August i8, 1904] 



NA TURE 



Zll 



accurately what will be the consequences of a given state 

 of things. We cannot help asking. How is it that such 

 rules are possible? A rule is invented in the first instance 

 to sum up in a compact form a number of past experiences ; 

 iDut we apply it with little hesitation, and generally with 

 success, to the prediction of new and sometimes strange 

 ■ones. Thus the law of gravitation indicates the existence 

 of Neptune; and Fresnel's wave-surface gives us the quite 

 \insuspected phenomenon of double refraction. Why does 

 Nature make a point of honouring our cheques in this 

 manner, or, to put the matter in a more dignified form, 

 "how comes it that, in the words of Schiller,' 



" Mit di-m Genius steht die Natur im ewigen Bunde, 

 Was der eine verspricht, leislst die andre gewiss " '; 



The question is as old as science, and the modern 

 tendencies with which we have been occupied have only 

 added point to it. It is plain that physical science has no 

 answer ; its policy, indeed, has been to retreat from a 

 territory which it could not securely occupy. We are told 

 in some quarters that it is vain to look for an answer any- 

 -where. But the mind of man is not wholly given over to 

 physical science, and will not be content for ever to leave 

 the question alone. It will persist in its obstinate 

 ■questionings, and, however hopeless the attempt to unravel 

 the mystery may be deemed, physical science, powerless 

 to assist, has no right to condemn it. 



I would like, in conclusion, to read to you a charac- 

 teristic passage from that .Address of Stokes in iSb2 which 

 lias formed the starting-point of this discourse ; — 



" In this Section, more, perhaps, than in any other, we 

 "have frequently to deal with subjects of a very abstract 

 character, which in many cases can be mastered only by 

 patient study, at leisure, of what has been written. The 

 ■question mav not unnaturally be asked. If investigations of 

 this kind can best be followed by quiet study in one's own 

 room, what is the use of bringing them forward in a 

 Sectional meeting at all? I believe that good may be done 

 by public mention, in a meeting like the present, of even 

 somewhat abstract investigations ; but whether good is thus 

 ■done, or the audience merely wearied to no purpose, depends 

 "upon the judiciousness of the person by whom the investiga- 

 tion is brought forward." 



It might be urged that these remarks are as pertinent 

 now as they were forty years ago, but I w-ill leave them on 

 their own weighty authority. I will not myself attempt to 

 ■emphasise them, lest some of my hearers should be 

 tempted to retort that the warning might well be borne in 

 mind, not only in the ordinary proceedings of the Section, 

 "but in the composition of a Presidential .\ddress ! 



SECTION B. 



CHEMISTRY. 



■OfENiNG .\ddress BY Prof. Sydney Young, D.Sc, F.R.S., 



President of the Section. 

 The researches of Hermann Kopp on the molecular volumes 

 and boiling-points of chemical compounds extended over 

 half a century, beginning with his inaugural dissertation 

 •on the densities of oxides in 1838, and concluding in i8Sq 

 with a review of the whole of the work done on the subject. 

 In his second paper Kopp considered the molecular volumes 

 ■of solid compounds, and arrived at the conclusion that truly 

 isomorphous substances have the same atomic or molecular 

 volume, but that in other cases the volumes are usually 

 different. Schroder also made the same observation at 

 about the same time. 



Now, isomorphous substances have analogous chemical 

 formulae, and are usually of similar chemical character, and 

 it is interesting to notice that at this early date the fact 

 was recognised that close chemical relationship is associated 

 with similarity in physical properties. 



For about the first six years Kopp was engaged in the 

 consideration of the results obtained by other observers, and 

 from these results he deduced the most important of his 

 generalisations. 



.As regards boiling-points, Kopp, in 1S42, concluded that 

 a constant difference in chemical composition is accompanied 

 fcy a constant difference in boiling-point, and he adopted 



1 Applied by Herschel to the discovery of Neptune. 



NO. 18 16, VOL. 70] 



the value 18° as the rise due to the replacement of the 

 ' inethyl by the ethyl group in organic compounds, although 

 the observed differences varied between 1 1'^o and 24^-8. 

 Two years later he found in sixteen comparisons differences 

 varying from 8° to 33° : but he doubted the correctness of 

 the extreme values, and took iq° as the true value ; he 

 further suggested that this is the constant difference for 

 an addition of CH, in any homologous series, and he pointed 

 out that the observed difference was most regular in the 

 case of the fatty acids. 



KoDO was also of opinion that isomeric compounds with 

 the same composition and the same vapour density have 

 the same boiling-point. 



The paucity of experimental data and the wide discre- 

 pancies between the results obtained by different observers 

 induced Kopp to undertake the determination of the boil- 

 ing-points of various compounds, and, later, their molecular 

 volumes at a series of temperatures, and it is interesting 

 to note the comparative crudeness of his first attempts and 

 the increasing attention which he paid to the purification 

 of his compounds and to the elimination of thermometric 

 and other errors. He first examined three pairs of esters 

 in order to find whether isomeric compounds have really the 

 same boiling-points. But he employed only calcium chloride 

 as a dehydrating agent, and this would remove neither water 

 nor the alcohol completely ; he was much troubled by the 

 " bumping " of the liquids, and the temperatures he actually 

 observed — with the thermometer bulb in the liquid — 

 fluctuated considerably, and he could only, in most cases, 

 take the lowest temperature observed as the most probable 

 boiling-point. By so doing, and by making a fairly liberal 

 allowance for residual errors, Kopp arrived at the erroneous 

 conclusion that the boiling-points of isomers were the same 

 in the three cases examined, and therefore, probably, in all 

 cases. 



The boiling-point of methyl alcohol was of great interest 

 to Kopp, because, taking that of ethyl alcohol— about which 

 there was general agreement — as correct, it should, accord- 

 ing to his law, be 78°-iq° = sq°, while the temperatirres 

 : actually observed varied from 60° to 66°. Kopp prepared a 

 i specimen of methyl alcohol, and found that it boiled at aboiit 

 65° ; but he had 'more faith in his law than in his experi- 

 mental result, and he concluded that the methods of deter- 

 mining boiling-points were not sufficiently accurate to give 

 results correct to within even 1° or 2°. 

 ' In 1854 he discussed the corrections which should be 

 I applied to thermometer readings, giving a table of correc- 

 tions for the unheated column of mercury, and adopting the 

 value 27 mm. per degree as the value of ipdt for all 

 substances, in order to reduce the observed boiling-point to 

 I that at normal pressure. He pointed out, also, that the 

 i height of the barometer should be reduced to 0° C. Taking 

 advantage of Delff's improved method of preparing and 

 purifying methyl alcohol, Kopp made a fresh specimen from 

 methyl o.xalate' and dried it with lime ; but while Delff 

 observed the boiling-point to be 60°, Kopp obtained the value 

 I 65°2-6s°8. He was still, however, inclined to think that, 

 ' owing to bumping, the observed boiling-point was too high 

 and that the true temperature should be about 60°. 



Meanwhile, in 1847 Kopp had examined sixteen liquids, 

 including water, two alcohols, three fatty acids, and seven 

 esters, and in 1854, as a result of his further determinations, 

 he was able to compare the boiling-points — and also the 

 ' molecular volumes — of a large number of substances, most 

 of which were either alcohols, acids, or esters, and he at 

 I first adhered to his previous value of iq° for the rise of boil- 

 ing-point due to the addition of CH,. Later in the same 

 year, however, taking a wider survey and including hydro- 

 carbons and their halogen derivatives, ethers, sulphides, and 

 other compounds, he was obliged to admit that the difference 

 is in some cases higher, in others lower, than iq", but he 

 1 still regarded these cases merely as exceptions to the law. 

 i In 1867 Kopp admitted that isomeric aromatic hydrocarbons 

 have not always the same boiling-point, and that the differ- 

 ence for an addition of CH, was not always ig° ; but he 

 '. still believed that the difference for CH, was constant in 

 any really homologous series — for example, 20°.5 for homo- 

 logues of toluene, i8''^5 for those of xylene, and i6°^5 for 

 those of trimethylbenzene. He also recognised the fact that 

 isomeric alcohols have widely different boiling-points. 



Kopp published no later papers on the boiling-points of 



