1888.] Effect of Chlorine on a Voltaic Couple. 



151 



and Melbourne, coincides with the evening maximum of the barometer. 

 A similar coincident minimum, even more strongly marked, charac- 

 terises the diurnal variation of the rainfall at Calcutta and Batavia 

 in their respective rainy seasons. In the author's opinion these facts 

 seem to point to a compression and dynamic heating of the cloud- 

 forming strata, and he points to the existence of a small irregularity 

 in the diurnal temperature curves of Prague, Calcutta, and Batavia, 

 which may possibly be due to such action. It is further remarked 

 that the evening maximum about coincides with the time when the 

 evening fall of temperature, after a rapid reduction between 6 or 7 

 and 10 p.m., becomes nearly uniform in rate, and it is suggested that 

 the former may possibly be determined by the check of the rate of 

 collapse of the cooling atmosphere. But it is observed that both the 

 morning and evening waves of pressure probably involve other ele- 

 ments than the forced waves, and are in part rhythmic repetitions of 

 previous waves. 



III. " Effect of Chlorine on the Electromotive Force of a Voltaic 

 Couple." By G. Gore, F.R.S. Received April 7, 1888. 



If the electromotive force of a small voltaic couple of unamal- 

 gamated magnesium and platinum in distilled water, is balanced 

 through the coil of a moderately sensitive galvanometer of about 

 100 ohms resistance, by means of that of a small Daniell's cell plus 

 that of a sufficient number of couples of iron and German silver of a 

 suitable thermoelectric pile (see ' Proceedings of the Birmingham 

 Philosophical Society,' vol. 4, p. 130), the degree of potential being 

 noted ; and sufficiently minute quantities of very dilute chlorine- 

 water are then added in succession to the distilled water, the degree 

 of electromotive force of the couple is not affected until a certain 

 definite proportion of chlorine has been added; the potential then 

 suddenly commences to increase, and continues to do so with each 

 further addition within a certain limit. Instead of making the ex- 

 periment by adding chlorine-water, it may be made by gradually 

 diluting a very weak aqueous solution of chlorine. 



The minimum proportion of chlorine necessary to cause this sudden 

 change of electromotive force is extremely small ; in my experiments 

 it has been 1 part in 17,000 million parts of water,* or less than a 

 70G0th part of that required to yield a barely perceptible opacity in 

 ten times the bulk of a solution of sal-ammoniac by means of nitrate 

 of silver. The quantity of liquid necessary for acting upon the couple 



* As 1 part of chlorine in 17,612 million parts of water had no visible effect, and 

 1 in 17,000 millions had a distinct effect, the influence of the difference, or of 1 part 

 in 500,000 millions, has been detected. 



