May 6, 1y15| 
NATURE 
259 


very different thing from maintaining that it will 
explain everything ; one might just as well search 
for a universal pill which will cure all the ills to 
which human flesh is heir. NVI Skt 
The Chemistry of Paints and Painting. By Sir 
A. Church. Fourth edition. Pp. ix+ 387. 
(London: Seeley, Service and Co., Ltd., 1915.) 
Price 7s. 6d. net. 
Tue facts that this is a fourth edition, and that 
the author has been before the world for more 
than fifty years as a student of the subject on 
which he writes, are sufhcient reasons for wel- 
coming it with respect. But the volume itself 
fully justifies its existence, and it is difficult, if 
not impossible, to suggest any change in it that 
would better fit it to serve the purpose for which 
it was originally issued. The temptation to use a 
material that facilitates or immediately improves 
one’s work without due regard to its lasting quali- 
ties is always very strong, and especially is this 
the case with those who are so absorbed in the 
study and practice of pictorial art, that the scien- 
tific aspect of their work becomes distasteful to 
them. But it is not right to accept ignorantly the 
opinion of the salesman, however honest he may 
be, or to trust to a few superficial experiments 
edition, 
made by one’s self or one’s friends. The author 
deals with painting grounds, vehicles, varnishes, 
pigments, methods, and results, giving just such 
details concerning them as the artist wishes, or 
ought to wish, to know. 
The previous edition of the treatise was trans- 
Jated into German and edited by Prof. Ostwald, 
who added a few paragraphs. The author has 
incorporated the substance of these in the present 
definitely indicating such paragraphs, 
and adding to their value by comments of his 
own. He gives, in short, the results of probably 
_all those who are known to have systematically 
tested pigments for permanency, and usefully, 
he 
and we think fairly, criticises the methods and 
results of these investigators. The preservation 
and restoration of pictures receive due attention, 
and throughout the volume the style of the author 
is such that a previous acquaintance with scientific 
principles and nomenclature is not necessary for 
the understanding of it. 
Machine-shop Practice. By W. J. Kaup. Pp. 
xii+199. Second edition. (New York: John 
Wiley and Sons, Inc.; London: Chapman and 
Hall, Ltd., 1914.) Price 5s. 6d. net. 
In this little book will be found descriptions of the 
hhand- and machine-tools employed in an up-to- 
date workshop, together with clear instructions 
for their use. The author has desired to lead 
the pupil in the shop to think, and not merely to 
do. For this reason the why of each step or 
operation is emphasised as much as the how. 
The function of college workshops is to fami- 
liarise students with the working properties of 
the materials employed and with the tools in 
general use. Such information cannot be ade- 

the verbal explanations of the instructor. It is 
not easy to make other than mental notes in the 
course of workshop practice, and it is often in- 
convenient to pull a machine, or part of a machine, 
to pieces for the purpose of explanation. 
The book before us will be found to be very 
helpful in such matters. Probably the most note- 
worthy feature in it is the clearness of the illus- 
trations. Where most books of the kind contain 
merely half-tone illustrations of machines (gener- 
ally from makers’ catalogues), the author has 
given perspective line drawings, and has named 
the parts clearly on the drawings. These draw- 
ings will be found to be of much value, even when 
the machine installed in the students’ workshop 
differs in detail from that in the book. Nomen- 
clature is not so serious a barrier in this volume 
as in some American books. We can heartily 
commend this book to workshop instructors. 

LETTERS TO THE EDITOR. 
[The Editor does not hold himself responsible for 
Opinions expressed by his correspondents. Neither 
can he undertake to return, or to correspond with 
the writers of, rejected manuscripts intended for 
this or any other part of Nature. No notice is 
taken of anonymous communications.] 
The Age of the Earth. 
WHILE reading through Dr. F. A, Lindemann’s 
defence of Lord Kelvin’s estimate of the age of the 
earth, I was reminded that in spite of the sympathetic 
spirit in which he always entered into any discussion, 
he would never allow the least doubt to be thrown on 
the correctness of his estimate of the earth’s age. 
Yet it is open to several objections: he assumed that 
the solidified crust, as it was being formed, would sink 
toward the centre of the earth until it was solid 
throughout, whereas there can be no doubt about its 
core being so heavy that the crust material could not 
possibly sink. He also assumed a diminishing rate 
of cooling, whereas the greater portion of the earth’s 
surface is covered by water the bottom temperature 
of which must have been practically constant for 
millions of years. He also cuts down the temperature 
in the earth’s centre from 410,000° F., which it would 
be according to his assumption, to 7c00° F.; whereby 
the available heat is reduced enormously. However, 
if radio-active processes can supply the earth’s radia- 
tion losses there is no need to deal with the older 
question. 
I notice that Dr. F. A. Lindemann draws the con- 
clusion that the sun’s radiation just compensates the 
amount lost by the earth, but this is not correct. 
The earth’s loss is estimated from the known tem- 
perature gradient in the earth’s crust; it is a net loss 
over and above any possible interchange of heat with 
the sun. Then, also, Dr. Lindemann limits the 
earth’s age by the sun’s age, but amongst the several 
possible sources of its heat supply he does not even 
mention the heat-producing power of a meteoric bom- 
bardment. Yet, as I have shown in my work, ‘‘ Unity 
in Nature,” in the chapter on matter (pp. 85-92), it 
is not at all unlikely in comparatively recent 
time the sun may have passed through a large cloud 
of heavy meteoric matter. One effect of a compara- 
tively slight addition of heavy meteoric matter would 
have been to increase its density from, say, I-00 to 
1-38, and the other effect would have been to raise the 
quately obtained from any book, but a book may ' sun’s surface to such a high temperature that it 
be very useful for the purpose of supplementing | would have evaporated and formed an atmosphere 
NO. 2375, VOL. 95] 
