A system of physical chemistry - Index of

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A SYSTEM OF PHYSICAL CHEMISTRY

NaCl, AgCl, KBr, and the triatomic substance PbClg. He supposes

the heat content **of** the body to be constituted **of** two parts, one due to

the vibrations **of** the molecules as a whole, and the other to the vibrations

**of** the atoms within the molecules. The first part can be obtained

by Debye's method, taking the upper limit **of** the frequency as deter-

mined by Lindemann's melting-point formula ; the second part can be

determined by Einstein's method, the characteristic frequency being

•determined by Rubens' "residual rays" method. The agreement with

observation is quite good ; details can be found in Vortrdge uber die

Kinetische Theorie dcr Materie und der Elektrizitdt (Teubner). Still

further extensions **of** Debye's ideas have been made by Born and

Karman and Thirring in papers contained in the Phys. Zeit., 14 (1913).

It was pointed out above that Planck's formula is derived from an

innovation in the treatment **of** statistical problems, the assumption, in

fact, that the elementary region **of** the two-dimensional condition diagram

for a **system** **of** oscillators is finite in area, and the replacement **of**

Maxwell's integral by a series. Planck has applied the same idea to the

six-dimensional condition diagram for an ideal monatomic gas, and

arrived at a result which had been already obtained by Nernst, who

bases his derivation **of** it on his well-known " heat theorem ".^ There

are several ways **of** wording this theorem, according to the particular

thermodynamic function which one elects to make use **of** in the state-

ment. As expressed by Planck in his Thermodynamics, it is as follows :

^'The entropy **of** a condensed **system**, which is.chemically homogeneous,

is zero at the absolute zero **of** temperature ". This law removes from the

•expression for the entropy **of** a body in a definite state that indefiniteness

in value which arises in the usual thermodynamic treatment and

which is due to the entrance into the formula **of** an undetermined con-

stant on integrating the differential **of** the entropy. For if clearly the

entropy **of** a body can be given definitely for one state, it is determinate

for all other states which can be conceivably reached from the former

by a reversible path. So if we can conceive that the body can be led

to the condition **of** a chemically homogeneous solid or liquid at absolute

zero, by a reversible process, Nernst's heat theorem makes it possible to

calculate its entropy without ambiguity, if sufficient experimental data

are available. By such means it can be shown that the entropy **of** one

gram-molecule **of** an ideal gas is given by the equation—

S = C^(i -f log T) - R log/ -fa . . (13)

(absolute units **of** temperature, pressure, and energy being used), where

the "chemical constant," a, indeterminate on pure thermodynamical

grounds, has, according to Nernst's theorem, the value—

R,„gQ + ^ .... M

in which p% is the vapour pressure **of** liquid condensed from the gas at

a very low temperature 6, and rg is the heat **of** vaporisation **of** the liquid

at absolute zero.

' Nachr. d. Ges. d. Wisseiisch zur Gd'.tingen Math. Phys. Kl. (1906).

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