﻿THE 

 LONDON, EDINBURGH, and DUBLIN 



PHILOSOPHICAL MAGAZINE 



AND 



JOURNAL OF SCIENCE. 



[FOURTH SERIES.] 



APRIL 1869. 



XXXIII. On Cometary Theory. 

 By John Tyndall, F.R.S. §c. 



To the Editors of the Philosophical Magazine and Journal. 



Gentlemen, 



ON the 8th of this month, in a lecture delivered before the 

 Cambridge Philosophical Society, I ventured to enunciate 

 a speculation regarding the origin and deportment of visible co- 

 metary matter. I had been led to reflect on the subject by those 

 experiments, on the decomposition of vapours by light, which I 

 have already described in abstract in the Proceedings of the 

 Royal Society. The speculation was introduced and communi- 

 cated to the Philosophical Society in the following words : — 



" In the course of my experiments on actinic action I have 

 been often astonished at the body of light which a perfectly infi- 

 nitesimal amount of matter, when diffused in the form of a cloud, 

 can discharge from it by reflection. I have been repeatedly per- 

 plexed and led into error by the action of residues so minute as 

 to be simply inconceivable. In order to get rid of these residues, 

 my experimental tubes, after having been employed for any va- 

 pour, are flooded with alcohol, sponged out with soap and hot 

 water, and finally flooded with pure water. Let me give you 

 some idea of the quantities of matter that here come into play. 

 The tube before you, which is 3 feet long and 3 inches wide, 

 was so thoroughly cleansed that when tilled with air, or with the 

 vapour of aqueous hydrochloric acid, no amount of exposure to 

 an intense light produced the least cloudiness. Having thus 

 assured myself of the perfect purity of the tube, I took a small 



Phil. Mag. S. 4. Vol. 37. No. 249. April 1869. R 



