Geological Society. 153 



It must be remembered that e depends on the rate at which 

 cold particles will come up to the hot surface, which is very slow 

 when it depends only on the diffusion of the particles of the gas 

 inter se and the diffusion of the heat amongst them. 



It will be much increased by convection-currents ; but these 

 will (as has been already explained), to a certain extent, produce 

 an opposite effect. It would also seem that this action cannot have 

 had much to do with Mr. Crookes's experiments, as one can hardly 

 conceive that much heat could be communicated to the gas or va- 

 pour in such a perfect vacuum as that he obtained, unless, indeed, 

 the rate of diffusion varies inversely as the density of a gas*. It 

 will be interesting, however, to see what light experiments will 

 throw on the question. 



GEOLOGICAL SOCIETY. 



[Continued from p. /6\] 



November 5, 1873. — Prof. Eamsay, P.R.S., Vice-President, 



in the Chair. 



The following communications were read : — 



1. " On the Skull of a Species of Halitlierium from the lied Crag 

 of Suffolk." By Prof. W. H. Flower, E.R.S., F.G.S. 



The specimen described, which is in the collection of the Rev. H. 

 Canham, of Waldringfield, is from the so-called coprolite- or bone-bed 

 at the base of the Red Crag, and presents the usual aspect of the 

 mammalian remains from that bed. It is of especial interest as 

 furnishing the first recorded evidence of the existence in Britain of 

 animals belonging to the order Sirenia. The fragment consists of 

 the facial part of the cranium, separated, probably before fossiliza- 

 tion, from the posterior part at the fron to-parietal suture, and in a 

 line descending vertically therefrom. It was afterwards subjected 

 to severe attrition, by which many of the projecting parts have been 

 removed ; but sufficient remains to enable its general relationship to 

 known forms to be determined. The whole of that portion of the 

 maxillae in which the molar teeth were implanted is preserved. 



The author compared the fossil skull with those of the existing and 

 extinct species of the order, and stated that, while it presents many 

 characters common to the Manati and the Dugong, there are others 

 by which it differs from both, the most striking being the more 

 normal development of the nasal bones and the outer wall of the 

 nasal fossae, and especially the dentition, in all of which it shows a 

 more generalized condition. The existence in it of maxillary teeth 

 removes it still further from Rhythm. In general character the 



* June 10. — Professor Maxwell lias shown that the diffusion both of heat 

 and of the gas varies inversely as the density ; therefore, excepting for con- 

 vection-currents, the amount of heat communicated from a surface to a gas 

 would be independent of the density of the gas, and hence the force /would be 

 independent of the density ; that is to say, this force would remain constant 

 as the vacuum improved, while the convection-currents and counteracting 

 forces would gradually diminish. It seems probable, therefore, that Mr. 

 Crookes's results are, at least in part, due to this force. 



