276 
MR. J. B. HANNAY ON THE MICRORHEOMETER. 
term “ capillary ” for tubes which adhere to this law. Poiseiulle was intending to 
apply his studies to the action of various substances when in the capillary vessels in 
the living body, and hence he examined such bodies as alcohol, blood, and various 
saline solutions. On examining' the rate of flow of various mixtures of alcohol and 
water, it was found that a mixture of about equal quantities of alcohol and water 
required about three times as much time for passage as either pure alcohol or pure 
water. The mixture corresponded to a hexhydrate of alcohol, so that, in this case, 
Poiseiulle discovered a chemical compound by purely physical means. Some results 
were obtained by dissolving various salts in water, and finding the rates of flow of the 
solutions; but as there appeared to be no connexion between the combining weights 
and the rates, Poiseiulle could offer no explanation of the numbers. One thing can 
be gathered from Poiseiulle’s work—that the flow of a liquid through a capillary 
tube is very seldom comparable with its physiological action in the veins. It was not 
for many years after this that any attention was paid to the subject, till Graham,* in 
1861, noticing the existence of the hydrate of alcohol discovered by Poiseiulle, made 
some experiments to find whether any hydrates of acids could be discovered by this 
method ; and his researches showed that several acids took up water by three mole¬ 
cules at a time, each addition being shown by a further retardation, but always fainter 
than the previous one. This addition to the work of Poiseiulle showed that the 
capillary tube might become an important instrument in the hands of the chemist. 
Lately,! M. Aug. Guerout has presented several papers on the subject to the Academy 
of Sciences, Paris, giving an account of some experiments on the rate of flow of various 
alcohols, in which he shows that the rate decreases while the molecular volume 
increases, but not in any definite proportion. Guerout, however, overlooks the 
most important factor in this kind of investigation, namely, the temperature. He 
experiments with all bis alcohols at the same temperature, while there is a difference 
of 60° between the boiling points. Now in such a substance as water, a difference 
of 60° means that the flow is three times as great at the high as at the low tempera¬ 
ture, so that some of the alcohols which by Guerout’s method seemed to offer the 
greatest retardation, may really, if compared under just conditions, have the least 
friction. As regards liquids, it would appear from the researches of Kopp that in 
order to obtain chemical deductions from their physical examination they must be 
examined when their vapour tensions are equal, as then they exist under somewhat 
comparable conditions as to mobility and expansion. Guerout has also in the second 
of these papers determined the effect of variation of temperature on the rate of flow, 
and comes to the conclusion that the rate for water varies by a constant factor 
whose value is 1'025. The experiments were, however, done within a very narrow 
range, viz. : 10° to 17°; and Guerout seems to have been ignorant of the fact that 
his countryman, Poiseiulle, carefully determined this some forty years earlier, and 
* Phil. Trans., 18G1, page 373. 
t ‘ Compt. Rend.,’ lxxix., 1201; lxxxi., 1025. 
