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popular science review. 
tail. Ill liis ‘‘Outlines of Astronomy,” Sir John Herschel, 
writing of the motion of comets’ tails, says, “ If there could be 
conceived such a thing as a negative shadoiu, .... this would 
represent in some degree the conception such a phenomenon 
irresistibly calls up.” The researches on which the new cometary 
theory is based show that we can do so, such a conception being 
fully justified by the physical facts now brought to our know- 
ledge. Suppose for a moment, in opposition to actual observa- 
tion of comets, that in the sun’s rays the head of a comet cast 
an ordinary or “ positive ” shadow. Then, wherever the comet 
moved, its shadow — like its tail — would always point away from 
the sun ; and when the comet passed through its perihelion, this 
shadow would be cast from the central cloud, or head, round 
upon the surrounding parts of the comet, with all the angular 
velocity actually shown by a comet’s tail at this period. The 
rapid motion of a comet’s tail round the head would therefore 
not be in the least degree strange, could the tail be taken to be 
not a thing of substance, but merely the shadow of the head. 
But we know that our supposition is contrary to fact. Suppose, 
however, the following modification of it. Give to the head of 
a comet the property of absorbing, not the illuminating rays 
of the sunbeams, but the heating rays, and it would throw, 
instead of (what we will call) a light-shadow, a heat-shadoiv. 
Further, let it transmit, besides the illuminating rays, any excess 
of chemically-acting rays over those used-up in causing changes 
in its own substance. Under these circumstances, a “ negative ” 
shadow, that is, an illuminated shadow, instead of a dark 
shadow, would be formed, and the luminous comet’s tail, so 
wondrously shadow-like in its motions, would no longer be a 
mystery to us. In the cometary theory of Dr. Tyndall such sup- 
[lositions are made, and, be it remembered, in full accordance 
with known facts. Thus, when, as is so often done for experi- 
mental purposes, a glass vessel of solution of alum is placed in 
the rays of the sun, the heating property of the rays is re- 
moved by the solution, while the chemical and illuminating 
properties pass on. Similarly, the luminous property can be 
withdrawn from the rays, and the heating property left to them 
by substituting for the aqueous solution of alum a solution of 
iodine in carbon disulphide. 
The liead of the cornet, being admitted to cast a heat-shadoiv, 
will keep all that portion of the cometary vapour on the side of 
it away from the sun at a much lower temperature than the 
portions whicli do not lie in this shadow: hence, when in these 
portions none of the products of the chemical change wrought 
l)y the sun’s beams are precipitated as spherules of liquid be- 
cause they are so heated, in the portion lying in the heat-shadoiu 
some of these products may assume the liquid state, and form a 
