402 
rorULAR SCIENCE REVIEW. 
(under certain circumstances, and acting on certain forms of 
matter) from the sun.” The truth of this demonstration is in a 
high degree improbable, even when not viewed in the light 
which Dr. Tyndall’s researches have thrown upon the nature of 
cometary phenomena; and, from the interest Sir John Herschel 
has taken in these researches, from the full recognition he has 
had of the profound difficulties of cometary theory, and from 
his admirable perception that cometary phenomena were of 
just that kind that they would be were some such reactions of 
matter possible as those discovered by Dr. Tyndall, we make no 
doubt he -no louger holds to his assertion that cometary pheno- 
mena demonstrate the existence of levitating matter. 
Notwithstanding the difficulties there have been felt to be in 
interpreting the behaviour of a comet as that of a material sub- 
stance, they have not been deemed sufficient to invalidate the 
claims of comets to rank as material bodies. These claims 
admitted, in what state or physical condition it exists is the 
next matter for inquiry. The answer to this has been tolerably 
clear, and this is, that it is in the state of mist or cloud. And 
here, as it is essential to the clear understanding of Professor 
Tyndall’s cometary theoiy that the distinction between “mist” 
or “ cloud ” and vapour ” is thoroughly recognised, it will be 
well to state these distinctions. A “ vapour ” is a gas which can 
be made to assume the state of liquid by cooling it, and being a 
gas, its particles are homogeneous and tend to separate from each 
other. A “ mist ” or a “ cloud ” is a gas in which are suspended 
minute particles of liquid, and its particles are therefore hetero- 
geneous. The consequence of this composite nature of mist is, 
that it intercepts rays of light cast on it, and reflects them ; 
surfaces of separation such as occur between its liquid and solid 
particles being the condition for reflection. In other words, a 
mist shines and is visible when light is thrown on it ; and this 
is a property not possessed by true vapours or other gases. 
Now, as the matter of a comet is obviously of a gaseous nature, 
there can be no hesitation in deciding that it is in the form of 
mist, because it is luminous, not of itself, but in the rays of the 
sun. The only difficulty that has been hitherto felt, has been 
its unparalleled transparency, already referred to. The shrink- 
ing which is observed to occur in the head of a comet when it 
aj»proaches very near to tlie sun, is quite in accordance with 
what is to be expected if the comet is a mist or vast cloud, the 
heat of the sun converting its outer portions of liquid pai*ticlcs 
into (invisible) vapour, and, being absorbed by so doing, not 
reaching the central portions. 
Such being the state of cometary theory, Dr. Tyndall has 
l;rouglit to bear upon it the knowledge we have gained of natural 
phenomena, by some exceedingly beautiful researches lately 
