Sept. 3, 1885 | 
advanced one inch during the time that the last blow has 
advanced 13 inches, and thus the distance between the 
two blows will be 12 inches, or one foot. If, therefore, an 
observer be standing on a railway platform and a railway 
engine be advancing at full speed whistling as it comes, 
the interval between the blows will be less than usual, or 
the note will be shriller than if the engine were at rest. 
On the other hand, when it has passed the station and is 
rapidly receding from the observer, the interval will be 
greater than usual, and the note less shrill. 
It is precisely the same with regard to light. If a 
luminous body emitting rays of definite wave length be 
moving towards the observer, the wave length will be 
lessened and the ray pushed forwards to the more re- 
frangible side of the spectrum. If, on the other hand, 
it be moving from the observer, the wave length will be 
increased, and the ray pushed backwards to the less re- 
frangible side of the spectrum. 
The only difference between light and sound is that the 
former moves so fast, that in order to get an appreciable 
alteration in wave length we must have a luminous body 
moving from or towards us with velocities much greater 
than we can produce experimentally, whereas in the case 
of sound we can make the experiment. 
Nevertheless if we go to the surface of the sun, or to 
the fixed stars, we shall find luminous objects moving 
from or towards us with velocities sufficiently great to suit 
our purpose. 
Let me now say a few words on the effect produced on 
some gaseous spectra by increasing the temperature of the 
gas. It is quite certain that at comparatively low tem- 
peratures such spectra are more complicated than they 
are when the temperature is high. In the former case 
they frequently present a fluted appearance, while in the 
latter we have spectra composed of a few bright lines on 
a dark background. 
In some cases an increase of temperature entirely 
changes the character of the spectrum, so that certain so- 
called elementary substances may be said to have two or 
more spectra. In general, however, we have, notwith- 
standing these remarks, the great feature already men- 
tioned of a persistence of the more permanent spectral 
lines, more especially in the case of metals, throughout 
a large temperature range. 
By means of spectrum analysis we have discovered the 
existence of several new elementary metals, all of which 
are very sparingly distributed. 
Bunsen was the first to detect two new elementary 
metals, ccesium and rubidium. Shortly afterwards Crookes 
discovered thallium, Messrs. Reich and Richter indium, 
and other elementary metals have since been discovered 
by the same means. 
It is now time that something should be said about the 
phenomena of absorption. Since gases have small 
radiating powers, they may naturally be supposed to have 
small powers of absorption. We know, for instance, how 
feeble is the absorption of pure air for luminous rays, or 
even for ordinary heat rays. Tyndall has studied the 
absorptive power of gases for low temperature heat, and 
has come to some very interesting conclusions. The 
following table embodies the results of his experiments :— 
Comparative absorptim of various gases, each of the pressure 
of 1 inch. 
PANT Paes cess: ven ae I Nitric oxide 1590 
Oxy Sens fos) ees 1) |) Nitrousjoxide =.) 7-3) 1860 
Nitrogen’ ... =. %.. I Sulphide of hydro- 
Hydrogen ... Tee gen... en ee ZLOO) 
Chlorine 60 | Ammonia... ... 7260 
Bromine ... ... 160 | Olefiant gas 7950 
Hydrobromic acid 1005 Sulphurous acid 8800 
Carbonic oxide ... 750 
By this we learn that the absorptive power of the three 
permanent simple gases for dark heat is very small, while 
that of compound gases is very considerable. Tyndall 
NATURE 
! 
425 
imagines that the molecule of a compound gas may be 
more inert and less nimble in its vibrations than that of a 
simple gas. That is to say, the compound molecule will 
vibrate more slowly than the simple one, and will thus 
give rise to rays of great wave length ; and inasmuch as its 
absorption and radiation are connected together, it will be 
peculiarly liable to absorb rays of great wave length. 
Its absorption for dark heat may therefore be very 
great, even although it may appear perfectly transparent 
for ordinary light rays. 
Tyndall has found, as the result of his inquiries, that 
aqueous vapour absorbs many more dark rays than dry air, 
and justly concludes that the aqueous vapour present in the 
atmosphere plays a very important part in terrestrial eco- 
nomy. Being transparent for rays of high temperature it 
stops but a small proportion of those which come to us from 
the sun; on the other hand, being comparatively opaque 
for rays of low temperature, it stops the radiation into 
space from the surface of tbe earth. To speak more 
accurately, it does not absolutely prevent this radiation, 
but absorbs it and returns as much or nearly as much 
again. Its action, in fine, is virtually the same as that of 
a cloud in preventing the refrigeration which accompanies 
dew. Tyndall remarks that in those regions where the 
air is very dry the nights are often intolerably cold, owing 
to this uncompensated radiation into space. 
Such regions are those in Central Asia and the great 
African desert, in the latter of which water can readily be 
frozen after the sun has sunk. The glass of a greenhouse 
acts in the same way as the aqueous vapour of the air. It 
allows the sun’s rays freely to penetrate and to heat the 
air within ; but it stops the dark heat of the plants and of 
the soil from being radiated outwards into free space. 
Even a loose frame of glass may save the tender blossoms 
of the peach, and other wall fruit, from being destroyed 
by nocturnal refrigeration. 
BALFOUR STEWART 
(To be continued.) 
NOTES 
ON Monday Prof. Michel Eugene Chevreul entered upon his 
rooth year. Apart from the fact that among men whose lives 
have been devoted to active scientific research no one has before 
attained such an age, M. Chevreul stands conspicuous for the 
vast amount of work he has done and for the great practical 
effect his work has had on the industries of the world. When 
Dumas in 1852 addressed M. Chevreul on the occasion of hand- 
ing to him the grzx of 12,000 frances accorded to him by the 
Société d’ Encouragement pour |’Industrie Nationale, he said :— 
“*Le prix consacre l’opinion de l'Europe sur des travaux servent 
de modele a tous les chimistes; c’est par centaines des millions 
qu’il faudrait nombrer les produits qu’on doit a vos découvertes.” 
More recently, in 1873, when the award of the Albert medal 
was made by our Society of Arts, the terms in which the Council 
expressed the grounds of the award were :—‘‘ For his chemical 
researches, especially in reference to saponification, dyeing, 
agriculture, and natural history, which for more than half a 
century have exercised a wide influence on the industrial arts of 
the world.” His scientific work, apart from its commercial out- 
come, was in this country recognised by the Royal Society as far 
back as 1826, when he was elected a foreign associate. In 1857 
the Copley medal was awarded to him. Other countries have 
also paid him honour, while the distinctions of his native land 
have showered upon him. Born in Angers in 1786 (on August 
31), where his father was a physician of note, he was but 
seventeen when he went to Paris to be ‘‘manipulateur” in the 
laboratory of the celebrated Vanquelin. At the age of twenty 
he published his first chemical paper, and in the next half dozen 
years he had published more than a score on different subjects. 
Then began that series of papers (commencing in 1813), 
