154 Prof. T. E. Thorpe uiid Mr. J. W. Rodger. [Feb. 22, 



whilst the change in the values of those atoms which are affected by 

 the mode of linkage is, when possible, expressed either by a new con- 

 stant the value of an iso-linkage, a doable linkage, <fcc., or by saying 

 that a particular atom has assumed a new value, e.g., carbonyl oxy- 

 gen, hydroxyl oxygen, <kc. In some cases the method of calculation 

 may lead to the result that a negative constant is ascribed to a particular 

 atom. In deducing the values of carbon and hydrogen, for example, 

 it is implied that in a CH* group and in the molecule of a paraffin 

 the individual effect of each atom of carbon or of hydrogen is the same. 

 The above facts, and the reasoning based upon them, show that this 

 is not the case, and although the absolute effect exerted by each atoi 

 upon the viscosity is positive, the fundamental constant of an atoi 

 may be negative, as it may involve a constitutive effect, which 

 present cannot be localised in a particular region of the molecule 

 For these reasons fundamental constants are to be regarded as em- 

 pirically ascertained magnitudes, which are merely intended to indi- 

 cate how far the observed results may be represented as the sum 

 partial values which are the same for all substances. They have 

 reference to the possible behaviour of the elements when in the ft 

 state ; they simply show how far definite changes in chemical uatui 

 correspond with definite changes in viscosity. 



Tables are given which show the concordance between the observe 

 molecular viscosity and those calculated by means of these constant 

 In the case of forty-five liquids the difference between the observed i 

 calculated values rarely exceeds 5 per cent. In the case of the isomei 

 ke tones and aromatic hydrocarbons, the differences are in part due 

 constitutive influences, which cannot at present be allowed for in ob- 

 taining the calculated values. 



In a second table are given those substances for which the diffe 

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

 nnsatnrated hydrocarbons, polyhalogen compounds, formic and aceti< 

 acids, benzene, water, and the alcohols. 



Similar fundamental constants for molecular viscosity work at . 

 boiling point have also been deduced. These are given in the tal 

 on p. 155. 



Tables are also given showing the comparison between the observe- 

 and calculated numbers, the substances being classified into two 

 groups, as in the case of molecular viscosity, according as the differ- 

 ences are less or greater than about 5 per cent. 



On taking a general survey of the comparisons at the boiling point, 

 it ia evident that for the majority of the snbstances examined the 

 paraffins and their monohalogen derivatives, the sulphides, the 

 ketones, the oxides, and most of the acids and the aromatic hydro, 

 carbons molecular viscosity and molecular viscosity work may be 



