640 
MESSRS. T. B. THORPE AND J. W. RODGER ON THE RELATIONS 
Influence of Double Linkage. 
The four available comparisons show that the molecular viscosity of a normal propyl 
comjDoimd is 41 units larger than that of the corresponding allyl compound. The 
average difference from this mean value is less than 2. The value is exactly the same 
as that brought about by the change of ethylene into acetylene bromide. This value 
corresponds with the addition of Hg and the loss of a double linkage of carbon 
atoms. Consequently if we take the value of H 2 as 89, we may assume that a double 
linkage of carbon atoms increases the molecular viscosity by 48 units. 
The iso linkage of carbon atoms, as already seen, exercises an opposite effect to 
the extent of 21 units. 
Values of the Halogens, 
Adopting the values H = 44'5, C = 31, iso linkage = — 21, double linkage = 48, 
the values of the carbon-hydrogen “rests” in the molecules of the various haloid 
compounds may be calculated, and the atomic effects of the halogens may in this way 
be obtained ; the data are given in the following tables ;— 
Value of Iodine. 
Iodide. 
)/(Z- (observed). 
“Rest” 
(calculated). 
I. 
IRetliyl. 
6.38 
164 
474 
Ethyl. 
778 
284 
494 
Propyl . 
903 
404 
499 
Isopropyl . 
878 
383 
495 
Isobutyl . 
1010 
503 
507 
Allyl. 
864 
363 
501 
Adopted value for I = 
499. 
The value for methyl iodide, the lowest member of the series, varies so much from 
the closely concordant numbers given by the other iodides that it is omitted in taking 
the mean. With this omission, which will be referred to subsequently, the average 
divergence from the mean is about 4 units. 
