PROCEEDINGS OF THE POLYTECHNIC ASSOCIATION. 475 



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

 the barometric column would be doubled, the boiling point of 

 Avater 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 cli- 

 matic conditions of the earth modified. Yet, as the amount of 

 carbonic acid required to produce these results is probal)ly but a 

 small proportion of that now fixed in the limestones of the earth's 

 crust, we should find this condition of things at a period geologi- 

 cally not very remote, and in still earlier times the earth nuist 

 have had a fjir 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 and chemical 

 jjrocesses, is not less worthy of consideration, as a single instance 

 will show. Some years since, I pointed out that the explanations 

 of the almost constant association of gypsum and magnesian lime- 

 stone in nature was to be found in the fact that solutions of bicar- 

 bonate 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 monio-carbonate of magnesia, this partially decom- 

 poses the gypsum, regenerating carbonate of lime, and the results 

 of the experiment in an ordinary atmosphere are imperfect. I 

 find, however, that by infusing into the dY\ ing atmosphere a large 

 proportion of carbonic acid the separation by evaporation goes on 

 regularly and the gypsum is deposited in a pui?e state, enabling us 

 thus to realize the conditions of earlier geologic periods when vast 

 beds of gypsum, with their accompanying magnesian limestones, 

 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. Hvmt, have considered it»s meteorological relations, 

 but the chemical history of this carbonic acid, considered with 

 reference to its orig-in, its fixation in the form of limestones, and 

 its influence on chemical processes at the earth's surface, are points 



