82 CHEMICAL CONSTITUTION OF VEGETABLES. 



M. Payen, the change tliat takes place in the state of the fecula it 

 owing to a swelling, a rupture, or disgregation of its granules. By 

 heating a drachm cf starch, mixed with about a couple of ounces of 

 water, to about 00° cent. (140° Fahr.) the microscope shows us that 

 the smallest or youngest gVains — those possessed of the least cohe- 

 sion — have absorbed a considerable quantity of water, and that the 

 expansion of the contents has caused a certain number of the glo- 

 oulds to burst ; at this temperature, however, some grains of fecula 

 are observed, which do not appear to have yet attained their maxi- 

 mum of enlargement, and whose contents consequently are not yet 

 diffused -through the liquid ; it is only between 72° and 100° cent. 

 (161.6° and 212° Fahr.) that the maximum of expansion becomes 

 general, and that the solution acquires its greatest consistency.* 



The remarkable property possessed by starch of making a gluti- 

 nous solution or thick paste with water under the influence of heat, 

 led M. Payen to conjecture that a contrary effect would be produced 

 by lowering the temperature — that the starch might be recovered in 

 its original state of distinct globules by suitable management : and 

 this he in fact accomplished by an ingenious procedure. Starch 

 appears to suffer no actual change when diffused in water by exposure 

 to a temperature of 212° Fahr. ; the granules have only swollen to 

 about thirty times their original dimensions by the imbibition of a 

 large quantity of water. 



We have already seen how starch may be extracted from wheat- 

 en flour ; this method, however, is not the one that is usually 

 followed to procure this useful substance, so large a quantity of 

 which is consumed in the arts. Formerly, starch was universally 

 obtained from grain, — wheat ; at present the potato furnishes a still 

 larger quantity than grain. In the equatorial regions of South 

 America, starch is aoundantly prepared from the YucOy (Jatropha 

 manihot,) and from several species of palm. 



To obtain starch from wheat, the grain is either coarsely ground 

 and mixed with water in large tubs ; or it is put to steep in sacks 

 until it is so soft that a process of kneading sufiices to set the starch 

 at liberty. 



Starch from potatoes. The potatoes are grated after having been 

 well washed, and the palp being thrown on a sieve, the starch is 

 carried off by the water and deposited in suitable vessels. The 

 washings in the manufacture of potato starch soon become putrid 

 by reason of the azotized matter which they contain, and until lately 

 occasioned much annoyance, until M. Dailly conceived the happy 

 idea of turning them to account as liquid manure. 



Starch of the Yvca, or Jatropha manihot. The manihot yields 

 very large foots, rich in starch. These are taken tip a little after 

 the flowering, when the fecula is most abundant. To extract the 

 starch, precisely the same process is employed as in the case of the 

 potato. In South America the manioc is distinguished into yuca 

 duke (mild) and yuca brava, (malignant ;) the latter epithet applying 



• Payen, M^r.oire cit p 96. 



