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Nature
(Thursday, November 4, 1869)

Physiology,   pp. 28-29


Page 28


NA TURE
[NO0V4, 1 869
would produce complete decolorisation. If, on the other hand,
the same solution be added in the same quantities to wine which
has been artificially coloured red, the deception will soon become
apparent by the speedy decolorisation of the liquid, or by the
communication of some different colour to the liquid and to the
precipitate. The following table exhibits the various colours
assumed by the liquid and precipitate produced under these
circumstances in wine coloured by different substances-
   Subsftaces added.  Co/our oJ Liqid.  Coltoer of Precvifiate.
 Pernambuco wood.   Light orange red    Reddish yellow
 Campeachy wood      Golden yellow        Orange yellow
 Archil  .           Very light red       Reddish yellow
 Laccamuffa          Very light green     Greenish-grey
 Prepared Cochineal  Nearly colourless  Grey
 Fitolacca           Nearly colourless    Yellowish
 Myrtle              Nearly colourless    Dingy-greenish
 Violets         .   Very light rose      Yellowish
 Colouring matter of
   normal wine        Persistent wine-red.  Bood-red
   Dye-woods resist decolorisation more strongly than vegetable
juices; and Brazil wood, when treated with the above-mentioned
reagent, aided by heat, acquires a crimson-red colour, due to the
formation of brazilin. -[Ann. di Chim. app. alla Med., September,
1869, p. 142.]
                         PHYSICS
          Professor Magnus on Heat Spectra
  PROFESSOR MAGNUS has recently contributed to the Berlin
Academy a memoir on the radiation and absorption of heat
at low temperatures. The results, which are of the highest
importance, are essentially as follows:-
  i. Different bodies, heated to 1500 C., radiate different kinds
of heat.
  2. Some substances emit only one kind, some many kinds, of
heat.
  3. Of the first class, perfectly pure rock-salt is an instance.
Just as its incandescent vapour, or that of one of its constituents
(sodium), is solitary in tint, so the substance itself, even at 1500,
emits heat of but a single ray. It is monothermic, just as its
vapour is monochromatic.
  4. Rock-salt absorbs heat radiated from rock-salt in larger
quantity, and more powerfully, than that derived from sylvine and
other kinds. It does not, therefore, as maintained by Melloni
and Knoblauch, transmit heat from all sources with uniform
facility.
  5. The amount of absorption effected by rock-salt increases
with the thickness of the absorbing plate.
  6. The high diathermancy of rock-salt, does not depend on its
small absorptive power for the different kinds of heat, but on the
fact that it only radiates (and, consequently, only absorbs) heat
of one kind; while almost all other bodies at the temperature
of 1500 emit heat which contains only a small fraction or none
of those rays which are given out by rock-salt.
  7. Sylvine (potassium chloride) behaves like rock-salt, but is
not monothermic to an equal extent. This circumstance is also
obviously in analogy with the incandescent vapour of the salt,
or of potassium, which is known to furnish an almost continuous
spectrum.
  8. Heat purely derived from rock-salt is almost completely
absorbed by fluor-spar. It might thence have been expected
that heat radiated from fluor-spar would also be energetically
absorbed by rock-salt; yet 70 per cent. of it traverse a plate
of rock-salt 20 mm. in thickness. If we remember that the
total heat emitted by fluor-spar is more than thrice as large as
that of rock-salt, this phenomenon is readily explicable; never-
theless, it is probably dependent upon some other property of
fluor-spar.
  9. If a spectrum could be projected of the heat 'radiated at
1500, and rock-salt were the radiating substance, such a spectrum
would contain only one band. If sylvine were employed, the
spectrum would be more expanded, but still would only include
a small portion of the spectrum which would be given by the
heat radiated from lamp-black.
  In a subsequent communication, Herr Magnus treats of the
reflection of heat radiated at the surfaces of fluor-spar and other
bodies.
  Having succeeded in obtaining the heat from different sub-
stances at 150° free from the rays of flames and other thermogenic
bodies, and afforded proof that there are some substances which
emit waves of one or but few lengths, while others present them
in more frequent variety, it next appeared interesting to solve
the problem how bodies behave with reference to reflective
power; whether, in bodies which act similarly upon light, dif.
ferences parallel to those which are observed in respect of the
absorption and transmission of heat do not also occur in its
reflection.
  Differences in reflective power are unmistakably apparent only
when rays are reflected which have a uniform, or but slightly
varying, length. Such rays have already been derived either
from a section of the spectrum furnished by a rock-salt prism,
or by transmitting the rays from a source of heat of many wave.
lengths through substances which absorb a number of them.
There are, however, but very few bodies that transmit rays of
only one or a few wave-lengths; moreover, such rays, obtained
by either method, have a very low intensity.
  In spite of this difficulty, MM. de la Provostaye and Desains
showed, as early as 1849, that different quantities of the heat
from a Locatelli's lamp were reflected from speculum metal,
silver and platinum, according as it had been conducted through
glass or rock-salt; and, for reflecting surfaces of all kinds, less
in the case of glass than in that of rock-salt.
  Soon afterwards, by an extended series of experiments, and
employing the prismatically dispersed heat of a lamp, it was
proved by the same physicists that heat from the different por-
tions of the spectrum is differently reflected. But, doubtless in
consequence of the low intensity of the incident heat, their re-
searches had reference solely to reflection by means of metallic
surfaces. Now, if in rock-salt we possess a substance that emits
waves of only one or but few lengths, and are acquainted with
other bodies which, at 1500, also radiate but a few kinds, re-
searches can be instituted on reflection at non-metallic surfaces.
It has thus appeared that the different kinds of heat or wave-
lengths are reflected from such surfaces in very different propor-
tions. One of the most striking examples may here be adduced:
it refers to the reflective power of fluor-spar.
  Of the heat radiated by a great variety of substances, unequal
(though but slightly differing) amounts were reflected at an angle
of 450; being in the case of-
               Silver between 83 and go per cent.
               Glass     ,,   6 ,, 14
               Rock-salt ,,   5,, 12
               Fluor-spar ,,  6 ,, so
But of the heat from rock-salt, fluor-spar reflected 28 to 30 per
cent., whereas silver, glass, and rock-salt returned no more of
this heat than in the preceding cases.
  Here, too, it was evident, as in the experiments on thermic
transmission, that sylvine emits, besides a large quantity of the
rock-salt kind, species of heat of another nature.  Fluor-spar
reflects 15 to 17 per cent. of the heat from sylvine; less, conse-
quently, than that from rock-salt, and more than that from the
other radiating bodies.
  Granted an eye that could distinguish different wave-lengths of
heat in Ithe same manner as wave-lengths of light, and when the
waves from rock-salt are incident upon different bodies, fluor-
spar will appear to it brighter than any. If the rays are derived
from sylvine, fluor-spar would seem still brighter than all the
above bodies, but not so bright as when submitted to the rock-
salt rays.
  Melloni has shown that different substances transmit heat
in very unequal proportions, and that the source of heat has a
marked influence on the facility of transmission. Still, the
sources of heat were only distinguished by degree; it was merely
recognised that an increased temperature corresponds to increased
variability of wave length. It now appears that at one and the.
same temperature, and that-viz., 150°-far bel6w incandescence,
different substances emit very different kinds of heat, and that,
within such a range, an extraordinarily large number of different
heat-rays or wave-lengths continually intermingle. This mani-
fold intermixture is particularly furthered by the selective re-
flection taking place at the different surfaces.
  It follows from what has been said that an eye capable of dis-
cerning the different wave-lengths of heat, as it can now dis-
criminate the colours of light, would perceive, with very little
warmth to itself, every possible variety of tint in surrounding
objects.
                     PHYSIOLOGY
                 Pettenkofer on Cholera
  NEARLY the whole of the second part of the Zeitschrift flr
Biologie, bd. v. (300 pages), is taken up by a long memoir by.
Prof. Von Pettenkofer on " Soil and Sock-water in their
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