October 9, 1891.J 



SCIENCE. 



203. 



lecular vibration, as supposed io the kinetic theory: nor 

 whether the atoms constituting the molecules "are in a state 

 of vibration or rotation motion, in shoi-t, comparable to the 

 bodies of the solar system," as suggested by Mr. William 

 Crookes in a recent article in The Forum. These interest- 

 ing qualities, supposed to be possessed by molecules, or by 

 atoms constituting the molecules, but not by particles com- 

 posed of an aggregation of molecules, are in no wise incon- 

 sistent with the obvious fact that molecules possess some of 

 the qualities of other particles of matter: they are subject to 

 the force of gravitation ; that is, they have weight, and 

 weight is simply the evidence and measure of the earth's 

 gravitational attraction. Considering, then, that a body of 

 water consists of molecules in the same sense that a body 

 of corn consists of grains, it is manifest that the molecules 

 below the surface must sustain the pressure caused by the 

 weight of the superincumbent molecules above, and that this 

 pressure must increase with the depth, because the quantity 

 of superincumbent molecules increases in the same ratio. 

 An increase of pressure not essentially different occurs when- 

 ever particles of any kind are superimposed, as in a grain 

 elevator or a brick wall. It is therefore obvious that noth- 

 ing but the weight of the superincumbent molecules is neces- 

 sary to account for hydrostatic pressure; and the molecules 

 being free to move in respect to each other, all the phenomena 

 of hydrostatic pressure must follow, under the general law of 

 the conservation of energy, and its resultant, that motion is 

 always in the direction of least resistance. 



But it is not necessary to consider in detail the phenomena 

 of hydrostatic pressure, for they are the secondary and not 

 the immediate results of molecular motion, that is, of the 

 motion of the Lr.olecules constituting the water. This motion 

 is the change of position of molecules which constitute the 

 mass or body of water, in respect to each other, and is con- 

 tra-distinguished from molar motion, which is the change of 

 position of the mass in respect to other masses, or of part 

 of a mass in respect to other parts of the same mass. Molec- 

 ular motion may occur from convection without molar mo- 

 tion, as when heat below the boiling point is applied to the 

 bottom of a vessel containing water; the heated molecules 

 rise to the suiface, and the colder molecules at the surface 

 sink towards the bottom, the body or mass of the water re- 

 maining stationary. £o there may be molar motion without 

 molecular motion, as when a vessel full of water is moved 

 from one place to another without agitating the water. But 

 in the phenomena of flowing or pouring, both of these mo- 

 tions uecessarilj' occur: there is a change of the position of 

 the molecules if the subject be water, or of the particles if 

 the subject be grain, seed, shot, etc., in respect to each other, 

 and there is alsa a change of position of the mass in respect 

 to other things, and of parts of the mass in respect to other 

 parts of the same mass. When corn is poured from one 

 vessel into another, we can see the grains change position in 

 respect to each other, and if this did not occur we would 

 know at once that the grain was sliding, not pouring or 

 flowing. We cannot see, even with the most powerful 

 glasses, the molecules of water: one grain of corn equals 

 in bulk many billions of them; but the results of the phenom- 

 ena of flowing and pouring water leave no more doubt that 

 the molecules do change position in respect to each other, 

 than if we saw the motion of each one separately. Indeed, 

 if the lower part of the water in a vessel be colored with 

 sediment or other matter, and the water ba poured into an- 

 other vessel, we have visible evidence of the change of posi- 

 tion of the molecules in respect to each other by the trans- 



fusion of the colored particles throughout the mass in the 

 second vessel. 



Flowing and pouring are terms used to express different 

 phases of the same phenomenon. What actually occurs in 

 every case of flowing or pouring is the transference of a 

 fluid or semi-fluid — that is, of a mass composed of small par- 

 ticles — from one place or vessel to another by the action of 

 gravitation or some other force acting directly on the mass 

 itself, and not merely on the vessel containing the mass. 

 We know by observation when this phenomenon occurs in a 

 quasi- fluid, consisting of grains or particles large enough to 



be observed, that each grain or particle has a motion of its 

 own, and is subject to the mechanical laws applicable to all 

 moving bodies; and assuming the same to be true in respect 

 to the invisible molecules constituting a fluid proper, we find 

 an esplanation of the phenomena of hydraulics, absolutely 

 simple and perfectly satisfactory. 



For example we will take the diminution in the diameter 

 of a jet projected from an orifice in a plain surface; and to 

 illustrate the phenomenon we will borrow the following ex- 

 planation and diagrams from the last edition of the "Ency- 

 clopedia Britannica": "When a jet issues from an aperture 



in a vessel, it may either spring clear from the inner edge of 

 the orifice as at a or 6 [Fig. 20], or it may adhere to the sides 

 of the orifice as at c. The former condition will be found 

 if tlie orifice is beveled outwards as at a, so as to be sharp- 

 edged, and it will occur generally for a prismatic aperture 

 like &, provided the thickness of the vessel round the aper- 

 ture is less than the diameter of the jet. But if the thickness 

 is greater the condition shown at c will occur. When the 

 discharge takes place as at a or 6, the section of the jet is 

 smaller than the section of the orifice. This is due to the 

 formation of the jet from filaments converging to the orifice 

 in all directions inside the vessel. The inertia of the fila- 



