1 24" Observatio7is on Licbig's " Organic Chemistry." 



alkalies. This shows, he says, how these trees thrive best after 

 the pine forests are uprooted in America ; and how plants, as the 

 /Spartiiim 5Coparium,&c., grow best where ashes have been burnt. 

 Wheat has more ashes in its straw than barley, and barley more 

 than oats, in the proportions 15, 8, and ^; and therefore three 

 crops of oats, or two of barley, will not exhaust the soil more 

 than one of wheat. The amount removed by hay is also con- 

 siderable ; he instances a stack of hay which was consumed by 

 lightning in the neighbourhood of Heidelberg; and Gmelin found 

 the meteor left in its place was a mass of silicate of potash. The 

 potash may, in many places, he says, be replaced by soda, mag- 

 nesia, and lime ; soda is the most powerful solvent of any, but 

 potash combines most readily in the plant : lime is much more 

 insoluble. Phosphoric acid, another ingredient wanted, is, he 

 says, a constituent of all land capable of cultivation ; even the 

 barren heath at Luneburg contains a small quantity; and from 

 the soil, and the grains derived from it, is thus produced the 

 phosphoric acid needed by animals in the formation of their 

 bones : fluoride of calcium seems to have supplied the place of 

 phosphates, in the bones found at Pompeii, and other earlier 

 formations. He quotes from De Saussure, to show that the 

 quantity of potash decreases in wheat as it ripens, which thus 

 restores part of its potash to the soil. The disintegration of 

 rocks restores potash. Beans, peas, tares, and some other crops, 

 he says, contain only a very small quantity of ashes, sometimes 

 none, and should alternate with wheat. The tobacco, containing 

 only as 16 to 97 of phosphates, may also alternate. 



From all that has been stated above, we may see the value of 

 the alkalies in all manures ; but it would, perhaps, be an ex- 

 aggerated estimate to lay it down as a principle, that the 

 temporary diminution of fertility is solely caused by the ex- 

 haustion of alkalies. That they are beneficial I have often seen 

 proof, in noticing that wherever the refuse of nursery plants 

 was burned the potato foliage of the ensuing crop was of a 

 deeper green, and more abundant, and the stems nearly double 

 the height ; the tubers were not always proportionably increased. 

 On turnips we have observed the ashes to produce an opposite 

 effect; they sometimes died altogether : perhaps the ashes were 

 in excess. The ashes would also yield carbonic acid ; at p. 210. 

 of the Appendix, it is stated that the charcoal in which the plants 

 were grown was ultimately reduced to a coaly earth, having 

 yielded carbonic acid abundantly to the plants. Though it is a 

 substance that does not decompose ([uickly, it cannot be ab- 

 sorbed in the solid state, as Sir H. Davy's experiments prove; 

 and those of Dr. Lindley, mentioned at the commencement, 

 confirm the above from the Appendix, that charcoal is reduced 

 by plants. If we trust to analysis of minerals for the quantity of 



