TRANSACTIONS OF SECTION G. 719 



draw attention only to some general considerations, and to point out to what 

 «xtent they practically interest the members of this Section. 



In a lecture delivered at the Royal Institution last May, Professor Mendel^efi' 

 attempted to show that there existed an analogy between the constitution of 

 the stellar universe and that of matter as we know it on the surface of the 

 earth, and that from the motions of the heavenly bodies down to the minutest 

 interatomic movements in chemical reactions the third law of Newton lield good 

 and that the application of that law afforded a means of explaining those chemical 

 substitutions and isomerisms which are so characteristic, especially of organic 

 chemistry. 



Examined from a sutricient distance, the planetary system would appear as a 

 concrete whole, endowed with invisible internal motions, travelling to a distant 

 goal. Taken in detail, each member of the system may be involved in movements 

 connected with its satellites, and again each planet and satellite is instinct with 

 motions which, there is good reason to believe, extend to the ultimate atoms, and 

 may even exist, as Sir W. Thomson has suggested, in the atoms themselves. ' The 

 total result is complete equilibrium, and, in many cases, a seeming absence of all 

 motion, which is, in reality, the consequence of dynamic equilibrium, and not the 

 repose of immobility or inertness. 



The movements of the members of the stellar universe are, many of them 

 visible to the eye, and their existence needs uo demonstration ; but the extension 

 of the generalisation just mentioned to substances lying, to all appearances, inert on 

 the earth's surface is not so apparent. In the case of gases, indeed, it is almost self- 

 evident that they are composed of particles so minute as to be invisible, in a condi- 

 tion of great individual freedom. The rapid penetration of odours to great distances 

 the ready absorption of vapour and of other gases, and the phenomena connected with 

 diffusion, compression, and expansion seem to demonstrate this. One gas will rapidly 



Senetrate another and blend evenly with it, even if the specific gravities be very 

 ifferent. The particles of gases are, as compared with their own diameters 

 separated widely from each other ; there is plenty of room for additional particles ; 

 hence any gas which would, by virtue of its molecular motion, soon diffuse itself 

 uniformly through a vacuum will also diffuse itself through one or more other 

 gases, and once so diffused, it will never separate again. A notable example of this 

 is the permanence of the constitution of tlie atmosphere, which is a mere mixture 

 of gases. The oxygen and the nitrogen, as determined by the examination of 

 samples collected all over the w-orld, maintain sensibly the same relative propor- 

 tions, and even the carbonic acid, though liable to slight local accumulations, pre- 

 serves, on the whole, a constant ratio, and yet the densities of these gases differ very 

 greatly. 



Liquids, though to a much less degree than gases, are also composed of parti- 

 cles separated to a considerable relative distance from each other, and capable of 

 unlimited motion w^here no opposing force, such as gravity, interferes ; for under such 

 circum.stances their energj- of motion is not sufficient to overcome the downward 

 attractions of the earth ; hence they are constrained to maintain a level surface. 



The occlusion of gases without sensible comparative increase of volume shows 

 that the component particles are widely separated. "Water, for example, at the 

 freezing-point occludes above one and three-quarter times its own volume of car- 

 bonic oxide, and about 480 times its volume of hydrochloric acid, with an increase 

 of volume, in the latter case, of only one-third, and sulphuric acid absorbs as much 

 as 600 times its bulk of methylic ether. The quantity of gas occluded increases 

 directly as the pressure, which seems to indicate that the particles of the occluded 

 gas are as free in their movements among the particles of the liquid as they would 

 ■be in an otherwise empty containing vessel. 



Liquids, therefore, are porous bodies whose constituent particles have great 

 freedom of motion. It is no wonder, consequently, that two dissimilar liquids, placed 

 in contact with each other, should interpenetrate one another completely, if time 

 enough be allowed ; and this time, as might be expected, is considerably greater 

 than that required for the blending of gases, because of the vastly greater mobility 

 of the particles of the latter. The diff'usion p*" liquids takes place not only when 



