and the different amylaceous Substances of Commerce. 285 



and clear; by evaporation it furnishes yellowish transparent 

 laminae, which dissolve in cold water without leaving any re- 

 sidue : although this solution becomes of a beautiful blue by 

 iodine, it is precipitated by the subacetate of lead, and by nut- 

 galls, all properties heretofore attributed to the solution of 

 starch in boiling water ; the vegetable matter which it contains 

 always differs from this last, since it redissolves in cold. Ac- 

 cording to this very evident fact, it appears to me very ra- 

 tional to conclude, that the action of boiling water on starch 

 modifies its nature, since it renders it soluble in cold water. 

 But how does the boiling water act here? Is it simply by the 

 caloric which it contains, that the modification of the fecula is 

 produced ? Does its dissolving property here go for nothing ? 



If the temperature of boiling water sufficed to produce 

 such a result, we should equally obtain it by keeping the starch 

 continually exposed for a long time to this heat : but experi- 

 ment proves the contrary. 



Whereas if the temperature is elevated to 212° or a little 

 higher, that is to say to a degree very near to that at which 

 the starch becomes decomposed, immediately this substance 

 takes a slight reddish tint, developes a smell of baked bread ; 

 and if then it is left to cool, and exposed to the action of cold 

 water, it dissolves in it ; and the liquor which results from it 

 possesses all the characters of that which has been already 

 treated of. 



Thus the action of the water is evident. It in some sort 

 stands in the place of an excess of temperature, by reason of 

 its dissolving property, which facilitates and determines a new 

 disposition in the constituent molecules of the fecula. 



When the temperature is still more elevated than the form- 

 er, and fit to subject the starch to a strong torrefaction, this 

 principle is then completely altered ; it dissolves with great fa- 

 cility in water, and instead of a blue colour it takes a purple 

 colour with iodine, as results from experiments published at 

 different periods by MM. Bouillon-Lagrange, Dcebereiner, 

 and Lassaigne. 



The second kind of empois is of a nature and composition 

 similar to the former : it only differs from it by a greater quan- 

 tity of pure starch which is found in suspension, or in combi- 

 nation, which produces the opacity and the great consistence 

 of this empois. Also this compound, treated with cold water, 

 leaves a residue much more considerable than the other. 



According to these facts, then, the empois is a ternary com- 

 pound of pure starch, of modified starch, and of water. The 

 presence of these three bodies is indispensable in order to 

 form good empois. Modified starch is insufficient of itself to 



produce 



