168 Mr. C. Tomlinson on the Bleaching of 



serve the same order, and nearly the same proportion, in the 

 aluminium, chromium, and iron series : the order is thallium 

 far the highest, methylamine, caesium, ammonium, rubidium, 

 potassium, and sodium lowest. This is the same order which 

 may be deduced from old observations on sulphates, nitrates, 

 chlorides, and acetates. 



Thirdly. That the order of the other metals is iron far the 

 highest, chromium, indium, gallium, and aluminium lowest. 

 This is also in accordance with observations on the simple 

 sulphates of those previously examined. 



Our knowledge on this part of the subject is not yet suffi- 

 ciently advanced to determine the dispersion-equivalents of the 

 separate elements. 



XIX. On the Bleaching of Iodide of Starch by means of Heat. 

 By Chaeles Tomlinson, F.R.S.* 



THE bleaching of iodide of starch by means of heat forms 

 a pretty experiment. An aqueous solution of iodine and 

 one of starch may be mingled together in a test-tube, when 

 the well-known densely blue colour is produced. If the tube 

 be held over the flame of a spirit-lamp, the blue gradually 

 becomes paler and paler, and disappears long before the liquid 

 has reached the boiling-point. If the hot tube be now plunged 

 into cold water, the blue colour immediately reappears, 

 starting up from the bottom of the tube, where the reduced 

 temperature is first felt, and quickly spreading through the 

 liquid up to the surface. The colour returns, but of course 

 more slowly, if the tube be left to cool in the air. 



Thenard, in noticing this experiment upwards of half a 

 century ago, remarked that it is, "sans contredit, la plus 

 remarquable de toutes les proprietes de Fiodure " f- 



It is surprising what a large number of papers have been 

 written on this apparently simple experiment. Gmelin (Hand- 

 book, Cavendish Society's Translation, xv. p. 99) has collected 

 most of them down to 1862 ; and they exhibit various contra- 

 dictions. For example, some w T riters maintain that the iodine 

 and the starch form a definite chemical compound, others 

 describe it as a mechanical mixture ; some maintain that iodic 

 and hydriodic acids are generated on heating the compound, 

 others deny that this is the case ; some say that the blue liquid 

 may be heated many times in a sealed tube and yet recover 

 its colour on cooling ; this also is denied. Such statements 

 as these led Professor Miller, in the i Text-book on Inorganic 

 Chemistry ' that he wrote shortly before his death, to remark 



* Communicated by the Author. 



t My copy of Thenard's Traite de Chimie is a Brussels reprint, dated 

 L836 (vol. ii.'p. 157). 



