82 PETER GUTHRIE TAIT 



experiences a steady measurable pressure on the blackened side. Many naturalists, I 

 believe, had truly attributed this fact to the blackened side being rendered somewhat 

 warmer by the light; but none before Tait and Dewar had ever imagined the 

 dynamical cause the largeness of the free path of the molecule of the highly 

 rarefied air, and the greater average velocity of rebound of the molecules from the 

 warmer side. Long free path was the open sesame to the mystery." 



I had the good fortune to be present in the Laboratory when some of 

 the experiments were being made. One especially struck me as being of 

 peculiar significance. I cannot remember if this was shown before the 

 Royal Society of Edinburgh ; but it is not referred to in the published report. 

 A transparent light vane of rock salt was suspended under an ordinary air- 

 pump receiver and placed in front of and fairly close to a fixed blackened 

 surface. The energy rays were directed through the transparent vane on to 

 the blackened surface. At very moderate exhaustions repulsion was set up, 

 whereas for the ordinary form of Crookes' radiometer a very high vacuum is 

 needed. The whole question was thus proved to be one of the relation 

 between the free path and the distance between the repelling surfaces. 



The following among other experiments are described in the Nature 

 Report. Two equal disks, one of glass and the other of rock salt, were 

 attached to the ends of a delicately suspended glass fibre. When the 

 radiation fell on the glass disk there was repulsion due to the heating of the 

 disk ; but when the radiation fell on the diathermanous rock salt there was 

 no repulsion the heat was not absorbed sufficiently to produce the necessary 

 rise of temperature. The back of the rock salt disk was next coated with 

 lamp black, and after sufficient exhaustion was produced in the enclosing vessel, 

 the radiation was thrown through the rock salt on to the blackened surface. At 

 first one might expect an apparent attraction due to the repulsive action on 

 the far-away side ; but the disk was repelled exactly like the glass disk. This 

 was due to the bad conducting power of the lamp black, so that the rock salt 

 on the near side became heated by conduction more quickly than the outside 

 parts of the lamp black layer on the further side. In these experiments it was 

 necessary to use a very thin-walled enclosing vessel within which the vacuum 

 was formed, otherwise the glass vessel would itself absorb so much of the 

 low heat rays that the differential action of the glass and rock salt disks would 

 not be great enough to make itself apparent. 



The next engrossing piece of experimental work was in connection with 

 the "Challenger" Reports. On the return of the "Challenger" Expedition 



