186 Mr. S. Bidwell on the Sensitiveness 



Now it is well known that the resistance of sulphide of 

 silver is greatly diminished by heat *, and it was therefore 

 important to ascertain whether the effect just described was 

 due to light or to heat. To speak more accurately — Is it an 

 effect of radiation or of temperature*? Exposure to radiation, 

 whether visible or invisible, is of course always accompanied 

 by a certain rise of temperature, and confusion has sometimes 

 arisen, especially in discussing the properties of selenium, from 

 failure to distinguish between the direct effects of radiation, 

 and the indirect effects which are primarily due merely to a 

 rise of temperature f. In the photographic processes it is 

 radiation per se that produces the observed results: in the best 

 known processes, the effective rays happen to be those which 

 correspond to the most-refrangible part of the visible spec- 

 trum together with the invisible rays beyond it. But by 

 more recently discovered methods the " obscure heat-rays/' 

 as they are sometimes called, have been made available for 

 photographic purposes J ; and these do not act by virtue of 

 any rise of temperature which they may cause, but exert 

 direct chemical action upon the sensitized plate. Again, if a 

 thermo-pile is exposed to radiation, an electromotive force is 

 generated. Here, however, the effect of radiation is indirect ; 

 it acts only through the medium of the heat which it pro- 

 duces j and if an equal and similarly distributed amount of 

 heat were communicated to the thermo-pile by any other 

 means (as by conduction), exactly the same effect would 

 follow. In an ordinary selenium cell radiation acts both 

 directly and indirectly, tending to produce opposite effects. 

 The direct effect of the radiation, whether it be visible or 

 infra-red or ultra-violet, is a diminution of the resistance of 

 the cell; at the same time the radiation slightly raises the 

 temperature of the cell, and so indirectly tends to increase its 

 resistance. If a selenium cell in circuit with a battery and 

 a galvanometer is suddenly exposed, by withdrawing a screen, 

 to the radiation of a black-hot poker, a momentary swing of 

 the galvanometer-magnet will at first indicate a fall in the 

 resistance; but this will be almost immediately followed by 

 a rise which will increase up to a certain limit as the tempe- 

 rature of the cell becomes higher. The same kind of thing 

 occurs when the cell is exposed to the infra-red or red por- 

 tions of the spectrum; but in the latter case the temperature- 

 effect merely diminishes, instead of overpowering, that directly 

 due to radiation. If the bridge method is used for measuring 



* Faraday, Exp. Res. §§ 432-439. 

 t See Moser, Proc. Phys. Soc. 1881, p. 348. 



[ ( 'aptain Abney is said to have 1 obtained a photograph of a kettle of 

 boilinjx water by means of the invisible radiations which it emitted. 



