382 



NA TURE 



[August i8, 1904 



boiling-points of one substance can be calculated if those of 



the other are known. Again, from the vapour pressures of 



the pure substances we can calculate the vapour pressures 



Pa 

 and the boiling-points of all mixtures; and, lastlv, if <==— ' 



we can make use of Brown's formula 



-vb 'Wb' '" 



calculate the composition of the vapour from all mixtures 

 without carrying out special experiments to find the value 

 of c. It is, therefore, a matter of considerable interest to 



ascertain whether c is really equal to ~ or not. 



When the equation 



p_otPa-(-(ioo-ot)Pe 



does not hold good, a modification of Brown's formula, or 

 that of Lehfeldt, or of Zawidski, must be employed to 

 calculate the vapour composition, and the constants for 

 those formula must first be determined experimentally. 



Other physical properties, such as the refractive power 

 of mixtures, might be considered, but I will only refer to 

 the critical temperature and pressure. In 1882 Pawlewski 

 stated that the critical temperature of a mi.xture could be 

 calculated from those of the components by the formula 



g _ wSa + (ioo- w)9b 

 100 



where m is the percentage fcy weiglit of A; and G. C. 

 Schmidt, in i8gi, carried out experiments to test the correct- 

 ness of the statement, purposely choosing substances of 

 widely different physical properties. The differences 

 between the calculated and observed temperatures were not, 

 as a rule, very great, rarely exceeding 4°, and Schmidt 

 considered that they might, to some extent, be accounted 

 for by partial decomposition of one or other component. 



Such determinations are, however, liable to serious errors. 

 It is exceedingly difficult to fill a tube with the required 

 amount of a liquid mixture of known composition quite free 

 from air, and although the composition of the very small 

 amount of liquid employed might be determined after the 

 experiment from its specific refractive power, it would be 

 necessary to know the specific refractive powers of the two 

 components and of mixtures of them. Schmidt does not 

 state how he prepared his mixtures and determined their 

 composition. 



Again, when a liquid mixture is heated in a sealed tube, 

 fractionation goes on, so that the more volatile component 

 tends to accumulate in the upper part of the tube, leaving 

 the less volatile component in excess below, and unless a 

 stirring arrangement, such as that devised by Kuenen, is 

 employed, many hours would elapse before complete 

 admixture by diffusion took place at the critical point. 



By far the most important and accurate experiments on 

 this subject have been carried out by past or present pupils 

 of Prof. Kamerlingh Onnes, notably by Prof. Kuenen ; and 

 it is quite certain that the formula of Pawlewski cannot be 

 generally true for mixed liquids, for, just as we may have 

 mixtures of minimum or maximum boiling-point, so also, 

 as Kuenen has shown, mixtures of minimum or maximum 

 critical temperature may exist. Thus the critical tempera- 

 ture of carbon dioxide is 3i°i, and of ethane, 32°o, but 

 that of a mixture containing 30 molecules per cent, of 

 carbon dioxide is i8°.8. The question remains, however, 

 whether some such law as that proposed by Pawlewski may 

 not hold good for closely related substances. In certain 

 cases, when the relationship is very close (for example, 

 C.HjCl and C,H,Br), the critical pressures are equal, or 

 very nearly so, and it seems probable that the critical 

 pressure would be the same for any mixture as for the com- 

 ponents. Such a case as this would be likely to give the 

 simplest possible relation between the critical temperatures 

 of a mixture and those of its components; and although the 

 critical temperatures of these substances are inconveniently 

 high, there are, no doubt, others which might be employed 

 —perhaps ethyl chloride and bromide, or possibly carbon 

 dioxide and carbon disulphide. I imagine, however, that 

 NO. 18 I 6, VOL. 70] 



Pawlewski 's formula would be more likely to hold if m re- 

 presented the molecular percentage, and not the percentage 

 by weight of A. 



In the case of homologous compounds, paraffins, ethers, 

 esters, and so on, the critical pressures are not equal, and 

 it would be necessary to find whether the critical pressures 

 of mixtures are represented by the formula 



P = "'Pa-H (i oo->")Pb 



ICX) 

 (where m is the molecular percentage of A), and also whether 

 anv such simple formula is applicable to the critical 

 temperatures. 



Kuenen has made some observations with mixtures of 

 ethane and butane containing 25 and 5 molecules per cent, 

 of butane, and at the conclusion of his paper he says: " If 

 there was a simple law connecting the critical constants of 

 mixtures with those of the constituents, we might calculate 

 the constants for the second substance [those of the first 

 being known]. But such is not the case. Pawlewski's law 

 that the critical temperature is proportional to the compo- 

 sition, expressed in weight units, is very inaccurate, the 

 deviations being sometimes considerable in both directions." 



It would, I "think, be of great interest if Prof. Kuenen 

 could find time to carry out further experiments with 

 mixtures of ethane and butane in order to settle this point, 

 or, perhaps, with n-hexane and ii-octane, both of which can 

 be more easily obtained in a pure state. 



From what has been said it may be concluded that, in 

 order to ascertain the normal behaviour of pure substances 

 under different conditions, or to find the simplest relations 

 between the boiling-points, molecular volumes, or other 

 physical constants of a series of substances, or, again, to 

 ascertain the normal behaviour of substances when mixed 

 together, and the properties of the mixtures as compared 

 with those of the components, it is undoubtedly advisable — 

 at first, at any rate — to confine our attention to substances 

 of which the molecules show no signs of association in 

 either the gaseous or liquid state. 



In the case of mixtures it is also best to begin with sub- 

 stances which are chemically closely related to each other. 



SECTION C. 



GEOLOGY. 



Opening Address by Aubrey Strahan, M.A., F.R.S., 

 President of the Section. 

 It is forty-two years since the British Association last met 

 in Cambridge, and we may turn with no little interest to 

 the record of what was taking place at a date when the 

 science of Geology was still in its infancy, and in a Uni- 

 versity where its promise of development was first recog- 

 nised. Dr. John Woodward, the founder of the Wood- 

 wardian Chair, had been dead 176 years, but his bequest 

 to the University had not long begun to bear fruit, for the 

 determination to house suitably the collection of fossils and 

 to provide for the reading of a systematic course of lectures 

 was not arrived at until 1818. In that year .Adam Sedg- 

 wick, on his appointment to the Woodwardian Chair, began 

 a series of investigations into the geology of this country, 

 which made one of the most memorable epochs in the 

 history of British Geology. At the Cambridge meeting of 

 1862 he had therefore held the professorship for forty-four 

 years, a period sufficient to spread his reputation throughout 

 the civilised world as one of the pioneers of geological 

 science. 



Towards the close of his life Sedgwick gave expression 

 to the objects which he had had in view when he accepted 

 a professorship in a science to which he had not hitherto' 

 specially devoted his attention. " There were three 

 prominent hopes," he writes, "which possessed my heart 

 in the earliest days of my Professorship. First, that I 

 might be enabled to bring together a Collection worthy of 

 the University, and illustrative of all the departments of 

 the Science it was my duty to study and to teach. Secondly, 

 that a Geological Museum might be built by the University, 

 amply capable of containing its future Collections; and 

 lastly, that I might bring together a Class of Students 

 who would listen to my teaching, support me bv their 

 sympathy, and help me by the labour of their li.inds." 



