TEE RADIOMETER AND ITS LESSONS. 



29 



undulatory theory, consists in the propagation, 

 through an ethereal medium, of systems of waves 

 of different lengths ; and it is in virtue of this 

 difference that their direction is more or less al- 

 tered by refraction, the longest waves being least 

 and the shortest most deflected by passing through 

 the prism. The solar beam is the composite re- 

 sultant of the whole aggregate of these undula- 

 tions. When falling on the eye, it excites the sen- 

 sation of colorless light ; when falling on the hand, 

 it affects us with the sensation of warmth ; when 

 falling on the bulb of the thermometer it causes the 

 expansion of the mercury; and, when falling on a 

 photographic surface, it produces chemical change. 

 But, when made to pass through a prism, it is 

 decomposed not only into that succession of col- 

 or-bands, formed by rays of different degrees of 

 refrangibility, which constitutes the luminous 

 spectrum, but into two other successions of rays, 

 one of much lower and the other of much higher 

 refrangibility, which lie beyond the two ends of 

 the luminous spectrum. 



These dark rays are not recognizable by the 

 eye, because the retina is no more sensible to 

 them than the ordinary cutaneous surface is to 

 luminous impressions ; but their heating power 

 can be measured by a thermometer or a thermo- 

 pile, and their chemical power by the action they 

 excite on a photographically-prepared surface. 

 The heating power is thus found to attain its 

 maximum a little outside the red end of the color- 

 spectrum ; and from that point it progressively 

 diminishes toward the violet end of the luminous 

 spectrum, beyond which it is scarcely traceable ; 

 while it diminishes in the contrary direction also, 

 until it dies out at about the same distance from 

 the maximum' on one side as that at which the 

 violet lies on the other. The illumiuating power 

 has its maximum in the yellow band of the spec- 

 trum, and shows a gradual reduction toward the 

 violet end, a more rapid toward the red. The 

 chemical power, on the other hand, has its maxi- 

 mum in the violet band ; and, while it gradually 

 diminishes toward the red end of the luminous 

 spectrum, beyond which it is scarcely traceable, 

 it diminishes at about the same rate in the oppo- 

 site direction, dying out at nearly the same dis- 

 tance from the maximum on one side as that at 

 which the red lies on the other. Thus, while the 

 rays of low refrangibility, whose wave-length ex- 

 ceeds 812 millionths of a millimetre, are charac- 

 terized almost exclusively by their heating power, 

 and those of high refrangibility, whose wave- 

 length is less than 400 millionths of a millimetre, 

 are characterized almost exclusively by their 



chemical power, the rays of medium refrangibility, 

 whose wave-lengths are between 400 and 800 

 millionths of a millimetre, combine these with 

 illuminating power, in proportions varying with 

 their respective wave-lengths. But there is no 

 more reason, as Mr. Crookes has justly remarked, 

 for attributing these several effects to different 

 rays, than there is for hypothetically splitting up 

 the element iron (for example) into a number of 

 components, of which one gives its specific grav- 

 ity, a second its chemical reactions, a third its 

 magnetic properties, and so on. 



But, to the three attributes of radiation uni- 

 versally recognized by physicists, Mr. Crookes 

 proposes, in the passage already cited, to add a 

 fourth — the power of producing an electric cur- 

 rent in a thermopile ; and & fifth — the power of 

 producing a mechanical motion when acting on 

 light bodies freely suspended in a vacuum. Now, 

 the notion that radiation directly excites the elec- 

 tric current of a thermopile is one, I apprehend, 

 which no well-informed physicist would indorse ; 

 for, as the name of the instrument implies, it is 

 by the disturbance of the thermal equilibrium be- 

 tween the two metals of which it is composed 

 that the electric current is produced. And, since 

 this disturbance may be produced in a variety of 

 ways (as by friction or conduction), and the po- 

 tency of the electric current is strictly propor- 

 tional to the amount of that disturbance, there is 

 no reason whatever for attributing to radiation 

 any other power of exciting an electric current 

 than that which it exerts mediately through its 

 power of heating the thermopile. And the ques- 

 tion which, after the first shock of novelty passed 

 off, has greatly exercised the minds of physicists, 

 is whether the mechanical motion, also, is not an 

 intermediate effect of some one of the previously- 

 known forms of radiant energy — that which first 

 suggests itself being the action of heat upon that 

 residual vapor of which it is impossible to get rid 

 entirely by any means at present known. 



This idea very early occurred to some of the 

 distinguished physicists who took most interest 

 in the experiments first communicated by Mr. 

 Crookes to the Royal Society. I more than once 

 conversed with Wheatstone on the subject ; and 

 he expressed a very strong belief that the swing- 

 ing round of the pith-bar was due to the disturb- 

 ance of the thermal equilibrium in the residual 

 vapor, dwelling very strongly upon the impossi. 

 bility of obtaining a perfect vacuum, since " even 

 glass," he said, very emphatically, " would give 

 off a vapor if all other vapor were withdrawn.'" 

 On the other hand, two of the most distinguished 



