A system of physical chemistry - Index of

CHAPTER I.

Introductory— Definitions— Probability — Statistical mechanics— Entropy and thermodynamic

probability — Principle **of** equipartition **of** kinetic energy among

degrees **of** freedom— Application **of** the equipartition principle to specific heats

and radiation phenomena — Necessity **of** modifying the principle **of** equipartition.

Definition **of** Statistical Mechanics.

In what we may call classical mechanics, developed in the first instance

by Newton, we become acquainted with the concepts **of** mass, length,

and time as the fundamental **physical** quantities, and from these we

pass on to derived concepts, such as velocity, acceleration, force, and

energy, by means **of** which we arrive at certain principles and laws

which govern **physical** phenomena. We say that we have " explained "

a **physical** or chemical phenomenon, when we can restate it in terms **of**

mechanics ; that is, when we can show that the phenomenon in question

is to be anticipated on the basis **of** a number **of** mechanical principles

logically applied. In Volume I. we have seen how the application **of**

mechanics to the small discrete particles, which we recognise as mole-

cules and atoms, leads to a reasonable explanation **of** many physicochemical

phenomena. We have restricted ourselves, however, hitherto

by certain simplifying assumptions, i.e. we have dealt with **system**s **of**

molecules as though all the molecules possessed exactly the same value

for their velocity and therefore for their kinetic energy, throughout the

given mass **of** material, an elementary gas, for example. It is known,

however, that such an assumption is by no means true. We have

already indicated this in Chap. I., Vol. I., when referring to the distribution

**of** velocities among a large number **of** gas molecules in terms,

**of** Maxwell's distribution law. It is true that all our experimental

measurements deal with average effects, and henc(j by regarding every

molecule as in an average state and applying the principles **of** mechanics^

we are able to arrive at a number **of** very important and useful conclusions

in terms **of** the elementary kinetic theory, for which we find

experimental evidence.

This mode **of** treatment, however, has its limitations. Certain problems

present themselves which we are quite unable to solve on the

basis **of** the elementary kinetic theory.

We have already met a number

**of** these in Volume II., and have shown how they may be dealt with

from the standpoint **of** thermodynamics. By way **of** illustration we

may cite : the relation between the lowering **of** vapour pressure, lower-

ing **of** freezing point, and rise **of** boiling point **of** a liquid as a result **of**

VOL. III.

I