82 
as he tells us, with the importance of this investigation, 
and then and there devoted the efforts of his whole life 
to carry it out. Accordingly, in 1832 he presented a very 
extensive work to the Academy of Sciences, a report on 
which, by MM, Gay-Lussac and Becquerel, is annexed to 
the volume before us. 
The ideas developed in this work were derived from 
two sources—crystallography and chemistry. Haiiy had 
endeavoured to explain the regularity of the forms of 
crystals by regarding them as built up of molecules, the 
form of each molecule being similar to that of the simplest 
solid which can be obtained from the crystal by cleavage. 
The absolute size of these integrant molecules, as they 
were} called, was left, of course, indeterminate. 
Wollaston preferred to regard the arrangement of the 
ultimate molecules in a crystal as resulting, not from 
their accurately fitting one another as bricks doin a 
wall, but from their tendency to crowd together into the 
smallest possible volume as peas doin a bag. The form of 
the molecules, according to Wollaston, was not polygonal, 
but spherical or ellipsoidal. 
At this point Ampére took up the theory. His atoms 
were no longer either closely fitted together, or even 
touching one another at isolated points, but were main- 
tained by attractive and repulsive forces at distances 
exceedingly great compared with their own dimensions. 
The forms of the atoms themselves were therefore no 
longer considered as of any importance ; the molecules, 
formed of groups of these atoms, were represented 
in diagrams as systems of points; and the explanation of 
the geometrical properties of the substance in the crys- 
talline form was sought in the geometrical arrangement 
of these atoms. 
The proportions in which the atoms of different kinds 
were to be represented in the molecules were determined 
in accordance with the atomic theory of chemistry, esta- 
blished by Dalton, and the absolute number of such 
atoms in the molecule was arranged so as to satisfy the 
law of gases, recently discovered by Gay Lussac, which 
asserts that the mass of every gaseous molecule is pro- 
portional to the specific gravity of the gas at the standard 
pressure and temperature. 
The theory of M. Gaudin may be regarded as founded 
upon that of Ampére, with certain modifications. Instead 
of assuming with Ampére, that when two molecules com- 
bine, the form of the compound molecule is the resultant 
of the forms of its components, he supposes that the atoms 
of the combining molecules are all thrown into a common 
stock, to be arranged, according to some principle of 
equilibrium or of symmetry, in a form having no neces- 
sary relation to the forms of the combining molecules. 
In the work before us M. Gaudin gives us, as the 
result of his long-continued meditation on compound mole- 
cules, actual diagrams of their supposed forms, showing 
not only their outward shape, but the arrangement of the 
molecules in each of the layers in which they are dis- 
posed. The ingenuity with which he has arranged in a 
symmetrical manner groups sometimes amounting to 
279 atoms must be seen in order to be appreciated. But 
the merit of these arrangements as an explanation of 
facts must be tested, first by a careful comparison of 
those forms whose chemical relations are similar, and then 
by a comparison of each diagram with the crystallo- 
NATURE 
| May 29, 1873 
graphic properties of the substance which it is supposed 
to represent. The author has, to the best of his ability, 
applied both these tests, and we shall not here pro- 
nounce sentence upon the result of such an examina- 
tion. 
We may remark, however, thatwM. Gaudin began his 
labours forty years ago, using the methods of investiga- 
tion which we have briefly described. Since that time 
he has been patiently arranging his atoms by rows and 
groups, and representing them in models by means of 
pearls of various hues. He has shown no symptom of 
being attracted towards any of those newer paths which 
Joule, Clausius, and others have opened up into the 
higher regions of kinetic molecular science. Indeed we 
not only find no mention of the names of any of these 
men, but we look in vain for any indication of a desire to 
pass beyond mere geometrical arrangements of atoms, 
and to inquire into the forces with which they act on 
each other or the motions with which they are agitated. 
There is a chapter, indeed, entitled “ Hémiédrie et pou- 
voir rotatoire,” but though there is something about 
hemihedry, there is nothing there at all about the power 
of rotating the plane of polarisation of light. The only 
piece of dynamics in the’ book is the theory of capillary 
phenomena at p. 197, about which the less we say the 
better. 
M. Gaudin is favourably known to science as an adept 
in the management of the blow-pipe. He has melted the 
most refractory bodies, and compounded the oriental ruby 
from its elements. He has not only established the 
chemical formula of silica and modelled its molecule, but 
he has fused quartz into beads, and drawn it into threads 
like spun glass. 
His experimental researches have displayed great in- 
genuity and manipulative skill, but have often been 
brought to an untimely end for want of funds to carry 
them on. In his theoretical speculations he has been 
guided by geometry alone, without the powerful if not 
absolutely necessary aid of dynamics ; and in the great 
work of his life he has met with very little encouragement, 
and has been sustained only by his conviction of the 
scientific value of the treasure of which he is in search. 
OUR BOOK SHELF 
A Manual of Photography. By George Dawson, M.A. 
Eighth edition. (J. and A, Churchill.) 
THE new edition of this excellent manual of photography, 
which is founded on and incorporates as much of Hard- 
wick’s “ Photographic Chemistry” as is valuable in the 
present further advanced stage of the art, retains its 
position as the best work on the subject for amateurs, as 
well as professionals. The many new methods and 
materials which are so frequently being introduced, make 
it essential that any book professing to keep up to the 
times must be frequently revised, and Dr. Dawson has in 
this work presented the subject in its most advanced posi- 
tion. The earlier chapters, after giving a short sketch of 
the history of photography, enter into a description of the 
most important experiments, the expansion of which make 
up the subject itself. This is followed by a review of 
the various lenses required for the many different purposes 
to which photography is applied, and their peculiarities 
are rendered more evident by the introduction of very 
clear diagrams of them in section. After a full descrip- 
tion of the various points connected with the wet-plate 
as 
