28 
NATURE 
| Vov 4, 1869 
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— 
Substances added. Colour of Liquid. Colour of Precipitate. 
Pernambuco wood . Light orange red . . Reddish yellow 
Campeachy wood . Golden yellow . , . Orange yellow 
Archil . . . .. . Very light red « - Reddish yellow 
Laccamuffa . . . Werylightgreen . . Greenish-grey 
Prepared Cochineal . Nearly colourless . . Grey 
Fitolacca . . . « ~- Nearlycolourless , . Yellowish 
Myrtle. . . . . « Nearlycolourless . . Dingy-greenish 
Violets . - - . Verylightrose . . . Yellowish 
Colouring matter of Persistent wine-red. . Blood-red 
normal wine 
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. 
ProressoR Maanus 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 :— 
1. Different bodies, heated to 150° 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 150°, 
emits heat of but a single ray. It is monothermic, just as its 
yapour 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 150° 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 
obyiously 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. 
g. If a spectrum could be projected of the heat “radiated at 
150°, and rock-salt were the radiating substance, such a spectrum 
would contain only ove 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 wayes 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 heatjfrom 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 150°, 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 yery 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 45°; being in the case of— 
Silver between 83 and go per cent. 
Glass » » Th yy 
Rock-salt  ,, Sy np 
Fluor-spar ,, tO. a 
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 jthe 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 ¢zat—viz., 150°—far below 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 fiir 
Biologie, bd. y. (300 pages), is taken up by a long memoir by 
Prof. Von Pettenkofer on ‘Soil and Sock-water in their 
