1866. J MEETING OF AMERICAN ASSOCIATION. 119 



probably retained by peroxyd of iron. Disregarding this, however, 

 and admitting that the carbonic acid, corresponding to a layer 

 8-61 meters of limestone [about twenty-eight feet] were present in 

 our atmosphere, the effect would be most remarkable. The height 

 of the barometric column would be doubled ; the boiling point of 

 water, raised to 121 ° Centigrade [250 ° Fahr.] ; and, as the 

 absorptive power of an atmosphere of carbonic acid is, according 

 to Tyndal, ninety times that of dry air, the temperature of the 

 lower regions of the atmosphere would be greatly elevated, and 

 the whole climatic conditions of the earth modified. Yet, as the 

 amount of carbonic acid required to produce these results is 

 probably but a small proportion of that now fixed in the limestones 

 of the earth's crust, we should find this condition of thino-s at a 

 period, geologically, not very remote, and in still earlier times the 

 earth must have had a far denser and more highly carbonated 

 atmosphere than that just supposed. The relations of such a 

 condition of things to the animal and vegetable world furnish 

 fruitful themes for conjecture and experiment ; and its influence 

 on chemical processes is not less worthy of consideration, as a 

 single instance will show. Some years since, I pointed out that 

 the explanation of the almost constant association of gypsum and 

 magnesian limestone in nature, was to be fouud in the fact that 

 solutions of bicarbonate of lime and sulphate of magnesia 

 decompose each other, with production of solutions of sulphate of 

 lime and bicarbonate of magnesia. By spontaneous evaporation, 

 the former may be in part separated as gypsum ; but as in this 

 process the bicarbonate is changed into mono-carbonate of 

 magnesia, this partially decomposes the gygsum, regenerating 

 carbonate of lime, and the results of the experiment in an ordinary 

 atmosphere are imperfect. I find, however, that by infusing into 

 the drying atmosphere a large proportion of carbonic acid, the 

 separation by evaporation goes on regularly, and the gypsum is 

 deposited in a pure state, enabling us thus to realize the conditions 

 of earlier geologic periods, when vast beds of gypsum, with their 

 accompanying magnesian limestone, were deposited in evaporating 

 basins at the earth's surface, beneath an atmosphere charged with 

 carbonic acid. 



Ebelman has speculated on the probable existence of a much 

 larger proportion of carbonic acid in the atmosphere of earlier 

 geologic times; and Dana, Tyndal, and anterior to them, the late 

 Major E. B. Hunt, have considered its meteorological relations ; 



