262 



NA TURE 



\_'jnly 22, 1880 



history, and language will hold their own adequate place 

 in its scheme of instruction, but the newer sciences of 

 animate and inanimate nature will certainly start from a 

 fairer platform than usual, in the North of England. The 

 Victoria University will not be hampered, like its elder 

 sisters, by the traditions of the past. There is a great 

 career before it, and the people of England will watch its 

 development with the deepest interest. They may be 

 reasonably confident of one thing, that the new educa- 

 tional "brand," to adopt Prof. Huxley's felicitous expres- 

 sion, will be of as select a character as any of the 

 "brands" with which they are familiar. 



ON THE RELATION BETWEEN THE MOLE- 

 CULAR WEIGHTS OF SUBSTANCES AND 

 THEIR SPECIFIC GRAVITIES WHEN IN 

 THE LIQUID STATE 



UNDER this title I have communicated to the Chenii- 

 cal Society the results of a prolonged investigation 

 on the connection existing between the weights of unit 

 volumes of liquid substances and their relative molecular 

 weights (see Jcurual of the Chemical Society for March, 

 April, May, and June, iSSo), and in obedience to a 

 request from the Editor of NATURE I will briefly indicate 

 the scope of the inquiry, and point out the main con- 

 clusions to which I have been led. The inquiry, I may 

 say in the outset, has resolved itself into a critical and 

 experimental examination of what are known as Kopp's 

 laws of specific volume. That some definite connection 

 between molecular weight and specific gravity would be 

 traced had been surmised more than forty years since, 

 but all our exact knowledge on the subject is contained in 

 the series of classical memoirs which we owe to Hermann 

 Kopp. Kopp first clearly recognised the necessity of 

 comparing the liquids when under strictly analogous 

 conditions. By dividing the specific gravity of a hquid 

 taken at the temperature at which its vapour-tension is 

 equal to the standard atmospheric pressure — that is, at its 

 ordinary boiling-point — into its molecular weight, we 

 obtain its specific volume. If the specific gravity be 

 referred to the point of maximum density of water, this 

 value represents the number of cubic centimetres occu- 

 pied by the relative molecular weight of the liquid 

 expressed in grams at its boiling-point under the standard 

 pressure. The numbers thus obtained were first shown 

 by Kopp to exhibit certain definite relations which may 

 be briefly stated as follows : — 



I. In many instances diffe7'ences in specific volume are 

 proportional to differences in corresponding chemical 

 formiihe. — Thus a difterence of CHj in a homologous 

 series corresponds to a difference of about 22 in the 

 specific volume, or (CHjXr = 22.1-. On comparing the 

 specific volumes of similarly constituted haloid com- 

 pounds, it is seen thr.l the substitution of n atoms of 

 bromine for an ecjiial number of chlofine atoms increases 

 the specific volume by 5«. 



II. Isomeric and mctameric liquids have, as a rule, the 

 same specific 7'olume. — Exceptions are exhibited by certain 

 oxygen and sulphur compounds. 



III. The substitution of an atom ofi carbon fior two of 

 hydrogen nial-cs 110 alteration, in the specific volume of 

 certain groups of organic liquids. 



On the basis of these conclusions Kopp was able to 

 calculate certain numerical values for the specific volumes 

 of the elements in combination. These values are as a 

 rule constant for the particular element : thus, accord- 

 ing to Kopp, carbon has invariably the value of 11, 

 hydrogen that of 5-5. Exceptions are observed in the 

 case of the chemical analogues oxygen and sulphur. 

 Each of these bodies has two values depending, it would 

 seem, on its mode of combination, or on its relation to 

 the remaining atoms in the molecule. For example, 

 acetone and allyl alcohol have each the empirical formula 

 C^HijO, but the specific volume of acetone is 78'2, whilst 

 that of allyl alcohol is 73'8. In the case of acetone the 

 combining power of the oxygen atom is wholly satisfied 

 by carbon ; that is, we have reason to know that the 

 oxygen atom is more intimately associated with one of 

 the carbon atoms than it is with any one of those of the 

 other elements ; whereas in allyl alcohol a moiety of the 

 combining value would seem to be satisfied by carbon 

 and the remainder by hydrogen. It appears, then, that 

 when oxygen is united to an element by both its affinities 

 its specific volume is i2'2 ; when it is attached by only 

 one combining unit its specific volume is 7'8. The corre- 

 sponding values for sulphur are 28'6 and 22 '6. 



I have already pointed out that these differences in the 

 values for the specific volumes of oxygen and sulphur 

 may be employed to throw light upon the constitution of 

 such bodies as the phosphoryl and thiophosphoryl com- 

 pounds, and that we may in this way obtain evidence as 

 to the particular affinity-value that an element such as 

 phosphorus, which is variously regarded as a triad and a 

 pentad, exerts, and in the present paper I give additional 

 instances to show that a knowledge of the specific volume 

 of a body is often calculated to furnish valuable informa- 

 tion concerning its constitution. 



The most accurate method of ascertaining the specific 

 volume of a liquid is (i) to determine its specific gravity 

 at some convenient temperature; (2) to ascertain its 

 boiling-point with the utmost exactitude ; and (3) to 

 determine with great care its rate of expansion, say 

 between 0° and this boiling-point. 



The space at my disposal forbids me attempting to 

 show how these various physical data were determined 

 for the purpose of the present inquiry. Full details of 

 the methods employed are given in the original paper, 

 and the errors incidental to the various processes are fully 

 discussed. The observations necessitated among other 

 things the frequent detemiination of the fixed points of the 

 thermometers employed, and the accompanying figure 

 shows how these were found to rise during the progress 

 of the investigation. The abscissa; represent the times 

 in months at which the several observations were taken, 

 and the ordinates the extent of displacement in hundredths 

 of a degree. A represents a thermometer ranging from 

 - 10° to 50° C, B from 50" to 105° C, and c from 98^ to 

 144° C. It will be seen that the extent of the displace- 

 ment is evidently dependent on, or at any rate is greatly 

 influenced by, the amount of molecular disturbance to 

 which the glass envelope is subjected. 



The accuracy of the results is of course in great measure 

 dependent upon the purity of the liquids employed, and 

 this fact to some extent limited the number of compounds 

 which could be investigated. Whene\-er the mode of 



