TEBNARY PRINCll'LES STARCH. 81 



tained varies with the temperature at which the drying was accom- 

 plished. Thus the feeuhi of the potato, which is moist and porous, 

 even when subjected to strong pressure, still retains 45 per cent, of 

 water. This is the green or raw starch of manufacturers. Dry 

 starch is very hygrometric. If after being dried it is placed in an 

 atmosphere saturated with moisture, at 20° centig. (68° Fahr.) it 

 will absorb nearly 36 per cent, of water, and its hulk increases in 

 the ratio of one to one and a half; in this state starch is brilliantly 

 white, and its grains adhere so closely that they foim a mass of 

 sufficient firmness to take the impress of a seal ; starch in this state, 

 however, pressed upon paper yields no perceptible trace of moisture ; 

 it is too hard and adherent to pass through a sieve ; and when 

 thrown on a metal plate heated to 125° (257° Fahr.) its particles im- 

 mediately unite and form a cake. The starch of commerce, in the 

 state in which it is usually found in shops, contains 18 per cent, of 

 water ; it is either pulverulent or readily reducible to powder, 

 though by slight pressure in the hand, it may be formed into a mass 

 or ball. After drying in vacuo at the ordinary temperature, starch 

 retains no more than 10 per cent, of moisture ; a temperature not 

 less than 140" (284° Fahr) is required to dry it completely ; the 

 water which it retains at this temperature belongs to its constitu- 

 tion, and cannot be taken from itexcept by combining it with bases.* 



MM. Collin and Gaultier de Claubry discovered the important 

 character of starch, that of yielding a fine blue or violet color on 

 combining with iodine. f According to M. Payen, the color is more 

 intense, nearer to blue and more lasting, in proportion as the starch 

 is more strongly compressed ; the effect of separation is to turn the 

 blue to shades of violet which approach redness as the substance is 

 looser. The same fecula, according to the degree of its aggregation 

 in plants, is seen to assume shades, which are first reddish, then 

 violet, and eventually of a more decided blue color, under the action 

 of iodine. J 



M. Lassaigne has noticed a very curious property of the combina- 

 tion of iodine and starch : if an amylaceous fluid, having the decided 

 blue color, be heated to 89° or 90=" C. (193° or 194° Fahr.) the solu- 

 tion becomes completely blanched ; but it resumes its former tint as 

 the liquid cools.^ 



This property which starch possesses of striking a blue color with 

 iodine, renders one of these bodies an excellent test for the other. 

 However, as the iodine must exist in the free state to produce its 

 effect, it is necessary, when the blue color does not show itself at 

 once, in a solution in which iodine is suspected, and to which starch 

 has been added, to add a few drops of sulphuric acid, so as to decom- 

 pose the hydriodic acid in cases where it may exist. 



It is familiarly known that if raw starch be mixed with boiling 

 water, the result will be a thick, paste-made starch. According to 



• Payen, W6moire citt, p. 88. 



t Collin et GaiUhier de Claubry, Annales de Chimie, t. xc. p. S8 



i Payen, M6moire cite, p. 105. 



i Lassaigne, JoiiraaKde Chimie M^dicale, t ix. p. 51Q. 



