262 Dr. A. Schuster on the Dynamical 



to the point at which bodies become white-hot, we should not 

 be better off. For our retinal and crystalline lenses would be 

 white-hot too ; and supposing they could sustain such a tem- 

 perature, all parts of the retina would radiate alike ; conse- 

 quently they would also receive the same amount of radiation : 

 the whole internal radiation being constant, there would be 

 nothing to distinguish one part of the retina from the other, 

 and we should be as effectively (perhaps more effectively) 

 blind as at the ordinary temperature. 



No observations being possible in an enclosure of uniform 

 temperature, we must try to adapt our theoretical conclusions 

 to cases in which we can subject them to the test of experi- 

 ment. We do this by making an assumption contained in 

 Prevost's law, and adopted by all subsequent writers. The 

 first part of Prevost's law says, that bodies are always both 

 radiating and receiving heat ; if their temperature remains 

 constant, as it does if they are placed in an enclosure of con- 

 stant and uniform temperature, it is because they radiate just 

 as much heat as they receive. The second part of Prevost's 

 law, which is always implied though not always distinctly 

 stated, says that the radiation of a body is a function of its 

 temperature only. Assuming, with • Pre vost, that a body of 

 given temperature radiates in the same way whether placed in 

 an enclosure of uniform temperature or not, we can apply the 

 results obtained by theoretical reasoning to the phenomena 

 which we observe every day. But if some of these conclu- 

 sions seem to be contradicted by experience, we are led to 

 inquire a little more closely again whether really the radiation 

 of a body is completely defined by its thermometric tempera- 

 ture, without regard to the fact whether, on the whole, it is 

 gaining or losing heat. In Prevost's time men of science had 

 nothing to guide them, and Prevost's assumption was the 

 simplest, as it was the only natural one. But we may well dis- 

 cuss whether the dynamical considerations on which the mole- 

 cular hypothesis is founded tend, or do not tend, to support 

 the view that the radiation of a body is only a function of its 

 temperature. 



In the first place, let us consider a case, not connected with 

 radiation, w r here the temperature alone certainly does not com- 

 pletely define the state of a gas at a given point. Imagine 

 at first, again, an enclosure of uniform temperature, and fix 

 your attention on a certain element of space in that enclosure. 

 The molecules of a gas are moving about within that space 

 with varied velocities; but, on the whole, as much energy 

 passes the element in one direction as in another, and the 

 average vis viva of the molecules is therefore independent of 



