March i, 1894] 



NA TURE 



421 



aliphatic acids, and to a much greater extent the alcohols, do 

 not conform with this rule). 



(3) Normal propyl compounds have, as a rule, slightly higher 

 values than allyl compounds ; in the case of the alcohols, propyl 

 compounds have much the higher value. 



(4) The efTect of molecular weight in some cases may be more 

 than counterbalanced by that of constitution, or of complexity. 



(5) The lowest members of homologous series frequently 

 exhibit deviations from the regularity shown by higher mem- 

 bers. 



(6) An iso-compound has in general a larger coefficient than a 

 normal compound, and the differences reach their maximum in 

 the case of the alchohols. 



(7) In the case of other metameric substances, branching in 

 the atomic chain and the symmetry of the molecule influence 

 the magnitudes of the coefficients ; the ortho-position, in the 

 case of aromatic compounds, appears to have a more marked 

 efTect on the coefficient than either the meta- or para-position. 

 Acetone and ether have coefficients that are less than half the 

 values given by the isomeric alcohols. 



(8) One of the most striking points thus brought to light is 

 the peculiar behaviour of the alcohols, and to some extent of the 

 acids, as contrasted with that of other liquids. 



Comparisons of molecular viscosity at the boiling point 

 show — 



(i) That, with the exception of the alcohols, dibromides, and 

 the lowest members of homologous series, an increment of CH., 

 in chemical composition corresponds with an increase in mole- 

 cular viscosity. 



{2) With the above exceptions, it is also apparent that the 

 corresponding compound having the highest molecular weight 

 has the highest molecular viscosity : the difference in molecular 

 viscosity between the corresponding members of two correlated 

 series is fairly constant. 



(3) The relationships shown in the other tables are substan- 

 tially of the same nature as those given by the viscosity co- 

 efficients. 



The comparisons which give the largest deviation from re- 

 gularity contain those substances which, as already shown, 

 exhibit a peculiar behaviour, namely, the alcohols, acids, 

 propylene dibromide, ethylene dichioride, &c. 



In order to indicate how molecular viscosity at the boiling 

 point is quantitatively connected with chemical nature, attempts 

 were made to calculate the probable partial effects of the atoms 

 on the molecular viscosity. Values were also assigned to the 

 eflfects of the iso-grouping of atoms, the double linkage of carbon 

 atoms, and the ring grouping. 



Tables are given which show the concordance between the 

 observed molecular viscosity and those calculated by means of 

 these constants. In the case of forty-five liquids the difference 

 between the observed and calculated values rarely exceeds 5 per 

 cent. In the case of the isomeric ketones and aromatic hydro- 

 carbons, the differences are in part due to constitutive influences, 

 which cannot at present be allowed for in obtaining the calcu- 

 lated values. 



In a second table are given those substances for which the dif- 

 ferences exceed this 5 per cent, limit. These may be roughly 

 classed as unsaturated hydrocarbons, polyhalogen compounds, 

 formic and acetic acids, benzene, water, and the alcohols. 



Similar fundamental constants for molecular viscosity work at 

 the boiling point have also been deduced, and tables are also 

 given showing the comparison between the observed and calcu- 

 lated numbers, the substances being classified into two groups, 

 as in the case of molecular viscosity, according as the difterences 

 are less or greater than about 5 per cent. 



On taking a general survey of the comparisons at the boiling 

 point, it is evident that for the majority of the substances ex- 

 amined — the paraffins and their monohalogen derivatives, the 

 sulphides, the ketones, the oxides, and most of the acids and 

 the aromatic hydrocarbons — molecular viscosity and molecular 

 viscosity work may be quantitatively connected with chemical 

 nature. The remaining substances — unsaturated hydrocarbons, 

 di- and poly-halogen compounds, formic acid, benzene, water, 

 and the alcohols — present marked exceptions to the foregoing 

 regularities. 



As regards the comparison of the viscosity magnitudes at the 

 corresponding temperature, it is found that, although the critical 

 data are too unsatisfactory to warrant us in laying any particular 

 stress on the relationships obtained under this condition of com- 

 parison, these relationships are similar to, even if less definite 



NO. 1270, VOL. 49] 



than, those obtained at the boiling point. For a property like 

 viscosity, which alters so rapidly with temperature, a corres- 

 ponding temperature is no better as a condition of comparison 

 than the boiling point. 



The third series of comparisons was made at temperatures at 

 which dy\ldt is the same for the different liquids. Or, graphi- 

 cally, the temperatures may be defined as those corresponding 

 with points on the viscosity curves at which tangents are equally 

 inclined to the axes of coordinates. The temperatures are 

 therefore those at which temperature is exercising the same 

 effect on viscosity, and for shortness may be termed lemperahircs 

 of equal slope. The temperatures were obtained by means of 

 Slotte's formula. 



It was apparent from the shape of the curves that all the 

 liquids could not be compared at any one value of the slope, 

 because the effect of temperature on the slope varied so much 

 from substance to substance. In some cases — the whole of the 

 alcohols for example— the slope at the boiling point was con- 

 siderably greater than that at o^ in the case of some of 

 the less viscous liquids. A slope was, therefore, selected at 

 which as many liquids as possible could be compared. Another 

 slope was then obtained at which the outstanding liquids could 

 be compared with as many as possible of the liquids used at the 

 original value of the slope. The relationships between the 

 magnitudes of the viscosities of these liquids which could be 

 compared at the two slopes were then found to be the satne at 

 either slope, so that general conclusions regarding the be- 

 haviour of all the liquids could be deduced. These are as 

 follows : — 



(i) Temperatures of equal slope tend to reveal much more 

 definite relationships between the values of viscosity coefficients 

 and the chemical nature of substances than are obtained at the 

 boiling point. 



(2) In all homologous series, with the exception of those of 

 the alcohols, acids, and dichlorides, the effect of CH^ on the 

 value of the coefficient is positive, and tends to diminish as the 

 series is ascended. 



(3) Of corresponding compounds the one of highest molecular 

 weight has the highest coefficient. 



(4) Normal propyl compounds have slightly larger coefficients 

 than the corresponding allyl compounds. 



(5) An iso-compound has invariably a larger coefficient than 

 a normal compound. 



(6) In the case of other isomers the orientation of the molecule 

 and branching of the atomic chain influence the magnitudes of 

 the coefficients. Similar effects of constitution are also exhibited 

 on comparing saturated and unsaturated hydrocarbons, and the 

 variable effects produced by successive substitution of halogen 

 for hydrogen. 



(7) The alcohols, and to some extent the acids, still give 

 results which are peculiar when compared with other substances. 



As regards molecular viscosity at equal slope the following 

 conclusions may be drawn : — 



(i) For the great majority of the substances molecular vis- 

 cosity at equal slope can be calculated from fundamental 

 constants which express not only the partial effects of the atoms 

 existing in the molecule, but also those due to different atomic 

 arrangements. 



The large effects which can be attributed to the ring-grouping 

 of atoms, to the iso-linkage, to double-linkage, and to changes in 

 the condition of oxygen in its compounds, as well as the smaller 

 effects due to the accumulation of atoms of halogen in a mole- 

 cule, render evident the quantitative influence of constitution. 



(2) Of the remaining substances the chlormethanes, terra- 

 chlorethylene, ethylidene chloride and carbon bisulphide give 

 deviations from the calculated values on account of constitutive 

 influences not allowed for in obtaining the fundamental 

 constants. 



(3) The alcohols and water exhibit no agreement with the 

 calculated values. The mode in which deviations vary indi- 

 cates, in the case of the alcohols, that the disturbing factor is 

 related to their chemical nature. 



The results obtained from the consideration of molecular 

 viscosity work at equal slope, are of precisely the same nature 

 as those discussed under molecular viscosity. 



The substances which give deviations from the calculated 

 values fall into two classes. In the first the deviations are to 

 be attributed to chemical constitution, inasmuch as similar 

 disturbing effects may be detected in the magnitudes of other 



