May 5, 1870 | 
NATURE : 
prevented from uniting. But in doing so he must bear in mind 
that the waters of Ulenge, made by him to be the recipient of 
the Kassavi, as well as of the Chambeze, are said by some of 
Livingstone’s native informants to flow N.N.W. into Chowambe 
or Albert Nyanza, and that for the westerly extension of this 
body of water we have the authority of Baker. 
On the subject of the Congo I have little to say. If it should 
be found that north of the fifth parallel of south latitude the 
basin of that river requires to be carried further eastward than 
the 20th meridian, I see no objection to it. Only I am 
bound to remark, that I do not consider there is any war- 
rant for representing the Congo as a river having a low 
alluvial valley extending some 500 or 600 miles inland. I 
have not examined the subject of this river very closely, but my im- 
pression is, that, like the other rivers of the West Coast of Africa 
south of the equator, the rise of the level of its bed is rapid, and 
that it becomes considerable within a short distance from the 
ocean ; so that there would not be sufficient fall for the waters of 
Ulenge in 27°30’ E. long. to join it. And, further, I cannot but 
entertain the opinion that the volume of water of the Congo has 
been greatly over estimated ; in support of which opinion I will 
cite the following passages, in pages 147 and 148, of Captain 
Tuckey’s Narrative :—‘‘ At the further end of the banza we 
unexpectedly saw the fall almost under our feet, and were not 
less surprised than disappointed at finding, instead of a second 
Niagara, which the description of the natives and_ their 
horror of it had given us reason to expect, @ comparative 
brook bubbling over its stony bed .... The principal idea 
that the fall creates is, that the quantity of water which flows 
over it isby no means equal to the volume of the river below it ; 
and yet, as we know there is not at this season a tributary 
stream sufficient to turn a mill below the fall, we can hardly 
account for this volume, unless we suppose, as Dr. Smith sug- 
gests, the existence of subterranean communications or caverns 
filled with water.” Doesthis look like the lower course of the 
supposed second great river of Africa, with a basin of which the 
area is estimated to measure $00,000 square miles ? 
On a reconsideration, then, of the whole subject, I see no 
reason whatever to go from the opinion I have expressed, that 
the rivers Kassavi and Chambeze unite to form the upper course 
of the Albert Nyanza; that is to say, the main stream of the 
Nile ; and as the former of those two rivers has the more direct 
course, and its source is the most remote of all, it is entitled to 
the honour which I claim for it, of being the hitherto undis- 
covered head of the great river of Egypt. 
Bekesbourne, April 22 CHARLES BEKE 
P.S., April 26.—Since the foregoing was written, Mr. Keith 
Johnston has obligingly sent me a copy of his ‘‘ Map of the Lake 
Region of Eastern Africa,” with notes, just published, in which I 
find a categorical answer to my question respecting the western 
limits of the Upper Nile Basin. He traces them as coming from 
the south of Lake Liemba and its feeders, and running close 
along the western side of Tanganyika as far as its northern end, 
where he gives them a curve to the westward not more than 
sufficient to include the south-western of the Albert Nyanza, 
and thence continuing along the high mountains on the west side 
of that body of water, the westerly extension of which, reported 
by Baker, he ignores entirely. To these views I need not 
reiterate my objections. Cabs 
Why is the Sky Blue? 
CAN any of your readers inform me why the sky is blue? Is 
it that the predominant colour of sunlight being orange, the 
regions devoid of sunlight appear of the complementary colour? 
If so, the planets of Sirius and Vega would have a black sky, 
those of Betelgeux a green sky, while those of the double stars 
would have different coloured skies at different times, according 
to their position with respect to their two luminaries. Or 
again, is the blueness merely the colour of our atmosphere, as 
Prof. Tyndall’s experiments have led some to believe? In favour 
of the former explanation, is the fact that the maximum intensity 
of the light of the solar spectrum is in the orange, and indeed 
that the sun /ooks orange, and if we close our eyes after gazing a 
moment at him when high up in the sky, we see a blue image. 
When the sun is low, his colour changes from orange to red, and 
this would explain the green tintsso often seen in the cloudless parts 
of the sky at sunset. Possibly Mr. Glaisher, who has seen the 
sky through a thinner stratum of air than most of us, could help 
us to a solution. ALN, 
Hampstead, April 24 
Curious Facts in Molecular Physics 
Some of the phenomena of photography present features of a 
very curious nature, yet seem to be very little knowr. to philoso- 
phers who devote their time to researches in molecular physics. 
For instance, when a glass plate coated with collodion con- 
taining an iodide—say iodide of cadmium—is dipped into a 
“bath” solution of nitrate of silver, strength twenty-five grains 
to the ounce, in from three to four minutes a good dense preci- 
pitate of yellow iodide of silver is formed in the spongy collodion 
film, and the plate is ready for photographic use. But, let a 
plate be covered with collodion containing bromide of cadmium, 
(ten grains to the ounce) instead of iodide of cadmium, an 
immersion of ten or fifteen minutes is necessary to obtain a good 
film of bromide of silver, though the collodion skin upon the 
glass surface is only of the same thickness as in the former in- 
stance, and not only is this much longer immersion necessary, but 
the nitrate of silver solution must be increased in strength to 
about sixty grains to the ounce to get the best results. When 
the strength of the nitrate of silver is only twenty-five grains to 
the ounce, the bromide of silver forms more on the surface of the 
collodion than within it, and sometimes breaks away in scales 
from the collodion, and falls to the bottom of the bath. 
Lastly, let chloride of cadmium be used instead of the bromide in 
the collodion, the strength of the nitrate of silver must be increased 
to about one hundred grains to the ounce of water, and an 
immersion of thirty or forty-five minutes is necessary to get a 
good photographic precipitate of chloride of silver. In this case, 
when a weak nitrate of silver solution is used, an uneven preci- 
pitate is formed upon the plate, and the tendency to burst out ot 
film in scales is seen as in the former instance. 
The three kinds of films just described vary in their photo- 
graphic properties. The iodide of silver film requires the 
shortest exposure in the camera to produce a good picture, the 
bromide of silver film requires a longer exposure, and the chloride 
of silver film requires the most prolonged exposure to light of all. 
Again, the iodide of silver film is more liable than the others 
to spots and markings, when there are particles of dust or 
other impurities on the glass plate or in the solution used ; 
bromide of silver is not nearly so delicately sensitive to such 
disturbing influences; the chloride of silver film is even less 
sensitive in this respect than the bromide surface. 
The reason of the differences of time of exposure just men- 
tioned may possibly be accounted for on the supposition that 
chlorine binds itself to silver with more force than is exerted by 
bromine, and that the atom of bromine clings to the atom of 
silver with more tenacity than iodine clings to the metal. Hence 
the waves of light have more work to do in beating chlorine 
from silver than in beating iodine from silver. One very beau- 
tiful experiment, first made by Mr. M. Carey Lea, of Philadel- 
phia, tends to prove that light will widen the distance between 
the hypothetical swinging atoms of iodine and silver, and that in 
darkness the atoms, with their attraction for each other thus par- 
tially overcome, will gradually fall together again. He prepared 
a film of absolutely pure dry iodide of silver, upon a glass plate, 
which film in the process of preparation had not been allowed 
to come into contact with the slightest trace of organic matter, 
in the washing water, or by any other means. On exposing 
such a film to light under a negative, and then applying what is 
known as the ‘‘alkaline developer,” a picture came out ; but if 
instead of developing the picture, the exposed plate were allowed 
to rest a day or two in the dark, the latent image died out, the 
film, so far as is known, returned to its primitive condition, and 
on expostue under another negative, a picture from it could be 
brought out, with no traceof the image impressed for a time 
through the first negative. The alkaline developer seems to 
‘drink up” the iodine where its cohesion to the silver is 
loosened, thereby leaving a dark deposit of metallic silver, but 
where the light has not somewhat beaten the atoms asunder, the 
developer has no action, unless its strength be increased till it 
blackens the whole plate, whether the light has acted upon the 
film or not. The alkaline developer consists of a weak solution 
of pyrogallic acid, rendered alkaline by the addition of a few 
drops of carbonate of soda, 
This is but one instance among many of the facilities offered by 
photographic phenomena to those who are trying to peer into 
the penumbral philosophical region of molecular physics. 
WILLIAM H. HARRISON 
