348 



Mr. W. Crookes. On the supposed [May 26, 



and thus give rise to motion, after the manner of the radiometer. In 

 this, as in the former case, the movement should be in the opposite 

 direction to what it is in reality, as it would be produced by mutual 

 repulsion acting between the sides nearest the source of heat. 



It seemed likely that information, decisive as regards one or other 

 of these two theories, might be gained by suspending the cylinder in 

 a glass tube attached to a Sprengel pump, and taking observations at 

 different degrees of exhaustion. 



In experiments tried by the author in 1875* the noteworthy fact 

 was ascertained, that two bodies of different temperature attracted 

 each other at normal atmospheric pressure ; the attraction rose as the 

 pressure diminished, until, at a tension of 1'15 mm., it was nearly four 

 times what it was in dense air. Above this exhaustion the attraction 

 suddenly dropped and changed to repulsion, which at the best vacuum 

 obtained was nearly thirteen times stronger than the attraction in 

 air. 



Two forms of apparatus are described by the author, wherewith 

 experiments were tried during exhaustion, and an exact parallelism 

 was established between the attraction or repulsion of the cylinder by 

 a hot platinum spiral, and the positive or negative rotations of the 

 cylinder under the influence of a warm body brought near. 



The two phenomena run absolutely in parallel lines ; when there is 

 attraction negative rotation is also produced ; when the exhaustion is 

 such that the attraction is nil, the rotation is nil also ; when the 

 attraction changes to repulsion the rotation changes from negative to 

 positive ; and when the vacuum is very good, so that the repulsion 

 between the two heated bodies is at its maximum, then also the 

 positive rotation is the strongest. It is impossible to resist the con- 

 clusion that the two sets of phenomena are due to the same cause, 

 and that as air currents did not produce the attractions observed in 

 the 1875 experiments, so likewise are they equally inoperative in 

 giving rise to the present rotations of the suspended cylinder. 



If the rotation is produced by a reaction between the suspended 

 and fixed body, it follows that were both free to move each would 

 rotate, but in opposite directions. To test this, another apparatus 

 was made, having two delicately suspended cylinders, 1 mm. apart, 

 in a glass tube capable of being exhausted. In a table the results of 

 twenty-two experiments are described, observations having been 

 taken at intervals during exhaustion. Down to 14 mm. pressure 

 the two cylinders rotate negatively (i.e., the right hand cylinder 

 rotates clockwise, and the left hand cylinder counter-clockwise). 

 Between 14 and 3 mm. there is no rotation, and below 3 mm. the 

 rotation is positive, the movement at an exhaustion of 0"0495 mm. 

 being five times as strong as it was originally. 



* ' Phil. Trans.,' 1875, Part II (pp. 528—532). 



