616 REPORT— 1894. 



5. Some Experiments on the Hate of Progress of Chepiical Change, 

 By Dr. J. H. Gladstone, F.R.S. 



In the last February number of the ' Philosophical Magazine ' Mr. VWey 

 pointed out four stages of a chemical reaction: 'First, the commencement; second, 

 the period of inertness or reluctance, followed by acceleration ; third, of constancy ; 

 fourth, of diminution of velocity.' This reminded the author of various old 

 experiments which had never been published, and he returned to the subject with 

 a view of seeing whether this period of inertness followed by acceleration occurred 

 in such simple cases as that of reciprocal decomposition of salts, and whether, 

 where it did occur, it was capable of any explanation on known principles. The 

 reciprocal decomposition of potassio-platinum-chloride and potassium-iodide slowly 

 produces the iodine salt which makes itself manifest by its deep red colour. In 

 examining this in various ways the action always appeared most rapid at first and 

 gradually slackened till a balance of the salts in solution was obtained. In such 

 cases, on the contrary, as that of the lormation of bitartrate of potassium or 

 calcium, where a larger amount of the products is foi'med than can be kept in 

 solution, it is some time before crystals make their appearance, and then they come 

 with a rush, gradually diminishing to the end of the reaction. This seems to be 

 due partly to the phenomenon of super-saturation and partly to the necessity of 

 rapid redistribution of the acids and bases when the bitartrate produced is thrown 

 out of the field of action and reaction. In cases where almost insoluble salts are 

 formed, such as strontium sulphate, the liquid becomes milky almost at once, a 

 constant redistribution being necessitated by the separation of the insoluble salt, 

 and the curves representing the course of the action closely resembled those of the 

 platinum salt given above. 



There are unquestionably many cases in which there is very little appearance 

 of action at first, but afterwards it comes on rapidly, and then, of course, diminishes. 

 The formation of zinc-methyl was quoted, but a more interesting instance was the 

 reaction between cuprous oxide and silver nitrate in rather weak solution. At 

 first little or nothing is seen ; after a while long filaments of metallic silver shoot 

 forth, the reaction becoming very rapid, until the silver solution is very much 

 weakened, when, of course, it proceeds more slowly. In explanation of this we 

 may conceive of some possible 'induction,' or charging up of the metallic oxide, or 

 the influence of the local rise of temperature, or the greater scope for voltaic action 

 between the growing silver and the copper compound. The author, however, did 

 not insist on any particular explanation, but gave the facts as a contribution to the 

 general subject. 



6. The Determining of the Freezing -jwint of Water, van't ffoff's Constant, 

 Arrhenius' Laio of Dissociation, Osticald's Law of Dilution. By Dr. 

 Mejer Wildermann. 



I have already given an account, in the Physical Section, of the method 

 devised, in concert with the late P. B. Lewis, for accurately determining the 

 freezing-point of aqueous solutions which freeze at temperatures just below 0° C. 



The depression of the freezing-point of a solution of any concentration is stated 

 in degrees below the freezing-point of water. The freezing-point of water and of 

 extremely dilute solutions is very difficult to determine with accuracy. Under 

 ordinary circumstances a cap of ice forms round the bulb of the thermometer ; if 

 the formation of this cap is prevented, the freezing-point of water determined by my 

 thermometer divided to 0°001 is higher by 0°-0015 to 0°-0017 than when the cap 

 exists and a constant error in the determination of the freezing-point is not removed. 



The method of determining the freezing-point of very dilute solutions which 

 was devised by my late friend P. B. Lewis, and my investigations of the freezing- 

 point of water, and of extremely dilute solutions, give us a means of submitting 

 van't Hoft's constant, Arrhenius' law of dissociation, and Ostwald's law of dilution 

 to a more accurate verification. 



It is well known that it was van't Hofl" who first drew attention to the fact 

 that the equations representing the generalisations arrived at by Boyle, Gay Lussac, 



