42 
Proceedings of Royal Society of Edinburgh . [sess. 
would permit of, blit the bank I have in my mind’s eye requires 
only the money advanced to be returned, neither more nor less. 
Well, then, it is clear that by the transactions of these two men, 
although one of them was making use of about four times more of 
the bank’s money than the other, the bank at any particular instant^ 
owing to these transactions, would be deficient in only <£200, each 
of the two traders owing it exactly £100. Now for the application 
of the parable, which the knowing reader already sees. The bank 
is the store of pressure, or force, in the medium ; the traders are 
the particles of the atoms ; the quick borrower is the nimble particle, 
and the slow borrower is the less nimble particle. The quickest 
moving particles borrow at a quicker rate from the bank of pressure, 
but also they repay at a proportionally quicker rate. Take, for 
example, two atoms of the same size but different weights, at any 
instant the pressure on each would be exactly as their weights ; 
but during any small portion of time, the pressure on each for that 
time would be proportional to their areas, providing the time were 
great enough to permit of the particles to each move once through 
its own domain or orbit. (Tor each particle would have its own 
little domain to protect from penetration, moving about in a space 
proportional to the size of the atom distributed amongst the number 
of its particles ; for, of course, no particle could complete an orbit 
right round the atom owing to collisions with its fellows, and would 
therefore be confined to one particular locality.) Well, then, each 
particle would be subjected during that time to an amount of 
pressure equal to that which would be received by a surface equal 
to the area of its domain. This pressure would not be on the 
particles all at one time, but would be distributed throughout the 
space through which they moved, a certain quantity being allotted 
to each portion of the space equal to that which they themselves 
would occupy when at rest, or equal to the disc-area of each 
particle. To sum up, then : Atoms are, some comparatively large, 
others comparatively small ; some comparatively open-grained, 
others comparatively close-grained in texture. The particles of 
the open or spongy kind will have a greater velocity than 
those of the close or rigid kind, they having, as it were, to 
perform, each of them, the duty assigned to two or more 
of those of the rigid kind, or at least to more than one. 
