402 
MESSRS. T. E. THORPE AND .J. W. RODGER ON THE RELATIONS 
observed time by the specific volume at that temperature. The values of Z, for so-called 
“ equivalent amounts,” were not given with the highest attainable accuracy, inasmuch 
as the thermal expansion of certain of the liquids was unknown. As the range of 
temperature over which Eellstab’s observations extended was only from 10° to 50°, 
and as the relative densities of all the liquids experimented upon were known at 20°, 
he employed in all cases the specific volume at 20°, instead of the true specific 
volume at the temperature of observation. The error thus introduced depends 
upon the difference between the coefficients of thermal expansion of the hquids 
under investigation, and may amount to three or four per cent, at the higher 
temperatures. The liquids investigated by Eellstab were alcohols of the 
series, certain of the fatty acids, a number of compound ethers (esters), aldehydes, 
and a few aromatic derivatives. 
Since the transpiration-time necessarily alters with the temperature, and at a 
rate varying with each liquid, it was of fundamental importance to determine the 
particular temperature at which the comparison between the individual results 
should be made. Eellstab assumed, with Kopp, that the temperatures at which 
the various liquids possessed the same vapour-pressure might be considered as 
comparable, and adopting Landolt’s values for the vapour-pressures, he compared 
the transpiration-times of “ equivalent amounts ” of the acids of the series 
at a number of comparable temperatures between 0° and 50°. 
The general result of the observations was to show that in the case of this series 
of acids the transpiration-time decreases with increasing molecular weight in passing 
from formic acid to acetic acid, and from acetic acid to propionic acid, but that the 
differences between the values for the several pairs of acids become less and less as 
the temperature rises until they become constant. On passing from propionic acid 
to normal butyric acid, from butyric acid to valeric acid, from valeric acid to caproic 
acid, the transpiration-times increase with increasing molecular weight, and the 
differences between the values for any pair of successive homologues at “ comparable 
temperatures ” become less and less with increasing temperature, as in the first case, 
and tend apparently to become constant. No simple relation either between the 
transpiration-times and the molecular weights or between these times and the vapour- 
pressures could be traced by Eellstab. Hence, in the rest of his memoir, Eellstab 
simply follows Graham’s suggestion, and compares the transpiration-times of 
“ equivalent amounts ” of the various liquids, whenever possible, at 50°, the highest 
temperature to which his experiments extended. 
The main conclusions which Eellstab deduces from his observations may be thus 
summarised :— 
1. The transpiration-time of all liquid substances decreases with the temperature. 
The decrease for equal intervals is most marked, the longer the time of efffux and the 
lowmr the temperature. 
2. An increment of CH 2 , in an homologous series, is in general accompanied by an 
