TRANSACTIONS OF THE SECTIONS. 31 



diminish ; that is to say, the amount which changes during a moment of time is 

 directly proportional to the total amount existing in solution at that time ; or, if 

 we regard the binoxide as doing work — oxidizing oxalic acid — then the statement 

 is that the amount of work done is du'ectly proportional to the amoimt of substance 

 which at any time is there to do it. It will be of interest to examine by similar 

 methods other cases of chemical change in solutions. If it is foimd, as appears 

 highly probable, that wherever the rate of change is measurable, — wherever, that 

 is, it proceeds slowly, and can be started and terminated at a given moment, and 

 the amount changed or remaining unchanged determined, — it follows the same law, 

 then we may pass inductively to a generalization covering those cases of chemical 

 change which take place with an immeasurable velocity, or which cannot be aiTested 

 at will, or for the determination of whose residues or products no exact methods are 

 known ; just as by the use of the pendulum or of Attwood's machine we may prove 

 experimentally the laws of falling bodies, when in the common case of bodies fall- 

 ing freely the velocity with which they move is too great for measiirement. 



On a New Method of detecting Arsenic, Antimony, Sulphur, and Phosphorus, 

 hy their Hydroqen Compounds, ivJien in mixed Gases. By Dr. W. Bibd 

 Herapath, M.i)., F.B.S. L. <Sf E., F.C.S., ^c. Sfc. 



Having to investigate a case of suspected poisoning by phosphorus, in which 

 the traces of free phosphorus had disappeared during the long interval between 

 administration of the poison and analysis, Dr. Herapath examined for phosphorous 

 acid by Scherer's method ; but as several of the hydrogen compounds of sulphur 

 and arsenic, for instance, have the property of blackening the salt of silver, he 

 eliminated these hydrogen compounds from the gas before its absorption by 

 ammoniacal nitrate of silver, or tested the gas, as it was being evolved, for any of 

 these compounds. He dissolved in dilute hot hydrochloric acid the organic matter, 

 stomach, intestines, and contents ; the room of operation being at the time quite 

 dark ; and to the flask an apparatus was fixed for exhibiting any phosphoric flashes 

 of light, as in Mitscherlich s experiment : no flashes appeared. The acid solution 

 might, however, have contained arsenic, phosphorus as phosphorous acid, antimony 

 as chloride, and sulphur as tam'ine, &c. No chlorate of potassa could be employed 

 in oxidizing the organic matter, or phosphorous acid would become phosphoric, 

 and all evidence be lost, for sulphates and phosphates are not reducible in the 

 hydrogen apparatus. To the liquid filtered there was added one-third of spirit of 

 wine, and it was then ready for use. A gas evolution bottle, with funnel and 

 pipe, armed with a tube containing chloride of calcium and chalk in coarse 

 powder, for the preparation of pure hydrogen gas, was arranged and tested, as 

 usual, for arsenic. To the exit-pipe was attached a green glass tube, well sup- 

 ported, passing over two or more spiiit-lamp flames. The exit-pipe was bent at 

 right angles, to go through a wide-mouthed bottle, containing slips of white 

 filtering-paper, dipped in a solution of nitroprusside of sodiimi, made alkaline by 

 ammonia, ti-om which the gas was carried to the next bottle, containing ammo- 

 niacal nitrate of silver ; and there was another exit-pipe leading to a bottle of some 

 salt of lead, or armed with a jet for burning. The apparatus being at this period 

 ready for use, pure zinc, sulphuric acid, and distilled water were placed in the hydro- 

 gen evolution bottle, and the stream of gas having been allowed to escape through 

 the apparatus, to expel atmospheric air, heat was applied to the tubes with spirit- 

 lamps. Now, if arsenic had been present it should have produced a crust in the usual 

 place ; and antimony would, if present, have been deposited at a spot near it ; whilst 

 sulphur would partly have been sublimed and deposited in front of the arsenic, and 

 the remaining undecomposed sulphuretted hydi-ogen gas have communicated a 

 deep purple-blue tint to the paper charged with the ammoniacal nitroprusside of 

 sodium ; whilst the phosphoretted hydrogen, passing unchanged through aU these 

 tests, would have been at once seized by the ammoniacal nitrate of silver and 

 have produced the black phosphide of silver, and the free hydrogen have escaped 

 through the lead solution without changing its colour, unless the evolution (sup- 

 posing phosphorus to be present) of phosphoretted hydi-ogen should have been too 

 violent for the perfect reaction of the silver salt. It was now possible to examine 



