126 Observations on Licbig's " Organic Chemistry" 



manure to replace it. Wheat does well to succeed potatoes, 

 and both are found to contain a great deal of alkalies ; as do 

 potatoes and turnips, according to Dr. Madden's analysis. The 

 fertility of the lava soils near Vesuvius comes the nearest to the 

 point. Modern lava, however, is more spongy than the old vol- 

 canic rocks, from being disrupted in the air, and not subjected to 

 the pressure of the sea above as the old trap rocks were. It is, 

 therefore, more easily reduced to soil ; and a few years only will 

 suffice, by the seeds of lichens and their decomposition furnish- 

 ing food for larger plants ; the decomposition and growth of 

 which, in a warm climate, may prepare a soil in which plants, 

 aided by the carbonic acid washed into the soil by rains, and 

 neutralised by the alkalies, may grow crops to great perfection, 

 having both alkalies and organic matter. In the instance of 

 good corn not being produced in Brazil, it has been attributed 

 to the heat of the climate expanding the straw till it becomes 

 feeble, as plants do in our hothouses that are not adapted to 

 them : the silicate of potash, the specific gravity of which, 

 1*26, is less than carbonate of potash and soda, is as likely to be 

 carried by rain as they are, and should exist in vegetable soils, 

 partly as silicates, seeing potash abounds in these soils. The 

 wheats originally drawn from the soils in Virginia differed in 

 nothing but climate from those of Brazil. The vegetable soils 

 he alludes to as not growing wheat in Europe are probably peat 

 soils, which, from the tannin they contain and stagnant water, are 

 unfit for food till fermented and reduced, when they are said to 

 be as good as manure. In the instance mentioned of pine trees 

 and deciduous plants, if these last shed their leaves annually, they 

 will again return them to the soil, and the alkalies with them. 

 The state of moisture to the roots, and the light, heat, and air 

 afforded to the leaves, are, perhaps, as much the causes of their 

 thriving, as any thing else. The pines thrive in general best 

 on bare dry soils, and are not usually fond of much moisture 

 to the roots. The genus y^^bies, or spruce, however, differs from 

 the other pines in this respect. The larch, a deciduous plant, is 

 said not to thrive on sandstone or limestone rocks unless the 

 soil is deep ; JScotch fir thrives well on such, though bare and 

 thin ; and both are ^bietinae. The beech will thrive on very 

 bare soil, the ash will not thrive unless near moisture; and 

 both are deciduous. Willows, poplars, and alders, and also ash, 

 will thrive in all moist soils, whether sandy or clayey, if the 

 moisture is not stagnant. Oaks, sycamore, and chestnuts re- 

 quire a dry and deep soil. The oak requires the soil deep; the 

 beech, from its spreading root, will thrive in a dry moor, and so 

 will the Scotch fir. If there is too much or too little moisture 

 for the particular species, or too little light and air, it seems to 

 affect them most readily. The Scotch fir grows well on granitic 



