282 



HISTORY OF THE VEGETABLE KINGDOM. 



lowing statement, from Mr Jacob's Corn Tracts, 

 contains all the facts that can be depended upon 

 for formings an opinion on this question: "If an 

 acre of land, -with the same degree of labour be- 

 stowed upon it, and the same portion of manure 

 applied to it, yields 300 bushels of potatoes, it 

 may yield 24 bushels of wheat. The food pro- 

 duced by the former, at 38 lbs. to the bushel, 

 will then be 11,400 lbs. in weight; the food from 

 the latter, at GO llis. to the bushel, will be 1,400 

 lbs., or the weight of the wheat will be one- 

 eighth that of the potatoes. It is difficult to as- 

 certain the quantity of nutrition in a given quan- 

 tity of cither wheat or potatoes. The chemical 

 experiments of Sir Humphrey Davy show that 

 wheat contains about three times the quantity 

 of mucilage or starch, and of gluten or albumen, 

 of what is contained in a like weight of pota- 

 toes; but that potatoes contain also about from 

 three to four per cent, of their weight of saccha- 

 rine matter, in which wheat is deficient, though 

 it abounds in barley. The difficulty of esti- 

 mating the nutritive power of the two substances 

 is not wholly removed by this appeal to chemis- 

 try, because we are still ignorant of the effect 

 which the combination of the saccharine matter 

 with the mucilage and gluten may produce when 

 used as aliment. A small addition of the former 

 to the two latter may communicate to the whole 

 mass a degree of nutritive power very far exceed- 

 ing its own separate proportion of weight. Some 

 inquiries have been made as to the actual quan- 

 tity of potatoes consumed per head in families 

 in Ireland, in Prussia, and in Saxony; but the 

 answers varied to such a degree as to be little 

 satisfactory. It does not appear to me to be 

 very far from the fact, if we estimate the propor- 

 tion of the nutritive power of wheat to that of 

 potatoes, as about seven is to two ; or that 2 lbs. 

 of wheat afford as much subsistence as 7 lbs. of 

 potatoes, though it may be doubtful if it affords 

 as much nourishment. We have seen before 

 that the mean weight of the two kinds of food, 

 from the same extent of land, is nearly as one to 

 eight; and now assume that the consumption of 

 an individual is yearly one quarter, or 480 lbs. 

 of wheat, or an equivalent quantity of potatoes 

 being 1680 lbs., then one acre of wheat will pro- 

 duce sustenance for three persons, or one acre of 

 potatoes will afford it to six and five sixths." 



The chemical composition of the potato is as 

 follows: 100 parts, deprived of the skin, con- 

 tain 



Parts. 



Water 68 to 72 



Starch, . . . . 17 to IS 



Fibrous matter, , . . 9 to 8 



Extract, or Soluble Mucilage, 6 to S 



Tlicre is also a small proportion of Potaali and of an 

 Essential Oil, 



The farinaceous matter of the potato may be 



preserved in a dry state by two processes. Tho 

 one consists in washing the roots well in water, 

 then subjecting them to the temporary action of 

 steam, by which the skins are readily detached, 

 and finally slicing them into thin pieces, drying 

 them, and grinding the whole into a powder. Of 

 this, bread may be made by an admixture of 

 wheaten flour or oatmeal. The other method is 

 to grate down tlie potatoes into a pulp, either by 

 a hand grater, or a machine constructed for the 

 purpose. This pulp is then repeatedly washed 

 with water till the whole of the fibrous and mu- 

 cilaginous matter is cleared off, and the pure fa- 

 rina or starch remains in a white and insoluble 

 powder at the bottom. If sufficient pains are 

 taken by repeated washings, this farina is of a 

 perfectly pure and unmixed nature, and equal 

 to that procured from wheat or the other farina- 

 ceous roots. This starch, mixed in certain pro- 

 portions with wheaten flour, forms a palatable 

 bread; or it may be used for all purposes for 

 which arrow root is employed. 



Sugar has also been manufactured in France 

 from the potatoe, and a syrup like treacle. This 

 treacle, says Mr Jacob, appeared to me as sweet 

 as any from the tropics, the only perceptible 

 difference between them was, that it had less 

 consistence. By fermentation a kind of spirit 

 may also be distilled from this root. 



Sweet Potato, (convolvulus batata;) an herb- 

 aceous perennial plant, belonging to the natural 



113. 



The Sweet Potato. 



iam\[yconvolvulacea!, and to the order pentandriii, 

 and class monogt/nia of Linnteus. The term Ba- 

 tata, according to Rumphius, is of Malay origin. 

 It is also called skirrets of Peru. It is a native 

 of both Indies, as also of China, in all which 

 countries it is generally cultivated; and much 

 esteemed, not only for its tuberous roots, but for 

 the young leaves and tender shoots, which are 

 boiled and eaten. 



The sweet potato is a low trailing plant; the 

 stems are creeping, jointed, and extend from the 

 central root about six or eight feet. They are of a 

 pale green colour, and at each joint give out roots, 

 which are oblong tubers of considerable size, ac- 

 cording to the soil and climate. The leaves arise 

 from the stem by long petioles; they are of aa 



