DECEMBER 31, 1903] 
NATURE 
195 
variables, the phase law, gaseous systems, dissoci- 
ation, and dilute solutions. 
‘As is well known to specialists in thermodynamics, 
Prof. Planck, instead of using the thermodynamic 
potentials of the majority of writers, prefers to deduce 
the conditions of equilibrium from the study of the 
function 
(energy) —(temp.)(entropy) + (pressure)(vol.) 
— (temperature) 
i.e. the ordinary thermodynamic potential correspond- 
ing to temperature and pressure as independent vari- 
ables, divided by temperature and reversed in sign. 
While this function has not the advantage of being 
an exact analogue of the potential functions in statics, 
the differential coefficients of which with respect to the 
position-coordinates are equal to the corresponding 
generalised force-components, its introduction un- 
doubtedly serves to bring the conditions of equilibrium 
and stability of thermodynamic systems into closer 
connection with the entropy properties. _ We should 
prefer to see the principle of degradation of energy 
instead of the entropy principle adopted as the basis 
of thermodynamics. This would obviate the introduc- 
tion of Planck’s function, the ordinary thermodynamic 
potentials taking its place. The compensating draw- 
back is that the available energy of a system is not a 
definite measurable quantity, but is dependent on the 
surrounding media. 
The method of introducing such notions as tempera- 
ture and entropy cannot be regarded as satisfactory. 
We find in chapter i. the usual juggling with the 
terms ‘‘ perfect gas’’ and ‘‘ absolute temperature.”’ 
Thus absolute temperature is defined in § 9 by the 
expansion of gases, while in § 24 these gases are shown 
to obey laws which are not rigorously consistent with 
this definition of temperature. The term ‘“ perfect 
gas’ is introduced in a vague sort of way in this 
chapter, but without sufficiently definite statements 
being made as to what is a perfect gas and what is 
not. To define absolute temperature by means of a 
perfect gas and then define a perfect gas by means 
of its laws of expansion referred to absolute tempera- 
ture is merely working round in a circle. 
Moreover, the entropy of unit mass of a substance 
is defined, in the first instance, by the formula 
p=c log 6+R/m log v+const., 
applicable to the case of a perfect gas. This definition 
is suggestive of the definitions of pole and polar given 
in many text-books, according to which ‘the line 
xx!+yy!=c is called the polar of the point xy’ with 
respect to the circle x?+y?=c?.’’? But while the effects 
of the latter definitions are made patent by the absurd 
answers sent up by a large proportion of examination 
candidates to pole and polar questions on (e.g.) a 
so-called ‘‘ general conic,’’ opportunities at present do 
not occur so frequently in this country of testing how 
an average student, after reading such a treatment, 
would “ define entropy.”’ To define a physical quantity 
in the first instance by means of its value in a par- 
ticular case, when the definition is not valid in the 
more general case, is certain to be misleading, and no 
amount of subsequent discussion, such as Prof. Planck 
NO. 1783, VOL. 691 
admittedly gives, can set matters right. We have 
marked instances of the same thing in the old- 
fashioned treatment of electrostatics and magnetism, 
in which bodies were stated without reservation to 
attract one another according to the law of the inverse 
square, and when dielectrics were subsequently intro- 
duced there seemed something wrong about the whole 
theory which the writer of this review never cleared: 
up until after his undergraduate days. 
From this it will be seen that if Prof. Planck’s 
treatise is no worse than many others on the same sub- 
ject, it is in some essential points no better. It is a 
book which will be read with great interest -by the 
physicist, generally in conjunction with other books. 
on the same subject, but it is scarcely the book for an 
engineer to refer to for information on the nature of 
“entropy.” G. H. B. 
GEOGRAPHY AS A SCIENCE. 
The Teaching of Geography. By Prof. J. W. Gregory, 
D.Sc., F.R.S. (Melbourne and London: Whit- 
combe and Tombs, Ltd.) 
The Austral Geographies. Classes ii., iii., iv., v. and 
vi. Same Author and Publishers. 
ROF. J. W. GREGORY is taking an active part 
in the promotion of sound geographical instruc- 
tion in the land of his adoption. In a lecture recently 
published he sets forth the scope of geography and 
the way in which it should be applied to education. 
In a series of school-books he shows practically how he: 
would do this for Australian children. 
For Prof. Gregory geography is not a science, but 
a branch of knowledge which may be taught scientifi- 
cally—its subject-matter is ‘‘ description drawn from 
observation; it is not a search for underlying prin- 
ciples, nor a discovery of ultimate causes.’”’ In 
applying this descriptive knowledge to education Prof. 
Gregory points out that descriptions must glide into 
explanations and awaken interests which cannot be 
satisfied without understanding this world of ours. 
The geographer must not hesitate to borrow from 
literature, history, or science that which will make 
his appeal to his pupil’s imaginations most stimu- 
lating. Prof. Gregory’s scheme, as developed in the 
““ Austral Geographies,’’ is to begin with a plan of 
table, school-room, school, &c., leading to a map, 
directions, seasons, clouds, rivers, land forms (in the 
first stage these are definitions), a brief description of 
Victoria, and a few lines about other Australian 
States and the continents. In each succeeding book 
some sections of physiography are discussed, and are 
followed by a description of (a) Australasia in Class 
iii., (b) the continents ending with Australia in Class 
iv., (c) the British Empire in Class v., (d) Europe, 
U.S.A., Japan, Pacific Archipelagoes, and world 
trade routes in Class vi. Both the physiographical 
and the geographical parts are so planned that each 
year more advanced conceptions, as well as greater 
details, are given. The books, in the hands of a good’ 
teacher who applies the hints given in Prof. Gregory’s. 
lecture, should yield useful results, and teach the pupil 
much about land forms and climate and descriptive- 
topography. 
