BETWEEN" THE VISCOSITY OF LIQUIDS AND THEIR CHEMICAL NATURE. 607 
forces in play between molecules. Now the preceding tables, and more especially 
those given later, go to show that an increment of CHj in chemical composition, or the 
substitution of an atom of chlorine, bromine, or iodine for an atom of hydrogen, brings 
about a definite change in the viscosity magnitudes. It is therefore made evident 
that viscosity or intermolecular attraction is in reality a property of the atoms of 
which the molecules are composed. 
But besides change in molecular weight, change in the mode of grouping of the 
same atoms also affects the values of the viscosity magnitudes. The observations 
show that iso compounds have values differing from those of isomeric normal 
compounds ; ring compounds have not the values which by the study of straight chain 
compounds they might be expected to have ; compounds containing hydroxyl oxygen 
give values differing from those containing carbonyl oxygen. The same atoms must 
therefore exert different effects when differently linked together. That the effects of 
the atoms in one portion of the molecule need not be affected by change in the mode 
of linkage of the atoms in another portion is proved by the fact that the effects of 
CHg, of iodine, of bromine, etc., are the same in normal and in iso compounds. In 
the present state of the question it is impossible, however, to ascertain to what extent 
the individual effects of each atom are influenced by an alteration in the mode of 
grouping in a given portion of a molecule. Hence the method adopted in deducing 
fundamental constants is to assume that certain atoms retain the same values under 
all conditions whilst the collective change in the values of those atoms which are 
affected by the mode of linkage is, when possible, expressed either as a new constant 
—the value of an iso linkage, a double linkage, etc.—or by saying that a particular 
atom has assumed a new value, e.g., carbonyl oxygen, hydroxyl oxygen, etc. In some 
cases the method of calculation may lead to the result that a negative constaiit is 
ascribed to a particular atom. In deducing the values of carbon and hydrogen, for 
example, it is implied that in a CHj group and in the molecule of a normal paraffin 
the individual effect of each atom of carbon or of hydrogen is the same. The above 
reasoning and the actual results show, however, that this is not the case. The effects 
exerted by carbon and hydrogen in a CHg group are different from those in a CH^ 
group. Since this constitutive effect cannot be correctly apportioned amongst the 
atoms concerned in it, the fundamental constant of an element may for this reason 
come out negative although tbe absolute effect exerted by any atom is doubtless 
positive. 
For these reasons fundamental constants are to be regarded as empirically 
ascertained magnitudes which are merely intended to indicate how far the observed 
results may be represented as the sum of partial values which are the same for all 
substances. They have no reference to the possible behaviour of the elements when 
in the free state; they simply serve to show how far definite changes in chemical 
composition correspond with definite changes in the viscosity magnitudes. 
