METEOROLOGY. 185 
‘remaining the same in all countries and seasons, without being affected by 
temperature, moisture, atmospheric pressure, or elevation above the level 
of the sea. Even in the vicinity of swamps, or in places where large bodies 
of men have congregated, as in churches, theatres, &c., the proportion of 
oxygen is the same; whence it follows that the unwholesomeness of such 
places lies not in the deficiency of oxygen, but in the predominance of some 
other substance. | 
Besides oxygen and nitrogen, and the constantly present watery vapor 
not reckoned as a constituent, there are other ingredients found in the air, 
some of which are quite variable in quantity. The first of these is 
carbonic acid gas, produced by the respiration of man and animals, 
putrefaction, fermentation. combustion, and other processes, and occurring 
everywhere, at least ‘on land. The amount, according to Humboldt’s 
observations, is greater in summer than in winter ; according to Saussure, 
greater at midnight than at noon; greater on mountains during dry, windy 
weather, than in plains when the air is still and damp. Dalton’s obser- 
vations determined the average amount to be one twentieth, and this was 
confirmed by Saussure’s experiments made in a meadow near Geneva in 
1816-28, who estimated it at 0.049, or nearly one twentieth. The air 
above the ocean appears to contain but little carbonic acid gas. 
Another ingredient of the air is ammonia, a gas generated during the 
putrefaction of animal matter. This has but recently been detected by 
Liebig, who found it in snow or rain-water. Wain-water always 
contains ammonia, and in summer more abundantly than in winter or 
spring. 
The third variable ingredient is hydrogen, hitherto only found as a 
constituent in the vicinity of volcanoes and swamps, although in the 
decomposition of animal and vegetable substances, and in the oxydation of 
metals, considerable quantities must be set free. The supposition of some, 
that this gas, on account of its extreme lightness, ascends in the atmosphere 
and forms its confines, is very improbable, as it diffuses itself readily through 
the pores of other gases. At a height of 20,000 feet Gay Lussac found no 
appreciable trace of hydrogen. 
2. Distribution of Heat on the Earth. 
The alternation of heat and cold, and the unequal temperature at 
different places on the earth’s surface, unquestionably constitute the most 
important and remarkable changes in the condition of the atmosphere. 
The sun is to be considered as the principal cause of the heating of the 
earth's surface and of the air, his rays producing an effect proportional to 
the greatness of the angle of their incidence, although this effect may be 
modified by the density of the strata and the absorbing power of 
terrestrial objects. It is necessary, however, to consider the double 
motion of the sun (the diurnal and the annual) in its meteorological 
‘elations. In the diurnal motion of the sun, he stands highest in all places 
359 
