14: 



NATURE 



[June 6, 1895 



lities of iodide range from 3-64 HI, lo 23 HI. the quantities of 

 hydrogen sulphate from 45 H'-SO^ tn 46S H-SO*. and the 

 quantities of hydrogen chloride from 70 HC! \o 547 HCK leads 

 lo the following law of connexion lielween chemical change and 

 temperature. 



If a, is the rate of chemical change at a teniiierature /,° in 

 a homc^cneous medium consisting of given constituents ]ier 

 unit volume, and o, is the rate at a tem|KTature /,° in the 

 same metlium. the ratio of a, to o, is ; (273 -r /i)/(273 -r /j))'". 

 m l>eing a constant iie]">entiing ufH)n the character of the con- 

 stituents iif the medium. When the tem|>enitures are measured 

 from the aljsolute zero - 273^. and arc denoted b) 1\. T,, the 

 formula assmnes the simpler form, 



a,/a., = IT,/ T,)"'. 



The constancy of the \alue of 111 for a |x»riicular medium is 

 securetl when the quantity of the j')redominant constituent of 

 the medium is sufticicntly lai^e in comparison with the quan- 

 tities of the other constituents to make the medium praclicallv 

 homogeneous. When this is not the case the \Tilue of m has 

 some value intermediate to the values which it has when one 

 or other of the constituents is sufficiently preilominant to secure 

 a constant value. 



In media in which hydrogen sulphate is sufficiently pre- 

 dominant, the value of w is 20'38 ; similarly for hydrogen 

 chloride the value of m is 21 "17. \\'hen the meditim consists of 

 water and hydrogen iodide, the value of in is 24"!. The intro- 

 duction of sodium sulphate In large quantity into a medium 

 oiherwi.se consisting mainly of hydrogen sulphate reduces the 

 value of ;ii from 20'38 to iSi. In a medium in which the main 

 ingredient is Mxliuni hydrogen carbonate, the value of m is 

 approximately to. 



A further contirmalion of the law of connexion l>et«een 

 chemical change and tem|K"rature is obtained from the iliscussion 

 of cx|)eriments on the rate of change of hydrogen chlorate and 

 poL-ussium iodide made by \V. H, I'endlebury and M. .Seward. 

 The value of /« is in the case of this chemical change 40"5. 



It follow s from the law emmciatcd alx)Ve that at the tem|)erature 

 of al>solute zero no chemical change can take place. 



If the smallest v.ilueof w, viz. 10, is taken, a chemical change, 

 which is c<im))leted in one minute al a temperature zero, would 

 require for its completion, at a tem]jerature of -200°, a little 

 more than a year. If 20 is taken as the value of m. the minute 

 woidd be increased to more than half a millioi\ of years by the 

 same re<luction of temperature. 



The law enunciated aliovc may also be statetl in the follow ing 

 form. 



The increntent ol each unit of chemical change due lo a rise 

 of lemiK-rature varies as the increment of each unit ol absolute 

 temperature. 



Thi.s law is expressed b)' the formula 



IDo/o = wDT/T. 



Chemical Ei/iiilihriiiiii. 



A case of equilibrium Iwtween the reactions 



U-W + 2UI =2ll«f) -I- I-. 

 2ir-() : I- = II-O- -i 2in. 



Ic-ads 111 a discussion of the general equation'- ol clieniical 

 ctjuilibrium, which is given in an appendix lo the iKqK'r. These 

 equations are ein[iloyed to interpret the results of eX)K'riments 

 published by Dr. (Hailstone in the Traiisaitioii) u{ the Royal 

 .S<icicly {Phil. Tram., vol. cxlv. ). They had been [weviously 

 applied to the case of chemical equilibrium in\e-.tigaleil !>)• I'rof. 

 Dixon, in a i>a|K-r publi.sheil in vol. cKxv. of the . raiisiu lions 

 of the Royal .Society, the reactions in that case lieing 



ll'O-l-CO = ll-'-f CO', 

 l{" + €()"■= IR)-F CO. 



Physical Society/May 24. —Captain W. de W. .\bney. Presi- 

 dent, in the chair. Dr. Kuenen read a jiaper entitled " On the 

 condensation and the <ritlcal phenomena of mixtures of ethane 

 and nilrou.s oxide.'' If the vapour of a pure substance is com- 

 prcMcd at cnn.stant temperature, then when a certain pressure is 

 reached the \a|H>ur commences to condense, and the pressure 

 remains ronslani until all the va|x)ur is liipiefieil. Taking the 

 prcMure anil lenqierature as coordinates the corres|xindlng 

 temperatures and pressure^ at which liipiefnction lakes place are 

 pl'iltcd, the curve obl.-iinc'i i> railed the vajxiur pre-sure curve. 



NO. 1336, VOL. 52] 



and this curve ends at the critical temperature and pressure ot 

 the given substance. On the other hand, if a mixture of two- 

 vapours is compressed at constant temperature the ]iressure no 

 longer remains constant while condens;>tion is takini; place, but 

 gra<lually rises. The points at which condensation commences- 

 and enits lie on a U-.shaped curve having its vertex turned to- 

 wards the direction of increasing temperatures. Such a curve 

 the author calls a "border curve." The point at which a line 

 parallel to the axis of / touches a border curve corresixmds to 

 the critical i>oint (R) of the given mixtiu'e. For all temperatures 

 higher than that corresjionding to R there is no cundens;ition 

 into liquitl (wssible, while for any temperature below the critical 

 temperature there are two va|x>ur pressures, one corresponding 

 to the commencement, and the other to the conclusion of lique- 

 faction. The envelope of all the border curves for mixtures, 

 containing diflerent jiroportions of the two bodies is a curve, 

 called the jilait-point curve, joining the critical points of the two 

 constituents. The point of contact (!') of a border curve with 

 the plait-point curve corresponds to the plait-point on van der 

 Waal's, thermodynamic surface. If when we go along the 

 border curve, starting from its lower branch, we first reach R 

 and then I', and if we indicate the temperatures corresponding 

 to I* and R by Ti' and T r, then fi>r temperatvtres between. 

 Ti- and Tk as the pressure is increased the ipiantity of liquid 

 firts increases, reaches a maximiun, and after tiiat decreases till 

 it disippears. This is called retrograde condensjition of the first 

 kind, and has been observed by the author in the case of mix- 

 lures of methyl chloride and carbon dioxide. If 1', however, 

 lies beyond R the process of condensation for temperatures be- 

 tween Ti' aiul Tit is different. In this case the volume of vapour 

 increases, reaches a maximum, antl then decreases. This con- 

 .stitules retrograde condensation of the second kind. It was with 

 a view to the exi>erimental (>bservation ol* this second kind ot 

 retrograde condensation that the author undertook his observa- 

 tions. .\ .series of observations were made with each of the 

 pure gases, and gave the following values for the critical tem- 

 perature : — 



Kthane 

 Nitrous oxide 



36°' I c. 



In the ca.se of the mixtures, the very interesting result is obtained 

 that the critical temperature is in some cases less th.an th.it ot 

 either of the constituent gases. Thus a mixtme containing 10 

 percent, of Cjll,-. has a critical tcnqieralure of 32% the same 

 critical temperature as for pure ethane. .Ml mixtures containing 

 more than 10 per cent, of ethane have a lower critical tempera- 

 ture than 32" : the lowest critical temperature obtained is 

 25^'S, and belongs to a mixture conlaining etpial volumes of 

 ethane and nitrous oxide, .\noiher important point is ihal the 

 border curves do not all lie lu'lween the vapour pressure curves 

 of ethane and nilrous oxide. Hence for any temperature there 

 is some mixture which gives a maximum vapour pressure. It 

 also appears from the curves, given in the paper that the maxi- 

 mum vapour |)ressure is obtained with almost the s;rme mixture 

 at all temperatures, and that this maximinn vapour pressure doe 

 not ilisq)pear willi increase of temperature, but remains even up' 

 to the critical region, for mixtures containing between 20 

 and 50 |x.-r cent, of C._,II|, retrogr,ade condensation of the 

 second kind takes place, but ihe authiu- has no been able to 

 observe it, since theditTerence between Ti* nd Tu for the two 

 substances experimented on caimot be more than o"" I, and the 

 temperalure could not be maintained suHiciently constant to hope 

 to lie able t<i ilelect any phenoniennn taking place o\er such a 

 small temperature range. The author showeil his arrangement 

 for stirring the litpiid and vapour in the experimenlal tube .so as 

 to prevent any retardation of ihe dilTerent phases due to slow 

 diffusicm in the long narrow tubes employed. A small piece ol 

 iriin with enamel beads on the ends is enclosed in the experi- 

 mental tuiie, and by meatis of a small magnetising coil which 

 suridunils the jacket useil to keep the temperalure of the lube 

 constant, this piece ot iron can be moved up an<l down the 

 lube so as to keep the licpiiil an<l vapom thiirou^;hIy stirred. 

 I'rof. Carey Kosler and I'rof. Ramsay complimented Ihe author 

 on the very luciil way he had expounded a iiy no means easy 

 subject. I)r. Siilney \oung congratulated the author on the able 

 use he hail made of his lucky discovery nf two bodies .sm h thai 

 their mixture should have a lower critical lemperature lliaii thai 

 of either of the pure substances. I'rof. Ramsay and he I Dr. 

 \oimgl had made experiments on the va|iour pressure of mix- 

 Uire> of alcohol and ether, and had fouiul great dillicully in pre- 



