Intelligence and Miscellaneous A i tides. 467 



ON THE ACTION OF CHLORINE ON METALLIC IODIDES. BY 

 A. T. THOMSON, M.D., PROFESSOR OF MATERIA MEDICA IN 

 THE UNIVERSITY OF LONDON. 



To Mr. Richard Phillips. 

 Dear Sir, 5th May, 1834. 



It has been long known that liquid chlorine, that is, a solution of 

 chlorine in water, added to solutions of metallic iodides, or, as they 

 then become, hydriodates, sets free the iodine, and thus enables it to 

 be detected in minute quantity on the addition of starch. But it is 

 also well known, that a very small excess of the solution of chlorine 

 destroys the colour of the iodide of amidine, and renders the test fal- 

 lacious. To remedy this disadvantage I have substituted chlorine gas 

 for the liquid chlorine, and find that it is capable of discovering the 

 minutest portion of any hydriodate in solution, even in mixed fluids. 

 The method of testing is to mix a small quantity of solution of starch 

 in the fluid to be tested, and pouring on the surface of the liquid 

 some chlorine gas ; as soon as the gas reaches the surface a thin film 

 of blue appears, and gradually pervades the whole of the liquid, if any 

 hydriodate be present. The advantage of the test is the impossibility 

 of adding too much, as the action commences on the surface, and the 

 superabundant chlorine, which is mixed with the common air in the 

 upper part of the test tube or the glass, is soon dissipated. 



As a proof of the delicacy of the test, I may add, that four minims 

 of a solution of hydriodate of potassa, containing one drachm in the 

 fluid ounce, were added to a fluid ounce of water, and tested by the 

 method above described, when the presence of free iodine was imme- 

 diately rendered evident. The proportion of the hydriodate in this 

 case being only mhrth, I conceive that the test is adequate for any 

 experiment in which it may be required to ascertain the presence of 

 a hydriodate in solution. 



The theory of the process is too obvious to require any comment. 

 Your making this test generally known through the medium of the 

 Philosophical Magazine will greatly oblige, 



Yours faithfully, 



A. T. Thomson. 



BRITISH ASSOCIATION FOR THE ADVANCEMENT OF SCIENCE. 



We have great pleasure in announcing the speedy publication of 

 the Report of the British Association for the Advancement of Science 

 for 1833, containing the proceedings of the meeting at Cambridge. 

 Its contents, we believe, will be not less interesting than those of the 

 Second Report for 1832, and will evince the steady progress of the 

 Association in the fulfillment of the objects for which it was esta- 

 blished. 



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