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THE POPULAR SCIENCE MONTHLY.— SUPPLEMENT. 



THE RADIOMETER AND ITS LESSONS. 



By WILLIAM B. CARPENTER, M. D., F. R. S. 



SOMEWHAT less than two years ago, the large 

 assemblage of scientific men gathered at the 

 soiree of the Royal Society was startled at the 

 sight of a phenomenon which was altogether 

 new and strange to the great majority of them. 

 In the interior of a thin glass globe, about the 

 size of a small orange, prolonged below into a 

 cylindrical stem by which it was supported on a 

 stand, Mr. Crookes presented to our view an hor- 

 izontal cross of four slender arms radiating at 

 right angles from a common centre ; the extrem- 

 ity of each arm carried a thin disk about the size 

 of a threepenny - piece, black on one side and 

 white on the other, the black sides all facing 

 alike; while beneath the centre was a pointed 

 steel pivot, resting on a cup that formed the 

 summit of a rod fixed into the cylindrical stem, 1 

 on which the cross with its terminal disks was 

 free to revolve horizontally — exactly after the 

 fashion (in miniature) of Dr. Robinson's cup-an- 

 emometer for recording the velocity of wind. 

 The globe, Mr. Crookes informed us, had been 

 exhausted of air to the utmost degree attainable 

 by the "Sprengel pump" as improved by him- 

 self, and had been then hermetically sealed. 

 Without any other perceptible agency than the 

 general light of the apartment, the cross slowly 

 rotated horizontally in the direction of the white 

 sides of the disks. When a candle was brought 

 within a foot or so of the globe, the rotation 

 became much quicker. When the candle was 

 approximated to within two or three inches of 

 the globe, the cross spun rapidly round. And 

 when a piece of magnesium wire was burned 

 close to it, the rapidity of the rotation became so 

 great that the disks could no longer be separately 

 distinguished ! 



The effect was not perceptibly diminished by 

 the interposition, between the globe and the can- 

 dle or other source of light, of a glass trough 

 containing a solution of alum, which, while trans- 

 parent to light, stops a large part of the radiant 

 heat which accompanies it. And — what was yet 

 more remarkable — if, while the cross was rotating 

 rapidly under the influence of a candle within a 



1 In the radiometer as now constructed, the arms 

 radiate from an inverted cup, which rests upon the 

 pointed pivot — an arrangement that is in many re- 

 spects more convenient. 



short distance, the flame of a spirit-lamp was 

 made to play over the surface of the globe, the 

 rotation was checked in a. very peculiar manner; 

 the cross being, as it were, pulled up with a jerk- 

 ing action, much as when the swinging of a com- 

 pass-needle is stopped by the attraction of a mag- 

 net brought near it. When, on the other hand, 

 the spirit-lamp was withdrawn, the candle re- 

 maining where it was, the rotation commenced 

 anew as the globe cooled. 



It is scarcely surprising, then, that a general 

 impression should have at once prevailed that a 

 capital discovery had been made — that of the 

 direct mechanical action of light ; which, though 

 not indicating the existence of a new force in 

 Nature, showed that the most universally diffused 

 of all forces, next to gravitation, has a mode of 

 action which was previously not merely unknown, 

 but altogether unsuspected. And this impres- 

 sion was not confined to those who had only a 

 general acquaintance with physical optics ; for it 

 was shared by the greatest masters of that de- 

 partment of science, who had followed the course 

 of the experimental researches on which Mr. 

 Crookes had been for some time engaged, and 

 of which this discovery was the culmination. 



The origin of these researches was rather 

 singular. In the course of the weighings made 

 by Mr. Crookes to determine the atomic weight 

 of the new metal thallium, his discovery of which 

 by spectrum analysis had acquired for him de- 

 served distinction as a chemist, he noticed that 

 when the balance was inclosed in a case, and 

 the substance weighed was of a temperature 

 higher than that of the surrounding air and ap- 

 paratus, there was an interference with the due 

 action of the balance, which seemed attributable 

 to the currents set up in the air within the case 

 by the inequality of its temperature. Experi- 

 ments were then made to render the action more 

 sensible, so as to discover and eliminate sources 

 of error ; and in the course of these it was dis- 

 covered that, when a small light body is delicately 

 suspended in the most perfect vacuum that can 

 be produced, it is repelled by radiant heat or 

 light, although the same body suspended in the 

 same vessel, from which the air has not been 

 exhausted, seems attracted by the same radiant 

 force. This can be demonstrated by suspending 



