474 Theory of the Nutrition [BOOK in. 



ology, physical geography, and lastly of minerals, plants, 

 animals and men. In accordance with his chief object, 

 general instruction, it is written in German and in a good 

 homely style, and contains the best information that was at that 

 time to be obtained on scientific subjects ; among these he 

 gives an account of the processes of nutrition in plants, in which 

 he made careful and intelligent use of all that had been written 

 on the subject, bringing together all the serviceable material 

 which he could gather from Malpighi, Grew, Leeuwenhoek, 

 Van Helmont, Mariotte and others into a connected system, 

 and occasionally introducing pertinent critical remarks. If we 

 consider the state of scientific literature in Germany in the 

 first years of the i8th century, we shall be inclined to assign 

 as great merit to comprehensive text-books of this popular 

 character as to new investigations and minor discoveries. 

 Wolff's chapter on nutrition has however a special interest for 

 us, because it contains several observations of value which 

 were lost sight of after his time. These refer chiefly to the 

 chemistry of nutrition and touch many problems which were 

 not solved before our time ; for instance, the statement that it 

 is a well-known fact that the earth loses its fruitfulness, if 

 much is grown on it ; that it requires much to feed it, and 

 must be manured with dung or ashes ; in these few words we 

 have the questions of the exhaustion of the soil, and the resti- 

 tution of the substances taken from it by the crop, brought 

 into notice by Wolff at this early period. ' It should be 

 particularly noted,' continues Wolff, ' how fruitful nitre makes 

 the soil ; Vallemont has praised the usefulness of nitre, and 

 has mentioned other things which have a like operation by 

 reason of their saline and oily particles, such as horn from the 

 horns and hoofs of animals ; dung likewise contains saline and 

 oily particles, which are present in the ash also, and we see 

 therefore that such particles should not be wanting, if a plant is 

 to be fed from water. The seed also, which supplies the first food 

 of the plant, shows the same thing, for there are none which do 



