478 



Mr. J. B. Hannay. 



[June 10, 



Fig. 7. Spores in the elongated rods ; a is an example o£ a single spore formed at 

 one end of each segment of protoplasm. 



Fig. 8. Yarious phases of spore (coccus) formation in the tubes ; a is an example of 

 spheres in the protoplasm of an unsegmented rod ; b, chains of spores 

 (or cocci), in which remains of the tube, although hardly to be repre- 

 sented clearly in a drawing without exaggerating the appearance, are 

 yet distinctly visible. In estimating the size of b, it is to be borne in 

 mind that both the spores and the sheath make up the dimensions as 

 they are shown. 



Fig. 9. Micrococci or spores set free from scum, which is partly formed by rods in 

 the yarious phases shown in the preceding figures. 



III. " On the State of Fluids at their Critical Temperatures." 

 By J. B. Hannay, F.R.S.E., F.C.S. Communicated by 

 Professor G. G. Stokes, D.C.L., &c, Sec. R.S. Received 

 May 24, 1880. 



In carrying ont the investigations which I commenced some years 

 since npon the phenomena presented by the flow of different liquids 

 through capillary tubes, the question as to what constitutes a liquid — 

 that is in what way it differs from a gas, and how the great variance 

 of the microrheometrical laws for the two fluids can be explained — again 

 and again presented itself to me. Seeing that solids are soluble in 

 gases as well as in liquids, one of the chief differences supposed to 

 exist between the two states has disappeared ; and I have been com- 

 pelled to adopt as the only definition of a liquid, that it is a fluid which 

 has cohesion. Professor James Thomson, F.R.S., has suggested to 

 me the use of the term contractility, instead of cohesion, and this 

 term admirably defines the liquid state, but as it suggests (in a 

 distant way perhaps) a voluntary power, and is used in connexion 

 with organised structures, I shall retain the term cohesion at present. 

 We have then the two states of fluids, first, the gaseous, in which the 

 vis viva or heat energy of the molecules has entirely overcome 

 cohesion, or their mutual attraction, and they are prevented from 

 grouping ; and second, the liquid where the attractive power is greater 

 than the vis viva, and the molecules are enabled to group themselves, 

 but still are in sufficient motion to prevent the grouping from being 

 permanent, hence we have cohesion, but no rigidity. We do not yet 

 know that all solids are not also fluids, as many of them are known to 

 flow, but this may be from other causes, but we know that the solid 

 state is characterised by so much cohesion as to produce more or less 

 rigidity. The most interesting point in the consideration of a liquid 

 is that at which it approaches to the gaseous state, where its cohesion 

 disappears, and we have what Dr. Andrews has termed the critical 

 point, which is the termination of that property which distinguishes 

 a liquid fluid from a gaseous fluid, or in other words the liquid be- 



