THE CHEMISTRY OF A COMET. 
4C3 
made by him. He has almost entirely removed all the diffi- 
culties in the way of accepting the material constitution of 
comets, by showing that the properties of cometary matter are 
not peculiar to it, but common to terrestrial matter, and are 
sufficient to explain all its appearances. “Throughout this 
theory,” to use his own words, he “ has dealt exclusively with 
true causes, and no agency has been invoked which does not 
rest on the sure basis either of observation or of experiment.” 
Before we can proceed, however, to the consideration of Dr. 
Tyndall’s elegant theory, we must give a brief account of that 
part of his researches on which it is based. 
He has found that the rays of the sun, or of the electric 
light, possess the property of generating a mist or cloud, in 
certain vapours mixed with gases, and that such a mist in a 
state of extreme tenuity possesses peculiar optical properties, 
similar to those shown by the visible matter of a comet. One 
mode of exhibiting the formation of this cloud is as follows : — 
A glass tube is taken, about three feet long and three inches in 
diameter, closed at the ends by glass plates, and having a lateral 
opening by which it can be charged or emptied. This is so 
arranged in a dark room that a beam of light from an electric- 
lamp can be sent through it at pleasure. The tube is exhausted 
of air by the air-pump, and then air, or some simple gas such 
as hydrogen, oxygen, or nitrogen, is allowed to flow in, charged 
with the vapour of some selected liquid. The charging of the 
air or gas with the vapour is accomplished by allowing it to 
bubble through the liquid yielding the vapour, or to pass 
through a pellet of paper or cotton moistened with the liquid. 
When the electric light is sent through the tube, the contents 
are at first invisible, and continue so if the air or gas has not 
been charged with a vapour, and has been freed from motes. 
But in the presence of certain vapours, a cloud begins to 
form in a very short time. If the tube is fully charged 
with air loaded with the vapour, the cloud begins to form 
almost instantly, and assumes a white colour. But when either 
by allowing a very little only of the air saturated with this 
vapour to enter the exhausted tube, or by filling the tube with 
air charged with the merest trace of vapour, such as it obtains 
by passing through a pellet of bibulous paper barety damped 
with the liquid, a highly attenuated vapour is made to fill the 
experimental tube, the production of the cloud is much slower, 
and its appearances are different. It is at first of a beautiful 
sky-blue colour, and only gradually changes to white. It begins 
to form in the vapour next to the light, and then gradually 
extends through it to the remote end of the tube. 
The appearance of the cloud or mist in the tube is evidence 
of the formation of particles of liquid — indeed, when the tube 
