1876.] Motion of a Body exposed to Beat and Light. 391 



theory, the communication of heat to a gas must cause a force reactionary 

 on the surface, viz. whether this reaction was adequate in amount to 

 cause the results seen to take place. 



He adds a suggestion as to a new form of light-mill to have vanes 

 inclined like the sails of a windmill, and not having one side white and 

 the other black, like the light-mills at present constructed, arguing 

 that the forces act perpendicularly to the surface, and in a direction in- 

 dependent of that from which the light comes ; so that such a mill would 

 turn like a windmill with the full and not merely the differential effect 

 of the light. Such a mill, he concludes, would furnish another test as to 

 whether or not the force is directly referable to radiation. 



II. " 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 B. Stewart, F.R.S., Professor of 

 Natural Philosophy in Owens College, Manchester. 



(Abstract.) 



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 the 

 phenomenon, 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 from which the force emanates. It was with the 

 view of finding the seat of this reaction that I have made a few experi- 

 ments. 



If the force is directly due 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 Prof. Reynolds, the reaction will be on the en- 

 closure of the moving bodies. I have been able to test this by experiment, 

 and I have found that the action and reaction is entirely between the light 

 bodies suspended in vacuo and the exhausted vessel. 



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 Dr. Q-eissler, of 

 Bonn, under the name of " light-mills." Thanks to the courtesy of Prof. 

 Reynolds, I have been enabled to work with such an instrument. The 

 " light-mill " was suspended by means of two cocoon fibres, forming a 

 bifilar suspension, from the top of a vessel which could be exhausted. A 

 slight movement of the enclosure could be easily detected by means 

 of a concave mirror attached to it. A beam of the oxyhydrogen lamp 



