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rorULAR SCIENCE REVIEW. 
THE CHEMISTRY OF A COMET. 
TvEcturek ox ^’atural Piiilosophy in Charing Cross Hospital. 
MI ERE are few facts in modern science more marvellous 
than that of the application of certain optical properties 
of gases to the determination of the materials of the sun and' 
fixed stars. But perhaps the application by Professor Tyndall 
of some chemico-optical phenomena to the elucidation of the 
nature of cometary matter is no less remarkable. In the belief 
that the experiments and speculations of our distinguished 
physicist on this subject are as yet but imperfectly known, 
we propose to give some account of them in the following 
The reasons which require us to regard the cause of cometary 
phenomena to be a material substance are two. In the first 
placo, a comet pursues a path the direction of which has been 
proved to be such as would result from the attractions of the 
sun and planets for a mass having a certain momentum. 
.Secondly, the light of a comet has been shown to be reflected light 
received from the sun, inasmuch as that it is ‘polarised light, and 
that it weakens or intensities as it respectively removes from or 
advances towards the sun ; for the light of a self-luminous body 
is never found to be polarised, and it does not alter in intensity 
or brightne.ss <as we move off from or approach the body, the 
variation in the quantity of light received being solely pro- 
portionate to the variation in the apparent size of the body by 
(li. stance. 
But the acceptation of the theory that a comet is constituted 
of matter has presented great difficulties because of its pro- 
perties and behaviour. Thus, if as matter a comet reflects light, 
its reflecting power has hitherto seemed to be utterly out of 
proportion to its power of intercepting light, according to our 
knowledge of all terrestrial matter. For while a comet may be 
visible in broad daylight (in consequence of the light it reflects). 
article. 
