MACK1E — 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 is 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 

 fixation of carbon, 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 the sunshine, and not the 

 volume of carbon, that is the cause of luxuriance ? 

 ' Mr. Hunt appends to his paper a notice of an article by the late 

 Major 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 argues that the temperature of the 

 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 the 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 consequence 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 



