IN SAIN OG ia 
THURSDAY, SEPTEMBER 158, 1902. 
PROF, LORENTZ ON THE SCOPE OF 
PH VSIGSs 
‘Sichtbare und Unstichtbare Bewegungen. Vortrage 
gehalten von H. A. Lorentz.. German Translation 
by G. Siebert. Pp. 123. (Brunswick: Vieweg und 
Sohn, rgo2.) Price Mk. 3. 
HESE lectures, published under the title of “ Motions 
Visible and Invisible,” were delivered at Leyden last 
year to what would, perhaps, be calledin this country a 
philosophical society. The object was to present toa 
general audience an outline of the picture of the external 
world which modern physics is constructing within its 
own province, without going so far into detail as to lose 
the general outlook in the interests or difficulties of 
special departments of knowledge. The keynote of the 
book, as the title implies, is that all physical explanation 
rests ultimately on dynamics; and accordingly the first 
three out of seven lectures are devoted to an exposition 
of the gist of the dynamics of rectilinear, curved and 
vibratory motions. Not much originality is to be ex- 
pected in such well-worn topics, but the freshness of some 
of the illustrations reveals the calibre of the author. 
Such, for instance, is the estimation, from Hertz’s 
analytical theory, of the intensity of the pressures set up 
in the ordinary impact of small balls ; when a ball of 800 
grammes falls on another such from a height of 12 cm., the 
total pressure between them mounts up to the order of 600 
kilogrammes. The results developed are applied to a 
brief review of the cardinal phenomena of sound and 
light, and to the explanation of gaseous pressure and 
temperature on the kinetic theory. Of personal interest 
is the remark, on the basis of Michelson’s having found 
that the breadth of a spectral line of a gas approaches a 
definite limit as the gas is rarefied, that, according to the 
writer’s belief, we have here as direct evidence for the 
motion of the gas-molecules as spectral displacements 
give for the motions of the heavenly bodies. We 
naturally turn to the sixth lecture, on electrical pheno- 
mena, in the elucidation of which the author occupies 
sO prominent a position. At the very beginning, the 
conception is introduced that the ultimate atoms of 
matter involve positive and negative electrons, which 
interact through the connection between them that is 
afforded by the ether, and thus originate all electric 
phenomena. In the terms of this theory, the main 
electric phenomena are described, including such as 
kathode rays, though not much attempt is made to in- 
dicate the links of the chain of deduction that bind 
together the various departments of electrodynamics ; 
finally, a rather more detailed sketch of the significance of 
the Zeeman effect is given. Thus one is led to see how 
it comes about that it is now a main effort of physicists 
to ascribe to these electrons a function in many types of 
phenomena, and to realise that there is much evidence 
for the view that they form a very prominent element in 
the constitution of matter. So far as is known, they are 
the sole links between ponderable matter and the ether. 
On them, too, a hope entertained by many is largely based, 
that gravitation and molecular forces will also turn out 
e processes in that medium. The last lecture is an 
NO. 1716, Vor. 66] 
489 
exposition of the principle of energy. The writer expresses 
his opinion that nothing short of careful experimental 
investigation can assure us of the impossibility of the fer- 
petuum mobile, which forms the foundation of the doctrine 
of energy. The ramifications of that principle are illus- 
trated from the action of adynamo. The principle thus 
forms a clue that connects the most varied phenomena, 
even when we are ignorant of the mechanisms which 
produce them ; we find it an unfailing guide in tracing 
relations, with a certainty to which no electron-theory or 
other special theory can pretend. But it is of limited 
scope ; the independence e.g. of gaseous viscosity and 
density, or the effect of motion on spectra, can only be 
elucidated by penetrating beneath the surface of things 
and resolving the phenomena into their elements. 
To some people, the other side of the question above 
referred to as to the nature of the negation of the Aer- 
petuum mobile will doubtless appeal more strongly. A 
fundamental principle in nature of simple and universal 
type can hardly be based exclusively on the empirical 
ground of experimental verification ; indeed, this very 
principle is constantly being applied without any hesita- 
tion to considerations so delicate as to be beyond the 
reach of present experimental confirmation except as 
regards their remote results. Not the least interesting 
and promising topic in abstract physical science is the 
origin and scope of the great natural laws which transcend 
all distinctions between different kinds of material, and 
form the frame into which the science is gradually built. 
The sketch here given will show that Prof. Lorentz’s 
little book forms a welcome addition to the expositions, 
popular yet exact, which we possess of the current pro- 
gress of physical science, even in this country which 
produces more than its share of such concrete and 
illustrative presentations; all the more so as the 
trend of scientific method abroad now seems to seek 
greater security in the purely abstract exposition of re- 
lations, between entities which appear only as pure 
unknowns by means of mathematical symbols, from the 
fear that any full-bodied analogy traced between them 
and other things better known may, by reason of its 
limitations, encourage views that may ultimately prove 
false. J. 1 
CELTIC MYTHOLOGY. 
Cuchulain of Muirthemne: the Story of the Men of the 
Red Branch of Ulster. Arranged and put into English 
by Lady Gregory. With a Preface by W. B. Yeats. 
*Pp. xvii + 360. (London: John Murray, 1902.) 
Price 6s. 
Ee) before the appearance of this attractive volume 
the general reader had no valid excuse for not 
knowing something of the story of Cuchulain and the 
other heroes of Ulster; henceforward he will be still 
more inexcusable if he persists in ignorance. No one who 
cares at all for early literatures can fail to enjoy this 
version of these old Irish tales, so unique of their kind 
and so full of varied interest. The serious student, no 
doubt, will prefer to go direct to the originals, or to close 
translations like those collected in Miss Hull’s “ Cuchullin 
Saga”; but Lady Gregory’s work is so different in aim 
from Miss Hull’s that there is ample room for both. She 
We 
