THE PROPORTION OF STARCH VARIES VERT MUCH. 521 



Larger quantities of starch than any of those above stated have been 

 obtained from potatoes by some experimenters. Thus from the 



Per cent, of starch. 



Kidney potatoe, Dr. Pearson obtained . . . 28 to 32 



Apple do. Sir H. Davy 18 to 20 



Shaw do. Vauquelin 18-8 



L'Orpheline do. ^ 24-4 



The first and last of these proportions are probably very rare in oui 

 climate. 



4°. Effect of keeping. — Those potatoes are said to keep best in which 

 the starch is most abundant, but in general keeping has an effect — 



a. On the proportion of starch. — ^By keeping till the spring, potatoes 

 lose from 4 to 7 per cent, of their weight, and the quantity of starch they 

 are capable of yielding suffers a considerable diminution. Thus, ac- 

 cording to Pay en, the same variety of potatoe yielded of starch in 

 October, 17-2 per cent. January, 15-5 per cent. 



November, 16-8 " February, 16'2 " 



December, 15-6 " March, 15-0 " 



April, 14-5 " 



This diminution is probably owing to the conversion of a portion of the 

 starch into sugar and gum. When potatoes are rendered unfit for food 

 by being frozen and suddenly thawed, the quantity of starch which they 

 are capable of yielding is said to have undergone no diminution. 



6. On the proportion of gluten. — The proportion of gluten also ap- 

 pears to become less when potatoes are kept. Thus, in new potatoes 

 Boussingault found the gluten amount to 2|- per cent., but in old potatoes 

 to only 1| per cent, of their weight. To this natural diminution of the 

 proportion of starch and gluten, is probably to be ascribed the smaller 

 value in the feeding of stock, which experience has shown very old po- 

 tatoes to possess. 



5°. Effect of soils and manures. — The potatoe thrives best on a light 

 loamy soil — neither too dry, nor too moist. The most agreeably flavour- 

 ed table potatoes are almost always produced from newly broken up 

 pasture ground, not manured, or from any new soil. [Loudon's Ency- 

 clopaedia of Agriculture, p. 847.] When the soil is suitable, they delight 

 in much rain, and hence the large crops of potatoes obtained in Ireland, 

 in Lancashire, and in the west of Scotland. No skill will enable 

 the farmer to produce crops of equal weight on the east coast where 

 rains are less abundant. It has not been shown, however, that the weighs 

 of starch produced in the less rainy districts is defective in an equal de- 

 gree. Warm climates and dry seasons, as well as dry soils, appear to 

 increase the per-centage of starch. 



Potatoes are considered by the farmer to be an exhausting crop, and 

 they require a plentiful supply of manure. By abundantly manuring, 

 however, the land in the neighbourhood of some of our large towns, 

 where this crop is valuable, have been made to produce potatoes and 

 corn every other year, for a very long period. 



6°. Influence of saline manures. — I have already drawn your attention 

 to the remarkable influence of certain saline substances in promoting the 

 growth of the potatoe crop in some localities. The most striking effects 

 of this kind hitherto observed in our island have been produced by mix- 



