170 Mr. C. Tomlinson on the Bleaching of 



colour. At 105° it became pale, at 125° of a light red, and at 

 140° entirely bleached. In cold water it became reddish a 

 little below 140°; at 120° a purplish tinge came over the red; 

 at 90° the colour was purplish, at 80° violet, at 78° reddish 

 violet by transmitted and blue by reflected light. 



A fifth variety of starch, namely that used for domestic 

 purposes in my house, was taken ; but this was soon recognized 

 as potato-starch by the method of Gobley (Journ, de Pharm. 

 for April 1844), in which various specimens of starch, in 

 watch-glasses, are arranged on a plate around a central watch- 

 glass containing iodine, the whole being covered with a bell- 

 glass. I found that some of the specimens, namely such as 

 were slightly moist, became coloured by the iodine vapour in 

 the course of a few minutes ; others in an hour or so, according 

 as they absorbed moisture, there being no action with dry sam- 

 ples. If known samples are first acted on, the colours assumed 

 by them may serve to determine other unknown specimens. 



The four varieties of starch above referred to were made up 

 into thin and thick solutions, the thin containing five and the 

 thick ten grains of starch in about half-an-ounce of water. 

 Equal quantities of the starch solutions were severally mixed 

 with equal quantities of the iodine solution, thereby producing 

 very dark blue or blue-black compounds. Each variety was 

 heated in a test-tube over a spirit-lamp flame and boiled during 

 two minutes, and then cooled by plunging the tube into cold 

 water. The colour was not in any case reproduced during the 

 cooling ; but on the addition of a few drops of an aqueous 

 solution of chlorine the colour was restored in each case, but 

 with very different degrees of intensity ; for while in the case 

 of potato- and rice-starch the blue was almost as intense as 

 before the boiling, it was faint in the case of maize or absent 

 in that of sago, or exhibited but a mere trace in several trials. 

 A specimen of arrowroot also behaved like the sago. Hence 

 it seems that different kinds of starch act on iodine with 

 different degrees of intensity, in character not like a chemical 

 compound so much as, according to Liebig and others, a pre- 

 cipitation of the finely divided iodine on the surface of the 

 granules of the starch. 



M. Personne (Comptes Rendus for 1872, p. 617, in answer 

 to M. Duchaux, p. 533 of the same volume) refers to a state- 

 ment of his in the Comptes Rendus for 1861, in which he 

 regards iodide of starch, not as a chemical compound, but pro- 

 duced by the fixation of iodine on starch in the same manner 

 as a colouring-matter is fixed on a tissue. The blue compound, 

 he says, must be regarded as a dye, not a true lake*. 



* Puchot {Comptes Rendus, 1883, p. 225) has noticed that albumen 

 poured on iodide of starch suspended in water causes the colour to 

 disappear. Whey ( petit- lait) ha^ a similar effect. Here, again, I found 



