139 
But the most interesting part of the lecture was the con- 
clusion; the former parts were not absolutely novel, but the 
latter had never been announced before. These clouds and the 
similarity of their appearance to the tails of comets, have led 
Prof. Tyndall to form a Theory of Comets, which he enun- 
ciated on this occasion for the first time. The theory, which 
at any rate collects and accounts for observed facts, is as 
follows :— 
1. The theory is that a comet is composed of vapour decom- 
posable by solar light, the visible head and tail being an actinic 
cloud resulting from such decomposition. The texture of an 
actinic cloud is exactly that of a comet. 
2. The tail, according to this theory, is not projected mat- 
ter, but matter precipitated on the solar beams traversing the 
cometary atmosphere. It can be proved by experiment, that 
this precipitation may occur with comparative slowness along 
the beam, or that its consummation may be practically momen- 
tary throughout the whole length of the beam. The amazing 
rapidity of the development of the tail would be thus accounted 
for without invoking any motion of translation save that of the 
solar beams. 
3. As the comet wheels round its perihelion, the tail is not 
composed throughout of the same matter, but of new matter pre- 
cipitated on the solar beams which cross the cometary atmo- 
sphere in new directions. The enormous whirling of the tail is 
thus accounted for without invoking a motion of translation. 
4, The tail is always turned from the sun for this reason. 
Two antagonistic powers are brought to bear upon the cometary 
vapour: the one an actinic power tending to produce precipita- 
tion; the other a calorific power tending to effect vaporization. 
Where the former prevails we have the cometary cloud; where 
the latter prevails we have transparent cometary vapour. Asa 
matter of fact, the sun emits the two powers whose agency is 
here invoked. There is nothing hypothetical in the assumption 
