and the different amylaceous Substances of Commerce. 287 



are, its solubility in cold water, and the property of acquiring 

 a blue colour by iodine. We have also proved that it may be 

 obtained in two ways ; either by the action of boiling water on 

 starch, or by that of a higher temperature to which we might 

 immediately expose this substance. We have seen by these 

 two methods, which lead to the same result, how much a very 

 weak chemical agent may sometimes stand in the place of the 

 excess of another much more energetic, especially with regard 

 to an organic body whose elements are very soft. — We will 

 now pursue the study of the phenomena. 



If we boil for a long time an aqueous solution of amidine, 

 it at length loses the faculty of becoming blue by iodine, al- 

 though it still preserves that of precipitating by galls and the 

 acetate of lead ; it takes with iodine a purplish colour : then 

 the amidine has changed its nature, and is become much 

 more soluble in water : starch or fecula may be immediately 

 brought to the same state, either by a pretty strong torrefac- 

 tion, or, as I have effected it, by putting this vegetable principle 

 when heated, in contact with sulphuric acid diluted with 

 twelve times its weight of water : there is an instantaneous so- 

 lution, and the liquor carried to ebullition, then cooled, takes 

 a purplish colour with iodine, and is no longer precipitated by 

 water. If the ebullition is continued longer, the iodine no 

 longer produces any phenomenon of sensible coloration. I am 

 ignorant whether by an immediate torrefaction of starch, con- 

 tinued with skill, we should obtain a gummy matter which 

 would not become purple with iodine, as in the preceding 

 cases, — I am inclined to answer in the affirmative. 



A part of these phenomena is again produced by empois 

 decomposed by time. Perhaps time is not sufficiently consi- 

 dered, which nevertheless presents in many cases a valuable 

 chemical agent. I took some empois, not with basis of fecula 

 of potatoes, but of fecula of wheat ; I left it to itself during 

 more than six weeks, in the heat of the summer it became 

 sour : diluted in water in this state and thrown on a filter, the 

 following phenomena are observed. If iodine be poured upon 

 the liquor, without colour as it was, it becomes a fine purple, 

 whilst the insoluble matter left on the filter takes at the same 

 instant by the same re-agent a beautiful blue colour. This 

 decided result cannot take place, unless all the amidine has 

 been decomposed into empois, and has passed either into the 

 state of gum or into that of sugar. Although I have not veri- 

 fied it, analogy induces me to think that the development of 

 the purplish colour is only owing to the sort of gum formed 

 in this case by the acescence, and that the sugar has no agency 

 in the phenomenon. 



These 



