52 
in low valleys in volcanic districts, and the gases that 
generate malaria in many low-lying marshy districts — such 
as the Maremma in Italy, the neighbourhood of Montpelier 
in France, &c. — seem to point in the same direction. The 
very diverse condition in regard to the amount of watery 
vapour contained in contiguous areas of the atmosphere, 
whether we examine it in different neighbouring localities 
or in the superimposed strata of air in any particular 
locality, show that the working of the law of diffusion is 
here greatly impeded. In regard to hydrogen the evidence 
is somewhat different. It is clear that if under certain con- 
ditions hydrogen be an exception to the general law of the 
diffusion of gases, and follows rather the more general law 
of gravitation, that it will exist in a stratum above the 
atmosphere and beyond the reach of direct observation. In 
his experiment upon the occlusion of gases, Mr. Graham 
examined several aerolites, and found that under the air 
pump they parted with a very large quantity of occluded 
hydrogen. If, as is probable, the gas was occluded by the 
aerolites when at a red heat, and this red heat was coin- 
cident with their passage through that layer of the upper 
atmosphere in which the phenomena of shooting stars and 
of the aurora occur, it seems more than probable that this 
stratum is a layer of hydrogen. This is confirmed by what 
we know of the spectrum^ of certain auroras, which re- 
sembles those of the zodiacal light and the solar corona. 
The spectrum of the corona has been the most attentively 
studied, and Jansen, perhaps the greatest authority on it, 
speaks most confidently about its distinguishing feature 
being the hydrogen lines, while a special line which charac- 
terises both its spectrum and that of the aurora, and which 
is different to that of any terrestrial substance, is considered 
by Father Secchi to be an abnormal hydrogen line. Dr. 
Dalton long ago argued, as Mr. Baxendell has reminded Mr. 
Howorth, that the peculiar features of the aurora could best 
