314 
stirs in the matter. We have already a permanent inter- 
national organisation, which, if its functions were to be 
extended so as to include the measurement of time as 
well as of space, might consider the question without any 
large increase of its numbers ; we refer to the Commission 
du Metre, which already largely consists of astronomers. 
We point this out to show that there are no real difficulties 
in the way of a preliminary consideration of the matter— 
nay more, that there are ways of reducing the difficulties 
by the choice of a body which already exists, and exists 
too in France, where the idea of a neutral meridian still 
lingers. We believe that a serious practical discussion 
would show that the idea which lies at the root of the 
contention for a neutral meridian is as impossible now as 
it has been in the past with regard to other international- 
isations, such as Roman letters and Arabic numerals. If 
this were so, a great step would have been gained. 
The writer in Sczezce, however, does not propose that 
the Governments should be urged forward by any idea of 
saving their share of the sum we have already mentioned, 
and quite rightly. The idea is thrown out that it should 
be spent in an international mountain observatory, where 
in turns astronomers of all countries could carry out their 
special researches. The idea is a most admirable one, 
and will commend itself to all who know how years, and 
we may even say centuries, are being lost by heart- 
breaking attempts to do at a low level important work 
which is really only practicable at a high elevation. 
PROFESSOR TAITT?S “PROPERTIES OF 
MATTER” 
Properties of Matter. By Prof. Tait. 
Black, 1885.) 
HE subject of this excellent little book includes the 
mechanical properties of matter, and much that is 
usually treated under the head of Chemical Physics, 
such as Diffusion and Capillarity. It might be difficult 
to give a reason why the electric and thermal conducti- 
vities of mercury, for example, should not be included 
among its properties as much as its density and its capill- 
arity; but the distinction is convenient, and to some 
extent sanctioned by usage. 
In the introductory chapters the author expounds some 
rather peculiar views with perhaps more insistance than 
is desirable inan elementary work. The word “force ” is 
introduced apologetically,and with the explanation that “as 
it does not denote either matter or energy it is not a term 
for anything objective.” No one will dispute the immense 
importance of the property of conservation, but the author 
appears to me to press his view too far. As Dr. Lodge 
has already pointed out, if conservation is to be the test 
of existence, Prof. Tait himself does not exist. I forbear 
from speculating what Dr. Lodge will say when he reads 
on p. Ir that “not to have its price is conclusive agains 
objectivity.” ; 
Chapters IV. to VII. form an elementary treatise on 
Mechanics, in which even the learned reader will find 
much that is interesting in the way of acute remark and 
illustration. Under the head of Gravitation are con- 
sidered Kepler’s laws, the experimental methods for de- 
termining the constant of gravitation (“the mean density 
of the earth”), and the attempts (such as Le Sage’s) 
(Edinburgh : 
NATURE 
[August 6, 1885 
which have been made to explain the origin of gravi- 
tation. 
The succeeding chapters on the deformation of solids 
and the compression of solids, liquids, and gases, are 
perhaps the most valuable part of the work, and will 
convey a much needed precision of ideas to many students 
of physics whose want of mathematical training deters 
them from consulting the rather formidable writings of 
the original workers in this field. The connection of 
Young’s modulus of elasticity, applicable to a rod subject 
to purely longitudinal pull or push, with the more funda- 
mental elastic constants expressing the behaviour of the 
body under hydrostatic pressure and pure shearing stress 
respectively, is demonstrated in full. Prof, Tait remarks 
that “ Young’s treatment of the subject of elasticity is 
one of the few really imperfect portions of his great work 
(‘Lectures on Natural Philosophy.’). He gives the value 
of his modulus for water, mercury, air, &c.!” A defi- 
ciency of explanation must be admitted, but I am not 
sure that Young’s ideas were really confused. The 
modulus for solids corresponds to a condition of no lateral 
force, that for liquids to no lateral extension. The dis- 
tinction should certainly have been pointed out; but the 
moduli are really comparable in respect of very important 
effects, which Young probably had in his mind—viz. the 
propagation of sound along a bar of the solid in one case, 
and in the other through a fluid, whether unlimited or 
contained in an unyielding tube. 
As a great admirer of Dr. Young’s work, I cannot 
resist adding that if in some respects his treatment of 
elasticity is defective, in others it is in advance of many 
modern writings. Witness the following passage :— 
“There is, however, a limit beyond which the velocity of 
a body striking another cannot be increased without over- 
coming its resilience, and breaking it, however small the 
bulk of the first body may be, and this limit depends 
upon the inertia of the parts of the second body, which 
must not be disregarded, when they are impelled with a 
considerable velocity. For it is demonstrable that there 
is a certain velocity, dependent on the nature of a sub- 
stance, with which the effect of any impulse or pressure is 
transmitted through it; a certain portion of time, which 
is shorter, according as the body is more elastic, being 
required for the propagation of the force through any part 
of it; and if the actual velocity of any impulse be in a 
greater proportion to the velocity than the extension or 
compression, of which the substance is capable, is to its 
whole length, it is obvious that a separation must be 
produced, since no parts can be extended or compressed 
which are not yet affected by the impulse, and the length 
of the portion affected at any instant is not sufficient to 
allow the required extension or compression.” 
The theory of “ bending” and of “torsion” are discussed 
in Chapter XI. When the section of the rod deviates 
from the circular form, the torsional problem becomes 
rather complicated ; but a statement is given of some of 
the interesting results of Saint Venant’s investigations. 
In his treatment of the compression of solids and liquids, 
the author is able to make valuable contributions derived 
from his own experimental work. 
In the chapter on “ gases,” a long extract is given from 
Boyle’s “ Defence of the Doctrine Touching the Spring 
and Weight of the Air,” in order to show how completely 
