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Dr. M. Wildermail. On Chemical Dynamics [Jan. 30, 



effected by light only, and not only be accelerated by the same ; it 

 must stop as soon as light is removed, i.e., the phenomena under con- 

 sideration must be studied in their purest form and not be com- 

 plicated by other phenomena as well. Bunsen and Eoscoe's reaction 

 Ho + Clo = 2HC1 belongs to this class of reactions, but it cannot be 

 used for a series of reasons, in the first instance because the chemical 

 reaction is accompanied by no variation in volume. The reaction 

 ultimately chosen by the author was that of Davy : CO + Clo = COClo. 

 This reaction is accompanied by a variation of the volume, 1 volume 

 of CO + 1 volume CL> giving 1 volume of C0C1.„ i.e., the velocny of 

 reaction could be uninterruptedly studied from the variation of the 

 manometer, connected with the glass bulb or quartz vessel containing 

 the two gases. The CO and CL were used for the reaction dry. 

 Under such circumstances the reaction goes on only with a small 

 speed, so that to be able to investigate this reaction the problem of 

 getting a powerful light (of 250 or 500 candle power), which would 

 remain constant in intensity and composition for any length of time 

 required, and which would be at the same time of great actinic proper- 

 ties, had to be solved. The light of the sun could not be used, and 

 the arc light changes both in intensity and composition. Dr. Ludwig 

 Mond suggested the use of the acetylene light for the purpose. Having 

 first modified the "Icanto" acetylene generator of Thorn and Hoddle, 

 so that it should not give greater variations in pressure than 1 J per 

 cent., instead of 10 per cent., and having further constructed a special 

 burner, which gives a perfectly pure, smokeless, circular flame of 250 

 (or 500 candle power or more) on the space of about 1 inch, the author 

 reduced the variations in the intensity and composition of the flame to 

 about 1 — 1J per cent., by the use of a balance governor, and by 

 taking special precautions to expel the air from the generator. By 

 means of a thermopyle, galvanometer, regulating tap, &c, the in- 

 tensity of the light could be easily adjusted to about 0*1 per cent. 

 The author used a Rubens' therinopyle, having taken special precau- 

 tions to make its indications concordant, and the thermoelectromotive 

 force of the same in the dark not to exceed 1 or 2 mm., even when 

 the room was heated by the powerful light of the acetylene burner for 

 many hours. The galvanometer used was a sensitive Crompton of the 

 D'Arsonval type. The adjustment of the flame was arranged to 0'1 per 

 cent., though it could be arranged with a very much greater accuracy 

 by the use of a more sensitive galvanometer (say, of a Xalder and 

 Bros), or of a thermopyle of 1 square decimetre surface instead of 

 2 square centimetres. 



The value of the read intensity of light was further checked by 

 reading the deflections of the Crompton by means of a Clark cell and 

 standard manganin resistances. 



As it was found that exceedingly small traces of air or of water 



