SCIENCE 



[Vol. XVIII. No. 453 



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MOLECULAR MOTION IN HYDRODYNAMICS. 



Professor JosiAH P. Cooke's "New Chemistry" has done 

 much to dissipate the mystery which hung around the sub- 

 ject of molecules in my mind before this light was turned 

 on. The physicist, says Professor Cooke, "may prefer to 

 define molecules as those small particles of bodies which are 

 not subdivided when the state of aggregation is changed by 

 heat, and which move as units under the influence of this 

 agent. To the chemist, on the other hand, the molecules 

 determine those differences which distinguish substances. 

 . . . Hence the chemist's definition of a molecure: 'The 

 smallest particles of a substance in which the qualities inhere, 

 or the smallest particles of a substance which can exist by 

 themselves,' for both definitions are essentially the same" 

 ("New Chemistry," pp. 99, 100). When we try to contem- 

 plate the magnitude of these small particles, the mind be- 

 comes bewildered by the immensity of the minuteness, in the 

 same way that it is bewildered by the immensity of the ex- 

 pansion when it tries to penetrate the uttermost depths of 

 the celestial spaces. But every child who sees the stars at 

 night peers into these depths, and every one who hears the 

 whistle or the rumble of a steam-engine is listening to 

 the sound of work done by the movement of these minute 

 particles 



In the long series of experiments which enabled Mr. Wil- 

 liam Crookes to develop the radiometer and Crookes's tubes, 

 he became very familiar with these small bodies: not quite 

 enough so to handle them as a boy does his marbles, or a 

 sportsman his shot; but he furnished abundant proof, if any 

 further proof was required, that the molecules are separate 

 bodies of matter, each with the capacity for its own proper 

 motion and of doing its own proper work. It is true that he 

 did not prove that one molecule by itself could be made to 

 do work like a rifle ball, because he failed to separate one 

 from all others; but he leaves no doubt that when moving 

 together, like shot from a smooth-bore gun, each molecule 

 has its own proper motion and does its own proper work. 



Applying this determination to the phenomena of hydro- 

 mechanics, the explanation it affords is astonishing for its 

 simplicity. This application is entirely legitimate; for while 



Mr. Crookes's operations were on matter in gaseous form, it 

 is now well known that all matter can be changed from one 

 form to another, and the change of the substance which is 

 the subject of hydromechanics from the solid form of ice to 

 the liquid form of water and the gaseous form of vapor, are 

 amongst the most obvious of all phenomena. Moreover, the 

 very fact that water flows, demonstrates its separation into 

 particles each capable of independent motion of its own. 

 When grain passes along a conduit in an elevator, or when 

 seed or shot are poured from one bag or vessel into another, 

 there is a flow, each particle having a certain motion of its 

 own; one moves faster and another slower, as it happens to 

 be more or less subjected to the impelling force. If the par- 

 ticles did not change position in respect to each other, the 

 phenomena would be sliding, not flowing. The essential 

 difference between sliding and flowing is that in the one 

 case, the particles, large or small, constituting the moving 

 body, do not necessarily change position in respect to eac'n 

 other, while in the other this change of relative position of 

 the particles really constitutes the movement of the mass. 

 This is beautifully illustrated by pouring corn into a hopper 

 or bin. When the bag or vessel containing the corn is 

 tilted the grains on top begin to move toward the lower side, 

 and presently begin to pour over, and are followed by the 

 others, each one moTing in obedience to its own gravitation 

 and the pressure, if any, from grains above it, and its move- 

 ment is determined by the resistance it encounters from other 

 grains and the sides of the containing vessel. When the 

 operation is completed no two grains probably occupy the 

 same position in respect to each other, in the hopper or bin, 

 that they did in the vessel from which they were poured. 

 It is said that no two grains are precisely alike in every par- 

 ticular, and it is certainly probable that when a mass of 

 grain flows from one vessel into another, no two of them 

 have identically the same motion both in direction and 

 velocity. The gravitational pull on each is the same, but the 

 variation in pressure and resistance to which they are re- 

 spectively subjected is practically infinite. 



This phenomenon of flow is impossible except in a mass 

 composed of particles free to move in respect to each other, 

 and, therefore, the flowing of water is itself sufficient evi- 

 dence that the water is composed of particles free to move in 

 respect to each other, and that this motion of particles actually 

 occurs whenever water or any other liquid flows. The de- 

 composition of water has demonstrated that the particles 

 composing it are molecules, as defined by Professor Cooke; 

 that is to say, the particles constituting the water itself are 

 the smallest in which the qualities of the substance inhere, 

 and not aggregations of these smallest particles. When a 

 molecule of water is subdivided, as it may be, there is no 

 water left; the water is destroyed, and the matter assumes 

 the form of oxygen and hydrogen, which in certain combina- 

 tion form the molecule of water. (Decomposition, ex m 

 termini, imports a separation of particles; thus when ice is 

 decomposed into water, the particles separate, and there is a 

 further separation of particles when water is decomposed 

 into vapor; therefore when further decomposition destroys 

 the substance itself, it is obvious that the subslance must 

 have been subdivided by precedent decomposition into the 

 smallest particles in which its qualities inhere.) It is obvious, 

 therefore, that a vessel full of water is filled with an aggre- 

 gation of molecules, in the same sense precisely that a bushel 

 measure full of corn is filled with an aggregation of grains. 



It is not necessary for us to determine whether the mole- 

 cules of water are held apart and kept separate by iutermo- 



