346 Mr. T. S. Thomson on the Diffiision of Gases. 



and especially on several of our modern theories, by their re- 

 ception. The long-established idea of the immutability of the 

 properties of elementary bodies would to a certain extent be 

 sacrificed; and it is probable that before these results are 

 conceded, more cogent evidence of the main principle will be 

 required. In the mean time, however, it is plain that the ad- 

 mission of these doctrines throws much light on theories now 

 extensively attracting the attention of men of science, and for 

 that reason they commend themselves to our consideration. 

 1 have offered no opinion here on the atomic mechanism which 

 is involved in these changes from an active to a passive state, 

 though it is impossible to deal with these things without the 

 reflection arising in our minds, that here we are on the brink 

 of an extensive system of evidence connected with the polarity 

 qf atoms, — an idea, which, under a variety of forms, is now 

 occurring in every department of natural philosophy. 

 University of New York, July 29, 1843. 



LI. On the Diffusion qf Gases. 

 By Thomas Starkie Thomson. Esq. 



To Richard Taylor, Esq. 

 Dear Sir, 

 f\^ perusing lately the highly interesting volume on Natu- 

 ^^ ral Magic by your distinguished co-editor. Sir David 

 Brewster, I was much struck by his description of natural 

 phaenomena which had for many years past occupied n)y con- 

 sideration. 



As one fact which was new to me bears directly upon the 

 subject of my previous communications to you on the Diffu- 

 sion of Gases, and will, I think, tend further to elucidate the 

 matter, I may be allowed to offer some observations upon it, 

 and shall introtluce them by the following quotation from the 

 learned author. 



" The great audibility of sounds during the night is a phae- 

 nomenon of considerable interest, and one which had been 

 observed even by the ancients. In crowded cities, or in their 

 vicinity, the effect was generally ascribed to the rest of ani- 

 mated beings, while in localities where such an explanation 

 was inapplicable, it was supposed to arise from a favourable 

 direction of the prevailing wind. Baron Humboldt was par- 

 ticularly struck with this phasnomenon when he first heard 

 the rushing of the great cataracts of the Orinoco in the plain 

 which surrounds the Mission of the Apures. These sounds 

 he regarded as three times louder during the night than du- 

 ring the day. 



