250 CALX INCREASING THE SUPPLY OF CARBON. 



tained in a "bushel of gypsum, the ordinary dressing for an acre; 

 But gypsum is easily soluble in enough pure water, and would 

 find enough in the earth furnished by rain for its speedy solution j 

 whereas carbonate of lime is insoluble in water, unless with the 

 addition of carbonic acid. Therefore it may follow, that Qven from 

 a bushel of the soluble gypsum, the crop may draw up lime more 

 readily and abundantly for the time, than from 100 bushels, or 

 more, of insoluble carbonate of lime. Boussingault ascribes the 

 great effect of gypsum to its easy solubility in water, and its thus 

 readily furnishing dissolved lime to the roots and to the body of 

 the plant. Though this is not at all a satisfactory cause for all the 

 wonderful operation of gypsum on clover (and still less to explain 

 its very frequent want of effect), there can be no doubt of the 

 authority for the fact that the gypsum (or its lime) may be easily 

 so received into the sap of the body of the clover. And, as analysis 

 has shown that 1000 Ibs. of dry clover hay contains 27 Ibs. of 

 lime (Sprengel, quoted by Johnston, p. 220), and 100 Ibs. of sul- 

 phate of lime freed from water contains 41.5 Ibs. of lime, if fol- 

 lows that this quantity would suffice for the healthful constitution 

 of as much clover as would be converted to more than 1500 Ibs. 

 of dry hay. 



The chemical facts which have been cited are well established, 

 and the agricultural facts have been observed by very many prac- 

 tical cultivators ; and both would seem sufficient to establish the 

 position that lime gives to plants greatly increased power for ab- 

 sorbing carbon from the air. But, in addition to these, some very 

 interesting and apparently accurate experiments have furnished 

 more direct and certain proof of the results above mentioned. 

 These will now be reported. 



When nearly all the sheets of the preceding edition of this 

 essay (1842) had been printed, embracing the whole except part 

 of the Appendix, I first heard of the discovery having been made 

 by Dr. Wm. L. Wight, of Goochland, of the important property of 

 calcareous earth now under consideration. Forthwith I sought 

 and obtained from him information of his experiments and deduc- 

 tions ; and with his permission, a concise report of their substance, 

 together with such introductory and explanatory remarks as I 

 deemed required, was published among the papers of the Appendix 

 which then remained to be printed. 



Soon after my publication as above stated, Dr. Wight placed his 

 discovery before the public more at length in his " Observations 

 on Vegetable and Animal Physiology," printed in 1843; from 

 which publication will be here copied all that applies to this subject. 



After referring to the previous edition of the " Essay on Calcare- 

 ous Manures" especially, and also to other confirmatory publications, 

 tending to establish both the fertilizing and health-preserving 



