316 Messrs. Stewart and Tait on the Heating of a [May 15, 



In all the experiments with aluminium disks the needle of the galvano- 

 meter vibrated 20 times in 20 seconds, and the rotation consisted of 30 

 turns of the handle in 40 seconds. 



Without pretending to explain all these experiments*, it must, we 

 think, be concluded from them that there are two effects which are 

 differently distributed over the particles of the disk. 



45. The relation between the effect for hyd. and that for ^ hyd. -f 

 air for the thin aluminium disk (galv. 20 in 20 seconds) is given as 

 follows :— 



In the preceding experiments the centre of the pile was at a distance 

 (along the radius) of 1-5 in. from the rim of the disk. In the following 

 experiment it was adjusted so as to be at a radial distance of 2*9 in. from 

 the rim of the same disk. With this alteration and with the thin alumi- 

 nium disk we obtained 



We thus see that the effect for ^ hyd. (which may be supposed to re- 

 present the residual effect) and that for -fiy hyd. + ^o" a ^ r (which may be 

 supposed to represent the gas-effect) are both diminished in very nearly 

 the same proportion, namely 100 : 77, by the above transference of the 

 pile to a position nearer the centre of the disk. It would thus appear 

 that the difference in distribution among the particles of the two effects 

 brought to light in art. 44 cannot be explaiued by a difference from 

 centre to rim, but would rather seem to be due to a difference in depth. 

 If this appear to be improbable, it must be remembered that these experi- 

 ments were not made on a naked metallic disk. These experiments 

 would therefore appear to show that in an aluminium disk covered with 

 varnish, as well as in a disk of ebonite, we may imagine the residual 

 effect to be more deeply seated than the gas-effect. 



46. We venture on the following as what appears to us to be the most 

 probable explanation of the whole body of experiments, including those 

 with radiation. 



(1) There is a temporary heat or cold effect (art. 26) which may be 

 supposed to arise in particles very slightly attached to the disk ; this is 

 radiated off chiefly during rotation, and probably does not greatly affect 

 the disk afterwards. 



(2) There is a surface gas-effect, which in an aluminium and even in 

 an ebonite disk is conducted into the interior as it arises, so that it does 



* May not this peculiarity of the aluminium disk in the hydrogen vacuum be due 

 .to some hygrometric surface-effect resident in the varnish ? In one experiment, but 

 only one, the peculiarity was absent. 



Effect for ^ F hyd. (mean 

 of 22 observations) 



Effect for hyd. (mean 

 of 7 observations) 



( Effect for -fij hyd.-f- ¥ 6 - air 

 [ (mean of 9 observations) 



: : 37 : 175. 



