422 
NATURE 
| Sept. 3, 1885 
and the photographer can operate as safely in his car as 
in his studio. It will also add to the resources of photo- 
graphy, for there are no places on the earth’s surface 
inaccessible to a balloon. 
The ascent here described had for its main purpose 
photography ; but it had also some meteorolegical 
interest. The ascent began at 1.20; and at 3.20, at an 
altitude of 1100 metres above Meaux, another balloon, 
which ascended some time after them, was met with. 
They were actually in a frequented aéronautical route— 
an aérian river. At Meaux Nadar descended in 1863; 
M. Tissandier himself landed at the same place in 1872, 
and several other descents were made there. A little 
farther, at Chateau-Thierry, on a prolongation of the line 
from Paris to Meaux, M. Tissandier and M. de Fonvielle 
made an extraordinary descent in a storm in 1869, when 
they were dragged along the ground four kilometres in 
five minutes. They travelled from Paris to Chateau- 
Thierry, a distance of 80 kilometres as the crow flies, in 
35 minutes—the most rapid balloon voyage on record. 
On the present ascent, at an altitude of 1000 to 1400 
metres, an aérial current of considerable speed prevailed ; 
it was estimated at about 40 kilometres an hour. At 1400 
metres a mass of white translucid clouds stretched across 
the sky and floated in the upper part of the aérial current. 
Above this, again, the air was calm; small white clouds 
remained immovable at 2000 metres, and the sun was 
very hot. After having descended close to the earth 
above Chateau-Thierry, it was decided to rise above the 
clouds amongst which the aéronauts had just been. At 
6 o'clock, at a height of 1900 metres, they observed the 
shadow of the balloon projected on a white ground of 
clouds ; the latter formed a small greyish circle, sur- 
rounded by an aureole of the seven colours of the rain- 
bow. When they approached the clouds, it was only the 
shadow of the car and of the lower part of the balloon of 
which the projection could be distinguished, and the 
aureole assumed a larger diameter. This remarkable and 
beautiful phenomenon resembles that of the spectre of 
the Brocken. At 6.10 the descent commenced; the balloon 
crossed the bank of clouds, and the surface of the earth, 
when it came in sight, looked grey and dull compared 
with the magnificent regions of the upper atmosphere. 
RADIANT LIGHT AND HEAT} 
Ill. 
Radiation and Absorption—Terrestrial Applications. 
E AVING now established the Theory of Exchanges, 
let us inquire at greater length into the nature of 
the radiation from bodies of different kinds. For this 
purpose we shall adopt the well-known classification into 
solids, liquids, and gases, and shall select as the type of a 
solid body (as far as radiation is concerned) a black sub- 
stance like carbon. We must do this because, in order 
to obtain the greatest amount of radiation from such a 
body at a given temperature, it must be of sufficient 
depth to be practically opaque, or athermanous, for the 
heat of that temperature, and it must have a non-re- 
flective surface. Now carbon or lamp-black possesses 
these properties, if not completely, yet to greater perfection 
than any other substance that we know of; and on this 
account we shall select it as the type of radiating solid 
bodies. 
Then as regards liquids, we have no doubt an amount 
of surface-reflexion, which will have the effect of diminish- 
ing the radiation, and also of polarising it, to some 
extent. In this respect a liquid surface may be regarded 
as equivalent to a polished solid surface, so that liquids 
and polished solids may be classed together as giving out 
an amount of heat somewhat less than that given out by 
the typical black surface. ¢ 
But while there is no marked distinction iu radiation 
t Continued from page 399. 
between solids and liquids, if only the depth of substance 
be sufficiently great, the radiation of gases is essentially 
different. This difference consists in the fact that while 
solids and liquids radiate all kinds of heat possible to the 
temperature, gases radiate only a few. We shall best 
perceive this distinction if we confine ourselves to rays 
which affect the eye, and view these by means of the 
spectroscope. 
We have already explained how this instrument draws 
out a thread of white light into a parti-coloured ribbon, 
red at the one end and violet at the other. Now if our 
thread of white light be a thread of platinum, or, better 
still, of carbon rendered incandescent by means of 
electricity, we shall no doubt obtain the spectrum above 
mentioned. But if our source of light be a row of in- 
candescent gaseous particles, we shall obtain something 
very different. Instead of a long, continuous, variously- 
coloured ribbon, we shall have a few discontinuous threads 
of light emerging from a dark background, each such 
thread or image having of course its proper spectral 
position ; that is to say, if the gas gives out a yellow ray, 
this will appear in the yellow region of the spectrum ; if a 
red ray, in the red region, and so on. Such spectra may 
either be thrown upon a screen, or viewed through a 
telescope—sometimes it is possible to throw them upon a 
screen and render them visible to a large audience, but 
sometimes this is not possible. In all cases, however, 
they may be thrown into a telescope and viewed by the 
individual observer. 
Weare thus in a position to formulate the distinguishing 
characteristic between the spectra of solids and liquids, 
and those of gases, the former giving out a continuous 
spectrum, consisting of all the rays of light possible to 
the temperature, while the latter give a discontinuous 
spectrum, consisting of a few bright lines on a dark 
background. 
We can, in an imperfect manner, assign a reason for 
this behaviour. In a solid, or even a liquid, the various 
molecules are near together, so that no individual is free 
from the trammels of its neighbour in its vibrations. On 
the other hand, it is not so in a gas, or at least in a gas of 
which the molecules are very far from one another. 
Here one individual is for the most part of its existence 
free from the trammels of its neighbours, and is able to 
vibrate after its own fashion and in a way to suit itself, 
just as freely as a bell, or the string of a musical instrument. 
It thus gives out, as it were, its own peculiar note, or 
series of notes, these notes being here, however, rays 
which have a definite place in the spectrum, instead of 
sounds which have a definite place in the musical scale. 
But whilst there is a great amount of freedom amongst 
the molecules of a gas, we must not carry this conception 
of things too far, or suppose that in a compound gas at 
ordinary temperatures we have nothing but a series of 
perfectly similar molecules practically independent of one 
another. 
The particles or molecules of such a gas are far from 
being in a state of rest, and we may imagine them to be 
running about in straight paths, except when they are 
deflected by dashing against a neighbour, or against the 
sides of the containing vessel. It will thus be seen that 
the molecules are not quite free. In fact, a molecule per- 
fectly remote from neighbours, travelling, for instance, in 
free space, and remote from the sun, would have no more 
inducement to vibrate than a bell would have under 
similar circumstances. It is the collision with its fellows 
that will generally cause it to vibrate, but it is sufficiently 
independent to vibrate according to its own laws. Indeed, 
Wwe are in a position to assert that a great portion of that 
energy which constitutes ordinary heat in a gas is derived 
from this motion of the molecules in straight lines, while, 
‘again, the radiation of the gas is caused by the vibrations 
of the molecules after they have been in collision with 
one another, or with the sides of the containing vessel. 
