56 
POPULAR SCIEIi^OE ^EWS. 
[Aprii,, 1891. 
seem as bland and harmless as the pure air of 
ocean or mountain does to us. 
Next to air, water is the substance which is of 
the most importance to living organisms. It is a 
necessity of life, and forms the greater portion of 
the substance of all animals and plants, but how 
perfectly adapted it is to its purpose. Although 
dissolving to a greater or less extent most chem- 
ical compounds, its action on animal or vegetal : 
tissues is practically nothing ; it is a bland, no . 
and chemically inactive liquid, with phys. 
'properties which fit it most admirably to eni 
into the structure and sustain the vital processes 
of animals and plants, which no other liquid 
known to us is capable of doing. Who can doubt 
that the countless generations of living beings on 
our globe have been so modified and adapted in 
substance, form, and function, as to be in pei-fect 
harmony, chemically and physically, with this 
omnipresent liquid with which they are necessarily 
brought into such intimate connection. Animals 
and plants were made to suit the water, not the 
water to suit the plants. A remarkable instance of 
this is found in the distinction between salt and 
fresh water animals and plants, each of which 
lives and flourishes under conditions which would 
be fatal to the other. 
Coming to the mineral constituents of the earth, 
some very striking examples may be found. The 
salts of calcium are everywhere present, and are 
entirely hannless, while the elements barium and 
strontium are extremely rare, and, at the same 
time, poisonous. Nitrogen gas is present in the 
air, and the phosphates of lime and magnesia 
aie widely distributed. All of these substances 
are not only harmless, but necessary to the animal 
economy, while the closely allied arsenic, antimony, 
and bismuth are unwholesome and poisonous, but 
are only found in a few scattered localities. Chlo- 
rine, in the form of common salt, is met with 
everywhere, but the salts of its rare chemical 
brethren, fluorine, iodine, and bromine, are, except 
In medicinal doses, highly injurious to health. If 
we accept the modern theorj' of the evolution of 
the elements from one primitive form of matter, 
we can see no reason why the water of the ocean 
might not have contained as much iodide of potas- 
sium or lithium as it does of chloride of sodium ; 
that the coral islands, marble quarries, and lime- 
stone strata might not have been formed of the 
poisonous carbonate of barium, or even that the 
atmospheric nitrogen or the iihosphates of the 
soil might not have been replaced by compounds 
of arsenic or antimony. Under such conditions, 
life as we know it could not exist ; but there seems 
to be no reason whj' organisms might not have 
appeared that could tolerate arsenic, antimony, 
barium, or iodine, as well as they do the present 
common elements so closely allied to them. 
While the above considerations are suggested 
by the theories of development, and hannony 
with the environment, it must be confessed that 
they are largely of a speculative nature. We can 
study existing forms of life, their structure, com- 
position, actions, and functions, but we do "not 
know the nature, origin, or destiny of that life. 
We are studying a chain, both ends of which 
stretch into infinity, and whether we can ever 
attain and comprehend these limits either here or 
hereafter, time only can show. We can only strive 
after "more light,"' trusting to the future to make 
those things clear to us which are now veiled in 
an obscurity' which appears to be beyond the 
power of the human intellect to penetrate. 
The Loudon Electric .Supply Corporation has 
succeeded in transmitting a 10,000-volt current. 
PIIOTOGKAPHY IN COLORS. 
A NOTEWORTHY and remarkable step has been 
made towards the solution of the most difficult 
problem of photographing in colors, by M. Lii'i'- 
MANN, of Paris, who has actuallj' succeeded in 
reproducing upon an ordinary gelatine dry plate 
an image of the solar spectrum in its natural col- 
ors. Like many other great discoveries it is of 
■'^.tonishing simplicity, and, with the aid of the 
companying illustrations, (reproduced from La 
,tture), the process can easily be understood by 
any one. 
The sensitive plate used in the process must be 
prepared so that the film of gelatine emulsion 
shall be very thin and smooth, and entirely free 
from the slightly granular texture of the ordmary 
films. The sensitive film should be sinipl)' opales- 
cent, and not of the creamy opacity of the films 
used in ordinary photography. Otherwise the 
plate does not difTer from those in general use, 
and the emulsion consists of the usual bromide of 
silver. 
In Fig. 1 (1), F is a plate of ordinary glass; C 
is a piece of hard rubber shaped like a horse-shoe 
magnet, or the letter U; G is the photographic 
plate, the sensitive surface being turned inwards. 
The whole arrangement is fastened together with 
clamps, and the cell thus formed is filled with 
mercury (M). 
sounded together produce the alternations of 
sound and silence known as beats. Now just 
where the layers of light are produced in the film 
the bromide of silver is acted upon, and, when 
developed, a layer of metallic silver is formed. 
As the wave-length of each color is different, a 
different number of layers of silver will be depos- 
ited in the film for each color acting upon it. 
Thus if the gelatine film is 1-20 of a millimeter 
(1-500 of an inch) in thickness, the red rays will 
form 156 layers of silver, the yellow 200, and the 
violet 250, with intermediate numbers for the 
other shades. 
Fig. 1. 
All being in readiness, an image of the solar 
spectrum is thrown upon the sensitive plate, and 
the plate exposed to its influence for a period 
varying from thirty minutes to two hours. 
The plate Is then removed, and developed, 
fixed, and dried in the usual manner, when 
it is found that a perfect image of the sisectrum 
— Fig. 1 (2) — has been reproduced upon the plate, 
all the colors beiug shown in their natural shades 
and brilliancy. Curiously enough, if the image is 
viewed by transmitted light, the colors are re- 
versed, being replaced by the complementary 
ones — that is, the green appears red, the red 
green, etc. 
The cause of this result is undoubtedly due to 
the principle of the interference of the light- 
waves, and the colors formed in the gelatine film 
are analogous to those formed in a soap-bubble. 
In Fig. 2 a nmch exaggerated section of the gela- 
tine film is given, which will aid in the compre- 
hension of the theory. 
A light-wave of any color — say red — passes 
through tlie glass (verre) and the sensitive film to 
the mercury, where it is reflected back again, 
meeting in the sensitive film other similar waves 
on their way to the reflecting mercurial surface. 
These incident and reflected waves interfere with 
each other, producing in the film alternate layers, 
as it were, of light {I) and darkness (o), just as 
two musical notes of nearly the same pitch when 
Fig. 2. 
Now when the plate is developed and finished it 
would seem that these microscopical layers of 
metallic silver will only reflect light-i-ays of a 
wave-length corresponding to the distance be- 
tween them, that is, rays of the same color as 
acted upon that portion of the film during ex- 
posure; or, in other words, the solar spectrum 
will be reproduced in its original colors and bril- 
liancy. 
This discovery of M. Lippmann is certainly a' 
most remarkable one, and, from a scientific point 
of view, of the greatest importance ; but we must 
admit that it seems as yet to be of little practical 
value. The actual photographic reproduction of. 
landscapes, groups, paintings, etc., in their natu- 
ral colors appears to be almost as far oft' as ever ; 
but a beginning has at least been made, and it is 
not impossible that this process may contain the 
germ of future discoveries which shall add the 
finishing touch to the photographic art, and ren- 
der it a perfect method of reproduction and pre- 
servation, not only of form but of color. 
[Special Correspondence of Popular Science News.] 
PARIS LETTER. 
The two topics of the day are the failure of the 
Koch anti-tubercular treatment, and Lippmann's 
researches on the photography of colors. On the 
first matter we may be allowed to pass quickly— 
not without expressing, however, our regrets that 
the haste of the Geiman Emperor has so seriously 
affected Koch"s fame. Koch, at all events, has 
done good work, and he is not to be blamed if ho 
has not discovered the remedy for tuberculosis ; 
his Emperor alone is to be blamed for having 
ol)liged him to speak before time — before his ex- 
periments were sufficiently advanced, and before 
the facts were ascertained. Such Is the general 
opinion in France concerning Koch, and it must 
certainly be the prevailing one in all circles. In 
the mean time numerous experimenters are work- 
ing out the matter, trying to get at some remedy. 
MM. Richet and Ilericourt are at present giving 
dog's blood serum to their patients. This serum 
is merely injected under the skin in a dose of 
one cubic centimeter every two or three days. It 
seems very probable that some one or other will 
find out some method by which tuberculosis may 
