Prof. Magnus on the Propagation of Heat 



propagated either by radiation alone, or by radiation and con- 

 duction. The thermometer is indeed protected from direct radia- 

 tion by the cork screen ; but this screen itself becomes heated by 

 a long-continued action of the rays, and then gives part of its 

 heat to the thermometer. I confess I at first believed that 

 the heat transferred in this manner to the thermometer would 

 be scarcely perceptible with a screen of 2 millims. thickness, and 

 would in any case be less than with a metal screen. Hence the 

 greater part of the experiments were made with a cork screen. 

 Afterwards, however, I found that a metal screen, although six 

 times as thin as a cork screen, is a better protection against 

 radiation. This doubtless depends upon the fact that a metal 

 screen absorbs fewer of the rays, and also radiates worse than 

 the cork screen ; for when the silvered copper foil was blackened 

 on both sides by a tallow candle, the thermometer was heated 

 more than by the cork screen. Hence the metal screen was 

 never used blackened. But whatever the nature of the screen, 

 even when it consisted of two metal plates with an interposed 

 layer of air, the thermometer after the lapse of a sufficient time 

 always attained an- invariable temperature, just as it did when 

 without a screen. Other circumstances being the same, this was 

 highest when the thermometer was without a screen. In an 

 apparatus similar to that represented in ABC, PL I. fig. 1, 

 but in which the thermometer was somewhat more distant from 

 the vessel of boiling water, the temperatures which it indicated 

 in atmospheric air under a pressure of 1 atmosphere were as 

 follows : — 



Cork screen Two copper foils No screen 



2 millims. thick. 1 millim. distant. i>0 screen - 



23° C. 21-5 25-5. 



It might be thought that the temperatures obtained in different 

 gases, with the use of different screens, would be proportional to 

 one another, since the different screens would absorb propor- 

 tional quantities of the heat incident upon them, and would again 

 part with proportional quantities. But the result has shown 

 that, although these temperatures do follow the same series (that 

 is, if in one gas the temperature with the use of one screen is 

 higher than in another, it is also higher when another screen 

 is used), yet that there is no proportion between the two cases. 

 This arises from the fact that, besides the screen, the side of the 

 vessel AB which becomes heated during the experiment, also 

 acts on the thermometer. Although the vessel is surrounded on 

 the outside with air at 15°, it continually receives heat on the 

 inside, partly from the air in contact, partly by radiation from 

 the thermometer fg, and partly from the heated base of the 

 vessel C. In consequence of this, the side, although of very thin 



