NATURE 
483 
It is therefore with great pleasure that we welcome Dr. 
vy. Monckhoven’s “ Traité Général,” which seems to omit 
nothing in the way of recent additions to the number of 
photographic processes. 
The Doctor commences his book with an historical notice 
of the origin of the art, in the course of which the irre- 
pressible Egyptians make their appearance as having un- 
pated observed the effects of light on certain bodies ; 
but, unfortunately, they have not handed their experience 
in the matter to posterity. The Egyptians an Greeks, 
however; having been disposed of, we have sixteen pages 
of really very useful historical matter, so arranged that a 
short paragraph is devoted to each of the more important 
processes, and which is rendered still more valuable by 
‘humerous references to the original papers of the various 
investigators to whom we owe the art. 
_ The author then proceeds to give a sketch of the nature 
of light. Perhaps in a treatise of this sort one cannot 
expect a very comprehensive definition of such a subject. 
‘Still, however, something more satisfactory than the fol- 
lowing might have been expected .. . . “il existe néces- 
sairement entre le soleil et nous, un certain mode de com- 
munication dont nos yeux sont l’intermédiare ; c’est ce 
mode de communication qui constitue ce que l’on appelle 
Ta lumitre.” 
__ We then have a sketch of the chemical action of light, 
and a very good description of what a photographic 
laboratory ought to be, but, we fear, very rarely is. Con- 
siderable space is devoted to a description of the method 
of preparing the various substances required, including 
gun-cotton and collodion ; and here we may observe that 
Dr. van Monckhoven makes use of the old system of 
chemical equivalents obsolete in England, and very nearly 
‘so onthe Continent, a proceeding which is to be regretted 
in a work which is likely to remain for some time a 
standard book on its subject. We have noticed that 
photographers are singularly conservative on this point, 
for, to the best of our belief, there is not even nowa photo- 
graphic journal which makes use of the present atomic 
system of notation, a system which even nine years ago 
was largely used by chemists. A really admirable 
chapter on photographic optics succeeds that on photo- 
graphic chemistry; one soon perceives how much 
the art has owed to the lenses constructed on the 
formule of Dallmeyer and Steinheil, and to the credit 
of English opticians we find that in the summary the 
lenses of the former are stated to surpass all others. 
After dealing with cameras, printing frames, studios, 
and every other photographic requisite, the various pro- 
cesses are dealt with at length. Here we may note that 
specimens are given of two of the more recent mechanical 
printing processes, the “ Woodburytype,” and “ Helio- 
type.” Both are pigment methods, and so are not liable 
to the slow fading inevitable to the ordinary prints con- 
taining silver. Of them we can only say that while it is 
difficult to imagine that any process can surpass the 
former for artistic effect, the latter seems equally unsur- 
passable for any purpose requiring excessively minute and 
faithful reproduction of fine detail, such as is required 
in copying maps, prints, or diagrams. 
A specimen of what is modestly termed the “ retouche 
des clichés,” is also given, but here we feel that we are 
treading on dangerous ground, as a portrait of a lady is 
the subject. Suffice it to say, that the ge~eral effect of 
this process seems to be like that of the elixir vite, and 
to make the happy patient young and handsome again. 
We find considerable information also on photographic 
enamelling, and on the production of enlargements, where 
we observe that the heliostat and its use are described. 
The work is illustrated with 280 woodcuts, executed itt 
a style which is only found in foreign scientific works, and 
three specimen photographs are also given: In conclusion 
we must congratulate Dr. van Monckhoven on the pro- 
duction of so useful a book, hoping only that the chemical 
portion will be modernised and extended in future edi- 
tions. Why do not some of our many amateur or pro- 
fessional photographers devote some attention to the 
chemical nature of their art? Of the rationale of many 
of the reactions we know absolutely nothing, and of the 
others our knowledge is not much greater. Such a 
research would not be of theoretical value only, but would 
materially aid in the attainment of that perfect application 
of means to ends by which alone the best results either in 
art or science can be obtained, 
OUR BOOK SHELF 
The Relations of the Air to the Clothes we wear, the 
Houses we live in, and the Soil we dwell on. Three 
popular lectures delivered before the Albert Society at 
Dresden. By Dr. Max von Pettenkofer, Professor of 
Hygiene at the University of Munich, &c. Abridged 
and translated by Augustus Hess, M.D., Member of the 
Royal College of Physicians, London, &c. (London: 
Triibner and Co., 1873.) 
Dr. HEss has done well in translating these lectures by so 
great an authority on hygiene as Dr. Pettenkofer. Though 
the author does not believe that any knowledge of real value 
can be imparted by means of popular lectures, still they 
serve a good purpose in the way of “ scientific edification 
and elevation, which are to raise our minds and hearts 
and to affect us like listening to good music.” Though 
we in this country have perhaps less need to be instructed 
in the rules of hygiene than the mass of people on the 
Continent, still, it will be universally admitted that very 
few are acquainted with the principles which underlie 
healthy living, and still fewer can be at the trouble to put 
them into practice. In the little volume before us, which 
is well translated by Dr. Hess, the author expounds in an 
interesting and yet thoroughly scientific manner, the 
rationale of healthy living so far as our relations to the 
air are concerned, and shows the scientific principles on 
which we should choose our clothes both as to’ material 
and make, and which should guide us in building our 
houses. In the third lecture he speaks of the relations of 
the air to the soil, or on the Ground-air, and shows how 
much remains to be done before the principles of hygiene 
and their practical application can reach anything like 
perfection. The following extracts will give an idea of 
Dr. Pettenkofer’s method of treatment :— 
With regard to Clothing, the author says :—“‘ When 
exposed to Zuminous heat, the materials of our clothing 
do not show very great differences, but in experi- 
menting on shirtings of different co/ours, the following 
result was obtained :—When white absorbed 100, pale 
straw colour absorbed 102, dark yellow 140, light green 
155, dark green 168, Turkish red 165, light blue 198, 
black 208. In the shade these differences nearly vanish. 
Krieger, in experimenting on tin cylinders filled with 
warm water, has found that a double tight covering by 
the same material does not retard the heat loss much 
more than a single one; but when the outer layer was 
