NEW BOOKS. 179 



PER CKNTAGE OF 



Earthy phosphates. Other inors^anic mailer. 



IJritisii liiisecil cako 2.86 '2.S(i 



Dutch do. 2.70 2.54 



Poppy cake 5.22 1.24 



Dodder cake 6.67 3.37 



The numbers in the first column, opposite to popp)' and dodder cake, show that 

 I these varieties of oil-cake contained a much larg'cr proportion of the piiosphates than 

 j the others ilid, anil consequently that an equal weight of tlicni would yield to growing 

 I stock more of those substances which are specially required to build up their increas- 

 ing bones. 



Familiar Lettp:rs on Chemistry, and its relation to Commerce, Phvsio- 

 LOGY AND Agriculture : By Justus Licbig, M. D. &,c.; Edited by Joel Gardner, 

 M. D.— James M. Campbell &- Co. Philadelphia. 



An interesting series of Letters, by Professor Liebig, on various 

 subjects and sciences, in which is to be found much information. 

 His peculiar opinions are of course very prominent. The Twelfth 

 Letter will be found below. 



My Dear Sir — Having now occupied several letters with the attempt to unravel, 

 by means of chemistry, some of the most curious functions of the animal body, and, 

 as I hope, made clear to you the distinctions between the two kinds of constituent 

 elements in food, and the purposes they severally subserve in sustaining life, let me 

 inow direct your attention to a scarcely less interesting and equally important subject 

 1 — tlie means of obtaining from a given surface of the earth the largest amount of pro- 

 iluce adapted to the food of man and animals. 



Agriculture is both a science and an art. The knowledge of all the conditions of 

 ihc life of vegetables, the origin of their elements, and the sources of their nourlsh- 

 iiicnt, forms its scientific basis. 



From this knowledge we derive certain rules for the exercise of the art, the prin- 

 •iples upon which the mechanical operations of farming depend, the usefulness or 

 lecessity of these for preparing the soil to support the growth of plants, and for re- 

 moving every obnoxious influence. No experience, drawn from the exercise of the 

 irt, can be opposed to true scientific 'principles, because the latter should include all 

 he results of practical operations, and are in some instances solely derived therefrom. 

 Theory must correspond with experience, because it is nothing more than the reduc- 

 ion of a series of phenomena to their last cause. 



.V field in which we cultivate the same plant for several successive years becomes 

 larren for that plant in a period varying witli the nature of the soil : in one field it 



f'ill be in three, in another in seven, in a third in twenty, in a fourth in a hundred 

 cars. One field bears wheat, and no peas ; another beans and turnips, but no tobacco : 

 third gives a plentiful crop of turnips, but will not bear clover. What is the rea- 

 m tliat a field loses its fertility for one plant, the same which at first flourished there? 

 Vliat is the reason one kind of plant succeeds in a field where another fails? 

 These qneslions belong to Science. 

 j What means are necessary to preserve to a field its fertility for one and the same 

 |lant? — what to render one field fertile for two, for three, for all plants? 

 These last questions are put by Art, but they cannot be answered by Art. 



