146 Royal Society .'= — Prof. 0. Reynolds on Burface-forces 



teristic of the Trias, with 'Crocodilia, many land-lizards, Anomo- 

 dontia, Deinosauria, and Marsupial Mammalia. This fauna, as 

 regards genera, Avith the exception of Labyrinthodontia and the 

 appearance of Pterosauria, is represented through the remaining 

 members of the Mesozoic formations, from Jurassic to Creta- 

 ceous inclusive. After this comes the Pachydermatous Mammalian 

 Eocene fauna, and after that the Miocene land-fauna, which, 

 in its main characters, is of modern type. Prom Jurassic to Cre- 

 taceous times, inclusively, there was therefore, as far as we know, 

 in this area a land-fauna chiefly Eeptilian, Saurian, and Marsupial, 

 and in Tertiary times chiefly Eeptilian and Placental. (Illustrated 

 by a Table.) 



In conclusion, the recent character of the early marine faunas 

 of the Cambrian and Lingula-flag series was pointed out, such 

 as Spongida, Annelida, Echinodermata, Crustacea, Polyzoa, Bra- 

 chiopoda, Lamellibranchiata, Pteropoda, Kueleobranchiata, and Ce- 

 phalopoda. This was long ago insisted on by Professor Huxley ; 

 and we find no evidence of its having lived near the beginning 

 of the zoological series ; for below the Cambrian series we are 

 at once involved in a sort of chaos of metamorphic strata. Of 

 the geological history, in the words of Darwin, "we possess the 

 last volume alone, relating only to two or three countries." The 

 connexion of this question with that of the comparative value of 

 different geological eras is obvious, especially in relation to the 

 palseontological part of the question. 



June 18. — Joseph Dalton Hooker, C.B., President, in the Chair. 



The f ollowing communication was read :— 



"On the Forces caused by Evaporation from, and Condensation 

 at, a Surface." By Prof. Osborne Eeynolds, of Owens College, 

 Manchester. 



It has been noticed by several philosophers, and particularly by 

 Mr. Crookes, that, under certain circumstances, hot bodies appear to 

 repel and cold ones to attract other bodies. It is my object in this 

 paper to point out, and to describe experiments to prove, that 

 these effects are the results of evaporation and condensation, and 

 that they are valuable evidence of the truth of the kinetic theory 

 of gas, viz. that gas consists of separate molecules moving at great 

 velocities. 



The experiments of which the explanation will be given were as 

 follows : — 



A light stem of glass, with pith-balls on its ends, was suspended 

 by a silk thread in a glass flask, so that the balls were nearly at 

 the same level. Some water was then put in the flask and boiled 

 until all the air was driven out of the flask, which was then corked 

 and allowed to cool. When cold there was a partial vacuum in 

 it, the gauge showing from | to f of an inch pressure. 



It was now found that when the flame of a lamp was brought 

 near to the flask, the pith-ball which was nearest the flame was 

 driven away, and that with a piece of ice the pith was attracted. 



