ce 
. 
- only 0'000085 milligramme of organic ammonia. 
as explained above. 
2 ane 
29 
5 
of good air sometimes contains as little as 0°085 milligramme of 
“organic ammonia. 
A gramme of air—that is about 700 cubic centimetres—contains 
Expressing 
the organic ammonia in its equivalent of dry albumen we have in 
700 cubic centimetres of air 0°00085 milligramme of dry albu- 
men. ‘Translated into volume this 0'00085 milligramme of dry 
albumen will fall short of a cube, the face of which is 4,th milli- 
metre in diameter. 
Expressed in English measures, the result is, that rather more 
than one pint of average atmospheric air does not contain so 
much organic nitrogenous matter as corresponds to a cube of dry 
albumen of the 335th part of an inch in diameter. 
Now is this quantity adequate to admit of the existence 
of the immense multitudes of germs, the existence of which 
in atmospheric air is assumed by the vitalists ? 
J. ALFRED WANKLYN 
Colour of the Sky 
Wiru reference to Mr, Brett’s observations on the colour of 
sea and sky, I have one or two remarks to offer which I think 
may be of interest. Smokers have all noticed that the smoke 
from the end of a pipe or cigar is bluer than that which they 
puff from the mouth, and many may have wondered, as I did for 
a long time, what the reason of this could be. The contrast may 
be well seen on a bright sunny day. ‘This is, in fact, the sim- 
plest form of the experiment of the condensation of vapours 
causing them to pass through a fine blue to a white condition, 
which Professor Tyndall exhibited about two years ago, and 
which he employed to explain the blue colour of the sky, and the 
remarkable polarisation of its light. The finer state of division 
in the freshly-formed smoke gives it its bright blue colour, as 
does the finely divided aqueous vapour give to the blue sky; the 
smoke which has passed through the pipe-stem and mouth has 
become more condensed, and consequently gives a whiter cloud. 
The colour of water is, it appears, to a great degree dependent 
on the same cause as that of the sky. The investigations which 
Mr. Brett asks for have been already commenced. M. Soret, of 
Geneva, soon after Professor Tyndall’s researches on the cause 
of the blueness of the sky were published, made similar re- 
searches on the waters of the lake of Geneva, and found that the 
light from the water, when blue, was polarised as the light 
from the sky, and, so far, there was the probability of the 
cause of the colour being similar in the two cases. (See 
Comptes Rendus (Paris), April, 1869.) That particles in a fine 
state of division are the cause of the blueness of water as well 
as of sky is also made evident froma comparison of the waters 
of different lakes, seas, and rivers. There are two popular 
theories as to the cause of the colour of masses of water, which 
have very deep root, and yet must, it seems, be abandoned. 
One is that seas or lakes are blue by reflecting the blue sky. 
On this ground I have heard Mr. Brett’s picture in the Academy 
this year of a deep blue sea, severely criticised, because the 
sky, which he has painted with it, is not correspondingly blue, and 
could not furnish the sea’s tint by reflection. Mr. Brett is, how- 
ever, quite right in his fact, as many people know well enough ; 
and the criticism was misplaced, if the blue colour of a mass of 
water is dependent on the reflection of light from within water 
containing finely-divided particles—not from the surface only— 
The second popular theory which seems to 
be ill-founded is that the green colour of lakes, rivers, and seas 
is due to plants growing on the bottoms and giving their colour 
by reflection. The green colour is produced in the same way as 
the blue in all probability, and may be due to a yellowness of the 
water in some cases, but it is less easily accounted for than the 
ue colour. M. Sainte-Claire Deville is quoted by M. Soret 
as stating that waters which give a white residue on evaporation 
are b/ue, whilst those which give a yellow residue are greev. 
Reflection of the colour of the sky, and of the plant colour 
from the bottom, does no doubt produce colour of water in 
some cases, but it is only in shallow pools that the latter can 
have any effect, or through perfectly smooth surfaces that the 
former can be effective. Some cases of water-coloration which 
I have noted will be not out of place here:—1. Intensely 
blue on a bright day, with pale sky and large cumulous clouds, 
was the colour of water in reservcirs twenty feet deep at Plum- 
stead, depositing chalk (by means of which the water is softened 
according to a patent process). 2. Intensely blue (the bluest 
here noted)—Mediterranean at Marseilles. 3. Bright blue— 
Lake of Geneva. 4. Darker blue, tending to Indigo—sea near 
Guernsey ; also the Laacher See, in the Eifel. 5. Pale blue—sea 
near chalk cliffs, being at a little distance from the coast green or 
greyish. 6, Pale blue or greyish blue—the Rhone, the Mosel, 
glacier streams, &c. 7. Green—the Rhine, the Scheldt (very 
markedly so at Antwerp, as testified in Belgian pictures), the 
Seine, Thames Estuary, &c. 8. Intense green—in patches on the 
Lake of Geneva ; in the evening, when the sun was just below 
the mountains, more frequently on the Lakes of Thun and 
Lucerne. 9. Bright green—the sea, on a windy day, with bright 
sun, off the Isle of Man. 10. The sea round the coral reefs of 
Florida is said to be intensely green; when away from the 
coast it is deep blue. 11. On a heavy, clouded day, with 
rain, gleams of sunshine out at sea give patches of green colour 
and reddish brown. 12, Water standing in an old copper mine 
at Killarney was intensely green, whilst the water in the lake 
at the side was black in the mass. 13. Red colour is produced 
in some seas by algz, in others and in some rivers by the break- 
ing up of soil coloured red by iron, 14. Opaque green colour 
is produced in ponds (Serpentine and ornamental waters) by 
unicellular organisms, which sometimes swarm in these waters. 
They may similarly become red. Perhaps the most remarkable 
instance of blue colour, due to the optical properties of water, 
is the blue grotto of Caprera, where, at any rate, the reflection 
of the sky is eliminated. A similar phenomenon is the glorious 
blue and green of the glacier fissures. 
Leaving the question of surface reflection aside, which can 
only come into play in the case of road-side pools and such 
mirror-like waters, and also leaving aside the appearance of 
vegetation in clear shallow streams and ponds, it seems that at 
the present time we may ascribe the blue colour of masses of 
water to a peculiar reflection of the light from within the water, 
accompanied with polarisation, and depending on suspended 
particles. Blackish, brownish, and yellow colour is due to 
vegetable matter in solution ; reddish brown to iron, sometimes; 
green, sometimes, to copper or alge, but the green commonly 
seen on seas, lakes, and rivers, like that of glacier-fissures, pro- 
bably admits of a similar explanation to that of the blue. I 
trust some physicist may be induced to enter into the subject 
in these pages. Has not the production of a series of tints at 
sunset an origin which may tend to explain the various tints 
of blue and green waters ? I find that Mr. Sorby in the Phzo- 
sophical Magazine, November, 1867, ascribed the blue colour of 
the sky and the successive yellow orange and red tints of the 
setting sun to the absorption of the red rays more than the blue, 
by the fine aqueous vapour of the higher regions of the atmo- 
sphere, and of the blue rays more than the red by the coarser 
vapours near the earth’s surface—as e.g. a fog. 
The foregoing notes may suggest to others similar observations 
of greater importance, which it would be interesting to collect. 
It would be very satisfactory, and of interest to many readers, 
if some one who could speak with authority on the physics of 
light, would discuss these phenomena, however suggestively, in 
your pages. E. Ray LANKESTER 
Poisonous Fishes 
In answer to your correspondent M.D.’s second question in 
your issue of June 3oth, I beg leave to refer him to Dr. Giinther’s © 
article in this Society’s ‘* Proceedings,” ona Poison Organ in a 
genus of Batrachoid Fishes. (P. Z.S., 1864, p. 155.) 
Zoological Society of London, Pi, 1s SCEATER: 
11, Hanover Square, London, W., July 1o 
Fall of an Aerolite, 1628 
Your correspondent T. W. Webb may be glad to know 
that a graphic account of the Aerolite he refers to (NATURE, 
July 14) as having fallen in Berkshire in 1628, will be found in 
Vol. 11. of Chambers’ Papers for the People, published 1850, in 
an article entitled ‘* Memorabilia of the 17th Century,” p. Io. 
This article also contains many other very extraordinary and 
well-described accounts of earthquakes, floods, mirages, and 
varicus startling atmospheric phenomena which occurred during 
the 17th century. Amongst the latter the accounts of parhelia, 
or mock suns, and haloes, and the falls of two or three aerolites, 
are worth noticing. 
Unfortunately, throughout the article the sources whence the 
various notices have been taken is uniformly omitted. 
Alderley Edge, Manchester, July 17 J. P. EARWAKER 
