1877.] 



Movements of Radiometers. 



549 



have to deal with the latent heat of vapour condensed on it, or obtained 

 from it by rapid evaporation. 



Hence, thin as is the glass of the bulb (about 0*02 in. thick), we must 

 still, in imagination, divide it into an outer and inner stratum, and exa- 

 mine the effects of these separately. The heat radiated by either stratum 

 depends only on its temperature ; but the radiation from the outer, on its 

 way to the fly, is sifted by passing through the inner, and the portion 

 for which glass is most excessively opaque is in great part stopped. It 

 appears from the observed results that the residue acts decidedly nega- 

 tively, while when the bulb is pretty uniformly heated there is positive 

 action. We may infer that if it were possible to heat the inner stratum 

 alone it would manifest a very decided positive action. 



15. In the struggle between the opposing actions of the outer and 

 inner strata we see the explanation of the strange behaviour of the pith 

 radiometer. In the experiment of § 7 the outer stratum at first shows 

 its negative action ; but quickly the inner also gets heated, partly by con- 

 duction from the outer, partly by direct radiation from the tumbler, and 

 then the inner prevails. In the experiment of § 5 the whole bulb cools, 

 partly by radiation, partly by convection, while the fly remains warmer ; 

 and the slightly greater coolness of the outer than of the inner stratum 

 makes up for the superiority of the inner when the two are equally cool, 

 so that the antagonistic actions nearly balance, and slight causes, such as 

 greater or less agitation of the air, suffice to make the balance incline one 

 way or other. That the inner stratum ivoidd prevail if the two were 

 about equally cooled may be inferred from the behaviour of the radiometer 

 when the bulb is pretty uniformly heated (§§ 4, 11), or shown more 

 directly by cooling the bulb with snow, when a negative rotation may be 

 obtained. 



16. The complete definition of a radiation would involve the expres- 

 sion of the intensity of each component of it as a function of some 

 quantity serving to define the quality of the component, such as its re- 

 fractive index in a standard medium, or its wave-length, or the squared 

 reciprocal of the wave-length*. The experimental determination of the 

 character, as thus defined, of a radiation consisting of invisible heat-rays 

 is beset with difficulties, at least in the case of heat of extremely low 

 refrangibility ; and in general we can do little more than speak in a rough 

 way of the radiation as being of such or such a kind. It is obvious that 

 the behaviour of radiometers by itself alone affords no indication of the 

 refrangibilities of the kinds of heat with which we have to deal ; never- 

 theless, by combining what we know of the behaviour of bodies in respect 



* A map of the spectrum, constructed with the squared reciprocals of the wave-lengths 

 for abscissse, would be referred to a natural standard, no less than that of Angstrom, 

 which is constructed according to wave-lengths ; while it would have the great advan- 

 tage of admitting of ready comparison with refraction spectra, the kind almost 

 always used. 



VOL. XXVI. 2 Q 



