook through which a cocoon-fibre could be drawn. A concave mirror 
was attached to this wire. The instrument was then suspended by a bifilar suspension 
from the top of a glass receiver which could be exhausted. It was found, however, that 
the amount of air in the receiver did not affect the experiments in any way. The lower 
