coffin 0 Sa eS 
ze ° ; 
May 22, 1873| 
NATURE 
67 
Kinetic Theory of Gases 
On page 300 of the second edition of Maxwell’s excellent 
little text-book on the ‘‘Theory of Heat,” it is stated, as a 
result- of the kinetic theory of gases therein set forth, that 
“gravity produces no effect in making the bottom of the column ” 
(of gas) ‘‘ hotter or colder than the top.” 
I cannot see how this result follows from the kinetic theory of 
gases. On the contrary, it seems obvious that thermal equi- 
librium can only subsist according to the kinetic theory, where 
the molecules encounter each other with equal average amounts 
of work or vis viva, and in order that this may be the case, the 
velocity of the molecules (and consequent temperature) of any 
upper layer must be less than that of the molecules in the layer 
next below; since, in order to encounter each other, the former 
must descend, and acquire velocity, while the latter must ascend 
and lose it. This would establish a diminution of temperature 
from the bottom tothe top of a column of air at the rate (in the 
absence of any counteracting cause) of 1° F. for 113 ft. of height, 
as can easily be verified from the fact that on account of the 
specific heat of air 1 lb. requires 183 foot-pounds to raise its tempe- 
rature 1° F. Radiation may diminish this and tend to produce 
equilibrium, but nevertheless it seems obvious from these two 
opposing tendencies a residual inequality of thermal condition 
would result, and that the top of a column would be cooler than 
the bottom. That this would be the case if the air were 
in general motion in the form of upward and downward 
currents, will not, I presume, be disputed ; and surely molecular is 
on the same footing. If the particles of air are moving in every 
direction with great absolute velocity, in what respect does this 
differ from air currents? In fact, all the particles which at any 
epoch of time are moving in any given direction constitute an 
air-current in that direction, mingled, it is true, with currents in 
otherdirections, but moving with accelerated velocity if descending, 
and with retarded velocity if ascending, and thus always tending 
to produce a diminution of temperature with height as a condition 
of gaseous thermal equilibrium. J. GUTHRIE 
Graaf Reinet, Cape Colony, April 2 
Kerguelen Cabbage 
I woutp like to know, through your paper, whether the 
naturalists of the Cha//enger have orders to attempt to collect 
the seeds of the Kerguelen Land cabbage (Pringlea antiscor- 
butica). It has often occurred to me that the attempt ought to 
be made to introduce this plant on the seashores of Northern 
Europe and America. Joun R. JoNEs 
Milwaukee, Wisconsin, U.S. April 14 
Yorkshire Terrier Story 
THE anecdote of the instinct of dogs given in the number of 
NATURE, May I, p. 6, is identical with one to be found in 
Bewick’s ‘‘ History of Quadrupeds,” p. 367, 1800, which he 
calls the well-known story of the ‘* Dog at St. Alban’s.” 
The same story precisely, with some dramatic embellishments 
and names, occurs in ‘‘ Bingley’s Animal Biography,” vol. 1, 
p- 223. A. 
Dorking 
BICHROMATE PHOTOGRAPHS 
A SINGULAR discovery has recently been made 
touching the action of light upon substances ren- 
dered sensitive by the bichromates of potash and 
ammonia, which threatens to revolutionise photographic 
printing altogether, at any rate so far as the production 
of permanent prints is concerned. The printing by means 
of silver salts in the ordinary way, which is still in vogue 
with nearly all portrait photographers, will always find 
application, by reason of the simplicity of the manipula- 
tions and the delicate and pleasing nature of the results, 
albeit all silver photographs enjoy the unenviable notoriety 
of being perishable. First of all, they lose their pristine 
brilliancy and freshness, then a sickly yellowness gives 
place to the glossy whites of the picture, and finally the 
deep bronze shadows become of a flat brownish tint, 
which grows weaker and weaker as time goes on. To 
secure permanent photographs, which shall possess all 
the beauty and detail exhibited by silver prints, has been 
for many years the aim of photographic experimenters, 
and it was not until Swan and Johnson had contributed 
their well-known improvements that the production of a 
delicate photograph in permanent pigments became at all 
possible. Mechanical photographic processes, where the 
pictures are printed off in a press, are still beset with 
-many difficulties of a practical nature, the most perfect 
of them—Woodburytype—requiring further elaboration 
before perfect prints of large dimensions can be secured. 
Pigment photographs, or carbon prints, as they are 
generally termed, require three elements for their produc- 
tion—a pigment (such as Indian-ink, lamp-black, or some 
such substance), gelatine, and bichromate of potash, or 
ammonia. A compound of these three substances is 
spread upon paper, and termed pigment or carbon tissue. 
This tissue is printed under a transparent negative in the 
sun, the light acting more or less energetically upon the 
sensitive pigment, and rendering it insoluble in parts, so 
that when it is immersed subsequently in warm water 
certain portions refuse to wash away, and these form the 
image ; during the exposure of the tissue to light, these 
parts have in fact become fixed by its action. This, as 
we all know, is what takes place in the formation of a 
carbon print. 
It has been found that the action of light upon a bi- 
chromate film is very different in its nature to the result 
produced by the sun upon iodide of silver. A film of 
pure iodide of silver, as Dr. Reissig and Mr. Carey Lea 
have abundantly shown, may be impressed with an image 
which will fade out altogether if the film is afterwards 
preserved for a sufficient time screened from light. In- 
deed it is possible to impress iodide of silver with an 
image, allow the same to fade away in darkness, and then 
impress the film with a second and different picture. The 
photographic image, therefore, on iodide of silver is of 
an evanescent nature, becoming weaker and weaker, and, 
if preserved for any time, ultimately fading away alto- 
gether. Now, with a photograph upon a bichromate film, 
the reverse is the case. If an impression of the slightest 
kind is produced upon a film of gelatine sensitised with 
bichromate, and put away in the dark, the action of the 
light still goes on, and progresses until the image has be- 
come a perfect and vigorous one. This continuation of 
tbe solar action has been turned to good account by 
carbon printers, who in winter time and busy moments 
have printed their photographs in darkness instead of 
light; that is to say, in lieu of exposing their sensitive 
tissue in the sun under a negative for hours and hours, 
they merely do so for a few minutes, the slight image 
thus impressed being allowed to gain in vigour sub- 
sequently by preservation for some time—half-a-day or 
so—in darkness, before development in warm water. In 
the ordinary way only half-a-dozen copies can be obtained 
from one negative during the day, if all of them are fully 
printed in the sun, whilst if only incipient prints are 
produced, a score of impressions may easily be secured. 
Within the last few days we have progressed a step 
further in carbon printing. M. Marion of Paris has dis- 
covered that if you take a bichromate image printed in 
the sun, and put it into contact with another bichromate 
surface, you produce upon the latter a similar impression. 
You can in fact take a carbon picture fresh from the 
frame and employ it as a printing block, from which any 
number of impressions are procurable. It is a most 
singular fact that a solarised surface should be capable of 
setting up an action upon another sensitive surface 
placed in contact with it. But soitis. The impression 
made by light upon a bichromate film is capable of 
transmission to another surface of like nature merely 
pressed against it. We have, as it were, stored up in 
the original print a quantity of sunlight which has been 
