490 
NATURE 
—_" 
[ Oct. 20, 1870 
of scattering in every degree of intensity. Exceed- 
ingly fine examples of dichroitic action on the part 
of this substance are to be seen in Salviati’s window 
in St. James’s Street.* By reflected light the dishes 
and vases there exposed exhibit a beautiful blue— 
by transmitted ligit, a ruddy brownish yellow. The 
change of colour is very striking when, having seen the 
blue, a white cloud is regarded through the glass. Where 
the opalescence is strongest, the transmitted light, as 
might be expected, is most deeply tinged. From these 
examples, where the foreign ingredient is intentionally 
introduced, we may pass by insensible gradations to 
M. Lallemand’s glass. The difference between them is 
but one of degree. Many of the bottles of our laboratory 
show substantially the same effect as the glass of Salviati. 
We can hardly ascribe to molecular action, which is 
constant, an effect so variable as this. It is also a sig- 
nificant fact that, in the case of pellucid bodies—rock 
salt, for example—where the powerfully cleansing force 
of crystallisation has come into play, M. Lallemind him- 
self found the scattering to be z/Z. Under severe exami- 
nation, rock salt itself would probably be found not alto- 
gether devoid of scattering power. I have examined many 
fine specimens of this substance, and have not succeeded 
in finding a piece of any size absolutely free from defect. 
A common form of turbidity exhibited by clear rock salt, 
when severely tested, resembles on a small scale “ a mack- 
erel sky,” Nor have the specimens of Iceland spar that 
I have hitherto examined proved absolutely wanting in 
this internal scattering power. 
In relation to this question, which is one of the first 
importance, the deportment of ice is exceedingly instruc- 
tive. Asa rule the concentrated beam may be readily 
tracked through ice, at least at this season of the year, 
when the substance shows signs of breaking up inter- 
nally. In some cases the sparkle of motes, which are 
evidently spots of optical rupture, reveals the track of the 
beam. In other cases the track appears bluish, though 
rarely of a uniform blue, By causing a previously sifted 
beam to traverse lake ice in various directions, we are 
soon made aware of remarkable variations in the intensity 
of the scattering, and we find some places where the 
track of the beam wholly disappears. The convergent 
beam is sometimes divided by a transverse plane, one half 
of the cone being visible and the other invisible. In 
other cases the cone is divided by a plane passing from 
apex to base, one half shining with scattered light, and 
the other showing the darkness of true transparency. 
Now, if the scattering were molecular, it ought to occur 
everywhere, hut it does not so occur, therefore it is not 
molecular. The scattering is, perhaps, in most cases 
due to the entanglement in the ice, when the freezing is 
rapid, of the ultra-microscopic particles abounding in the 
water. Itis only by excessively slow freezing that such 
particles could be excluded from the ice. Purely optical 
ruptures of the substance itself, if minute and numerous 
enough, would also produce the observed effect. 
The liquids which I examined in 1868 all showed in a 
greater or less degree the scattering of light, to which 
was added in many cases strong fluorescence. In no 
respect did the deportment of the non-fluorescent liquids 
which showed a blue track differ from that of the blue 
* Mentioned to me by a correspondent. 
actinic clouds with which I was then occupied. I ex- 
amined water from various sources and found it uniformly 
charged, not only with particles small enough to scatter 
blue light, but with far grosser particles. Tested by the 
concentrated beam, our ordinary drinking water presents 
a by no means agreeable appearance; some of the 
water with which London is supplied is exceedingly thick 
and muddy. Nor does distillation entirely remove the 
suspended matter. Soret vainly tried to get rid of it; 
he diminished its effect, but he did not abolish it. I 
was favoured a few days ago with specimens of dis- 
tilled water from four of the principal London labora- 
tories. Looked at in ordinary daylight the liquid in each 
case would, in ordinary parlance, be pronounced “as 
clear as crystal,” but when placed in the concentrated 
beam of the electric lamp, the notion of purity became 
simply ludicrous. No one who had not seen it would be 
prepared for the change produced by the concentrated 
illumination. There were differences of purity among 
the specimens, arising, doubtless, from the different modes 
of distillation, but to an eye capable of seeing in ordinary 
light what was revealed by the concentrated beam each 
of the specimens would appear as muddy water. I also 
examined a specimen of extra purity distilled from the 
permanganate of potash and liquefied in a glass con- 
denser. It contained a large amount of foreign particles ; 
not of those which scatter blue light, but grosser ones. 
Such must ever be the case with water distilled in 
the laden air of cities and collected in vessels contami- 
nated by such air. These facts amply justify the language 
applied by Mr, Huxley to the statement that solutions 
without particles can be obtained by the processes hitherto 
pursued, Such a statement could only be based upon 
defective observation, In the number of this journal 
for the 17th of March, an experiment is described in which 
water was obtained from the combustion of hydrogen in air, 
the aqueous vapour arising from the combustion being con- 
densed by a silver surface of unimpeachable purity. In — 
this case, though the floating particles of the air were, in 
the first instance, totally consumed, the water was still 
well lajen with foreign matter. The method of obtaining 
water here referred to had been resorted to by M. 
Pouchet with a view of utterly destroying all germs, and 
my especial object in repeating the experiment was to 
reveal the dangers incident to the inquiries on which 
M. Pouchet and others were then engaged. But the 
warning was unheeded. Itis not for the purpose of adding 
to the weight of calamities, already sufficiently heavy, that 
I allude to this, but rather to advertise the adventurous 
neophyte, who may be disposed to rush into inquiries 
which have taxed the skill of the greatest experimenters, 
of some of the snares and pitfalls that lie in his way, 
JOHN TYNDALL 
Royal Institution, Oct. 18 
as 
NICHOLSON’S MANUAL OF ZOOLOGY 
A Manual of Zoology. By H. A. Nicholson,D.Sc. Vol. 1. 
Invertebrate Animals. (Blackwood and Sons.) 
BOOK such as this aims at being was wanted—one 
which should give a little more of systematic detail 
than is to be found in Professor Huxley’s “Outlines” or 
4 
