TRANSACTIONS OF SECTION A. 563 



were contained in similar glass tubes, which were connected together by end 

 tubes, one of which was connected to a Sprengel pump, and by means of the 

 Sprengel pump a very high vacuum was obtained, so that the energy lost was 

 entirely due to radiation; the amount of heat lost by convection being negligible. 

 The lowest temperature at which the strip becomes visible in a darkened chamber 

 to an observer who has remained in the dark for some time in order that his eyes 

 may attain perfect sensitiveness is about 400° C. At this temperature the 

 blackened strip loses nearly seven times as much energy as the polished strip. 

 As the temperature rises the ratio seems to fall, while the light given oft 

 passes to dull red, then to cherry red, and finally to bright red approaching white 

 lieat. 



Experiments are also referred to, of an older date, in which pairs of wires 

 one polished and the other sooted, were compared at the same temperature 

 (inferred from the resistance of the wires). 



It is concluded from all these experiments that the production of lio-ht is 

 vastly more economical when the surface of the light-giving body is bright and 

 highly polished than when it is dull or coated with lampblack. 



TUESDAY, SEPTEMBER 17. 

 The Section was divided into two Departments. 



Department I. — Physics. 



The following Papers were read : — 



1. On the Clustering of Graintational Matter in any part of the Universe 

 By Lord Kelvin, G.C.V.O., F.R.S. 



Gravitational matter, according to our ideas of univei-sal gravitation, would be 

 all matter. Now, is there any matter which is not subject to the law of oravita- 

 tion ? I think I may say with absolute decision that there is. We are all 

 convinced, with our President, that ether is matter, but we are forced to say that 

 the properties of molar matter are not to be looked for in ether as o-enerally 

 known to us by action resulting from force between atoms and matter, ether and 

 ether, and atoms of matter and ether. Here I am illogical when I sav between 

 matter and ether, as if ether were not matter. It is to avoid an illogical phraseo- 

 logy that I use the title ' gravitational matter.' Many years ago I gave strono- 

 reason to feel certain that ether was outside the law of givavitation. '\\'e need 

 not absolutely exclude, as an idea, the possibility of there being a portion of space 

 occupied by ether beyond which there is absolute vacuum — no ether and no 

 matter. We admit that that is something that one could think of; but I do 

 not believe any living scientific man considers it in the slightest degree probable 

 that there is a boundary around our universe beyond which there is no ether 

 and no matter. Well, if ether extends through all space, then it is certain 

 that ether cannot be subject to the law of mutual gravitation between its 

 parts, because if it were subject to mutual attraction between its parts its equi- 

 librium would be unstable, unless it were infinitelj- incompressible. But here 

 again, I am reminded of the critical character of the ground on which we stand in 

 speaking of properties of matter beyond what we see or feel by experiment. 

 I am afraid I must here express a view different from that which Professor 

 Riicker announced in his Address, when he said that continuity of matter implied 

 absolute resistance to condensation. We have no right to bar condensation as 

 a property of ether. While admitting ether not to have any atomic struc- 

 ture, it is postulated as a material which performs functions of which we know 

 something, and which may have properties allowing it to perform other functions 



