550 
MESSRS. T. E. THORPE AND J. W. RODGER ON THE RELATIONS 
paraffins the curves are all of the same general shape; their order and disposition are 
such as to exhibit a direct connection between the chemical nature of the substances 
and the magnitudes of their viscosity coefficients. The curve for any paraffin always 
lies below and to the left of that of its next higher homologue ; and, further, the curve 
for the isoparaffin always lies below and to the left of that for the normal paraffin. 
All the curves tend to approach as the temperature rises, but it is noticeable that at 
the same temperature the distance between the curves of isomeric paraffins is larger 
the higher the molecular weight. At the same temperature the viscosity is greater 
the higher the molecular weight and is lower for an iso- than for a normal compound. 
Another striking feature in the series is that all the curves stop almost exactly on a 
line drawn parallel to the horizontal axis. This means, of course, that at their respec¬ 
tive boiling points the paraffins have almost exactly the same viscosity coefficient. 
Unsaturated Hi/drocarhons. 
Only three unsaturated hydrocarbons were investigated, viz., isoprene, /3-isoamyleue, 
and diailyl. Fig. 6 represents the results. As in the case of the paraffins, the 
curve for isoprene CgHg lies to the left of that for diailyl CgH^Q; each of these 
curves is also disposed in the same way with reference to the corresponding iso¬ 
paraffin ; that of isoprene being to the left of that of isopentane ; and that of diailyl 
to the left of that of isohexane. The curve for yS-isoamylene lies also to the left of 
that of isopentane, and is very close to that of isoprene. Isoprene CgHg gives 
a curve, however, which lies uniformly to the right of that for isoamylene CgH^g, 
although the molecular weight of the latter compound is very slightly the higher. 
Hence it would appear that differences in constitution may, in some cases, modify 
the influence of molecular weight. When, however, we compare strictly homologous 
hydrocarbons, it would appear that the relative position of the viscosity curves is 
mainly dependent on molecular weight. 
