80 Aids to Microscopic Inquiry. 



that is introduced. The particles of water in a tumbler are 

 all acted upon by gravity, and all have great freedom of motion 

 from the highly fluid nature of the liquid of which each par- 

 ticle forms a part. The force of gravity pulls all the particles 

 downwards with equal power, and, as the whole mass of water 

 is liquid, the cohesion existing between the particles is not, as 

 it is in the case of a solid, sufficient to sustain any one set 

 of particles at a higher level than the rest. Consequently, 

 unless a new force operating in another direction is introduced, 

 a fluid surface will conform to the demands of gravitation, and 

 be level — that is, equidistant at all points from the centre of 

 the earth. At the sides of the tumbler a new force appears, 

 and the glass, having an attraction for the water, raises certain 

 particles above the rest. If any floating object be thrown in, 

 it will either depress the fluid level in its vicinity by repelling 

 the particles that are near it, or elevate the fluid level by the 

 opposite action of hoisting certain particles up its own sides. 

 In either case motion must be produced. You cannot pull up 

 or down a certain layer of fluid particles without causing a 

 wave, of a size proportioned to the amount of disturbance 

 originally occasioned. 



When a layer of particles is raised above a fluid level by 

 the surface attraction of a partially-immersed body, the force 

 which pulls the layer upwards is acted against first by gravity, 

 and secondly by the attraction which the liquid particles have 

 for each other; consequently, the amount of displacement 

 actually produced depends upon how much greater the surface 

 attraction is than the other two, and this again depends not 

 only upon the actual intensity of the forces, but also upon the 

 directions in which they act. If we take a rod of any sub- 

 stance easily ivetted, that is to say, of a substance that has an 

 attraction for the particles of water, we shall be able, if we 

 first immerse such a rod in water and then withdraw it, to 

 carry some of the water away. In so doing we have made the 

 surface attraction of the rod overcome gravity, which was op- 

 posed to any elevation of the particles above the fluid level, 

 and also the cohesion of the fluid, which was opposed to dis- 

 severing any of the particles from the general mass. 



Endosmose and exosmose lead to currents in and out of 

 porous bodies, and such currents may be made the means of 

 moving bodies floating in a liquid. If we suppose the force 

 arising from the currents to be equal in all directions, no motion 

 will result ; but, if the ingoing or outgoing currents are stronger 

 at certain points of the surface of the floating body than at 

 others, it will move more or less. It may oscillate if exposed 

 to opposite and alternate impulses, or progress in one direction 

 when the impulse is one-sided and continuous for a time. A 



