TRANSACTIONS OF SECTION B. 659 



this is so, the iodide is readily decomposed, even when jjerfectl}- pure, when 

 exposed to lin^ht in the presence of cellulose, the most suitable" form of this 

 material being filter-paper, which has been extracted by hydrochloric and hydro- 

 fluoric acids. If a solution of the ordinary pure salt is" sealed in a bulb and 

 exposed to light, whilst in another bulb is placed an equal quantity of the same 

 solution, together with pure cellulose, it is found that considerably more iodine is 

 liberated in the latter tlian in the former: this difference in many cases amounting 

 to 800 per cent. The solution not containing cellulose gives an alkaline reaction 

 with phenolphthalein, whilst one sealed with sufficient cellulose is quite neutral : 

 the action of the cellulose is therefore probably due to its combination with any 

 potassium hydrate produced by the oxidation of the iodide in presence of light and 

 moisture. 



.If_a slieet of note-paper containing starch is saturated with a solution of 

 potassium iodide, and exposed to light in a printing frame under a negative, it 

 will become printed in a period varying from ten minutes to four hours, the colour 

 of the exposed paper being pink or chocolate; this changes, however, to blue when 

 placed in water, the alteration being doubtless due to the formation of the so-called 

 starch iodide, for the production of which the presence of an excess of water is 

 necessary. It was found impossible to imitate this chocolate colour by any solu- 

 tion of iodine: if the solution was aqueous, a blue stain was produced, whilst, if 

 anhydrous, a brown stain resulted. At last, however, the colour of the exposed 

 paper was obtained by the action of a very concentrated solution of potassium 

 iodide upon paper previously coloured blue by starch iodide. This appears to show 

 that the colour is due to the formation of potassium triiodide, or some similar 

 compound. 



The prints obtained in this manner were fixed by rapid washirg in water, 

 followed by treatment with a dilute solution of lead acetate ; if subsequently sized 

 and varnished, they appear to be quite stable. 



The iodides of sodium, calcium, strontium, barium, iron, and zinc, all behave 

 like the potassiurn salt; the two latter, however, yield prints difficult to see, owing 

 to tlie decomposition of the salt upon the portions of the paper unexposed to the 

 light. 



Cadmium iodide differs from tlie other soluble metallic iodides in yielding a 

 print which is blue, and uDt chocolate coloured; from which it appears that this 

 element is alone unable to form a higher iodide. 



6. Second Report on Quantitative Analysis hy means of Electrolysis. 



See Reports, p. 235. 



7. Report on Wave-length Tables of the Spectra of the Elements. 

 See Reports, p. 273. 



FRIDAY, SEPTEMBER 1.3. 

 Joint Meeting with Section A. — See p. 609. 



SATURDAY, SEPTEMBER 14. 

 The Section did not meet. 



U u 2 



