90 Dr. A. R. Leeds on the Decomposition of Equivalent 



Baumert, they were rediscovered by Payen*, who extended 

 their application to the action of nitric, acetic, oxalic, and other 

 acids upon dilute solutions of potassium iodide, in and out of 

 contact with the air. 



In former papers!, a summary of which is given in the 

 Philosophical Magazine (loc. cit.), I have shown that the pre- 

 sence of oxygen not merely facilitates, but is absolutely essen- 

 tial to, the occurrence of the reactions in all cases, and whether 

 the reaction occurs in open or closed vessels, in the heat or 

 cold, in darkness or in light. The only exception to this law 

 is the case of an acid like nitric, which under the influence of 

 light (the action of heat alone in the absence of light is being 

 investigated) spontaneously breaks up, and supplies by internal 

 change the essential oxygen. The experiments made to de- 

 termine the rates of decomposition when various iodides in 

 the presence of different acids were submitted to the influence 

 of magnesium, electric, and solar rays, were made with solu- 

 tions of known though not of chemically equivalent strength. 

 To supply this defect a new series of experiments were per- 

 formed immediately after the publication of those cited above ; 

 and it is to make known certain remarkable laws of actino- 

 chemical change deduced from these latter experiments that 

 the present article is written. 



The solutions were of such strength that 1 cubic centim. of 

 each of the acids employed was chemically equivalent to 12*6 

 cubic centims. of a normal caustic soda solution ; the iodides 

 were each exactly equivalent to the 20-per-cent. solution of 

 the potassium iodide. 1 cubic centim. of the iodide and 1 

 cubic centim. of the acid were used in each trial, the volume 

 of the test being made up to 100 cubic centims. by the addi- 

 tion of distilled water. The tests were contained in " compa- 

 rison-tubes " made of thin perfectly colourless glass, of uniform 

 bore and dimensions, which were supported on frames in such 

 a manner that each tube should be normal to the incident rays, 

 and, in the case of magnesium and electric lamps, 6 inches 

 from the focus of the light. 



The first set of trials was made to determine whether the 

 addition of starch as an indicator facilitated the decomposition 

 under the influence of light, as had been originally supposed, 

 or whether, as later on there had been reason to think, it re- 

 tarded the reaction. In this, as in succeeding experiments, 

 the amounts of iodine liberated are given in milligrams. 



* Comptes Hendus, lxii. p. 254. 



t Proc. Amer. Chem. Soc. 1878, ii. no. 4 j Journ. Amer. Chem. Soc. 

 1879, i. p. 18 ; ibid. p. 65. 



