of Edinburgh, Session 1883-84. 
531 
Monday, IWi February 1884. 
Sheriff FOEBES IEVIKE, Vice-President, in the Chair. 
The following Communications were read : — 
1. On Eadiation. By Professor Tait. 
[Abstract.) 
The first part of this communication was devoted to a recapitulation 
of the advances in the Theory of Exchanges made by Stewart in 
1858, and published in the Transactions of the Society for that 
year. Such a recapitulation it will he seen is necessary; as 
Stewart’s papers seem either to have fallen into oblivion or to be 
deemed unworthy of notice. It was pointed out that Stewart 
showed in these papers that the radiation within an impervious 
enclosure containing no source of heat must ultimately become, like 
the pressure of a non-gravitating fluid at rest, the same at all points 
and in all directions ; but that this sameness is not, like that of fluid 
pressure, one of mere total amount ; it extends to the quantity and 
quality of every one of the infinite series of wave-lengths involved. 
For, as one or more of the bodies may be blach, the radiation is 
simply that of a black body at the temperature of the enclosure. 
Any new body, at the proper temperature, may be inserted in the 
enclosure without altering this state of things; and must therefore 
emit precisely the amount and quality which it absorbs. This 
remark contains all that is yet known on the subject. For we 
have only to assume for the purpose of reasoning, the existence of a 
substance partially, or wholly, opaque to one definite wave-length, 
and perfectly transparent to all others ; or with any other limited 
properties we choose ; and suppose it to be put (at the proper 
temperature) into the enclosure. If we next assume that its 
temperature when put in differs from that of the enclosure, the ex- 
perimental fact that, in time, equilibrium of temperature is arrived 
at, shows that the radiation of any particular wave-length by a body 
increases with rise of temperature, Apd so forth. 
Yet in the latest authoritative work on the subject, Lehrbuch dev 
2 M 
VOL. XII. 
