MACKIE — ON ANCIENT CLIMATES. 
3 
of free carbonic acid ? and if so, whence did it come ? Carbonic acid 
one would regard as the result of animal or igneous combustion. 
We could scarcely derive it from the piscine and mollusk life of the 
early geological life-periods ; nor i.^ it a bit clearer that there were 
any really igneous causes at work to produce it in such volumes. 
We would rather regard it as the resultant from the undoing of 
something else ; if so, it was probably generated continuously, not 
in volumes. In nature too little remains free for any length of 
time. Length of time, with small quantities of carbonic acid, would 
accomplish the same result for vegetation as large quantities and 
very rapid action ; and we have no right to conclude that the Coal 
period was not a long period. Moreover, we have no experiments 
to teach us that plants grow more rapidly and solidly in an abun- 
dance of carbonic acid than in purer air with only a slight amount of 
carbon in it. The luxuriance of vegetation is synonymous with 
jixationoi cario/?, and there is much to make us think that this takes 
place more freely and perfectly in proportion to the intensity of 
light and the purity of the air. Is it not tlie sunshine, and not the 
volume of carbon, that is tlie cause of luxuriance ? 
Mr. Hunt appends to his paper a notice of an article by the late 
Mtijor E. B. Hunt, of the United States Engineers, " On Terrestrial 
Thermotics," published in 1849, in the Proceedings of the American 
Association, in which the Major ar^jnes that the temperature of tlie 
earth's surface increases with the barometric column, and that the 
atmospheric mass must have been greater in the earlier geological 
periods by the amount of carbon and carbonic acid since extracted 
from it, and that therefore the general temperature of the earth's 
surface must have been higher. To this effect of the carbonic acid 
Professor Dana adds the suggestion that the excess of moisture in 
the Carboniferous age would also have contributed to increase the 
weight of the atmosphere. Given the premises that the atmosphere 
consists of the residual gases remaining after tlie consolidation of the 
globe and the reduction to the liquid state of its seas and of the 
greater volumes of water evaporated from it in cont^equence of its 
previous supposed higher temperature, we might grant the values 
of these additions as considerable, especially the last. As it is, we 
admit the idea of Major Hunt is clever. 
But a question or two may, however, be asked with advantage. 
We have, in former speculations, contested for a consideration of the 
possible interference of grand physical operations in effecting the 
