AT THE BOUNDARY OF A LIQUID lY MOTIOY. 
563 
The apparatus was then again set up. 
Temperature. 
Time of flow. 
18-4 
/ 
7 
52-6 
18-3 
7 
52-2 
18-2 
7 
52-4 
Means 18 30 
7 
.52-4 
The tube was then disconnected, dried, and weighed. Weight = 16'3931 grins., 
practically the same as before. The silver was then dissolved off, and the tube cleaned 
and dried. Weight = 16’3916. 
The weight of silver deposited is, therefore, ’0015 grni., and its thickness 
•000014 cm. 
This gives a change in equivalent to O'12 per cent. The time of flow for the 
unsilvered tube, corrected for change of radius, is 7' 5l"‘4, while the observed time for 
the silvered tube is 7' 52"‘4. The difference of about 0’2 per cent, is probably due to 
slight irregularities in the thickness of the silver layer. 
Two objections may be raised to this experiment. The first is that with such a 
thin film, the action between the water and the glass might still be effective, and 
prevent any slipping. When we remember, however, that the sphere of action of 
molecular forces is only about 10“® cm., we see that no direct action can occur across 
a distance of 10“® cm., and it is exceedingly unlikely that a layer of silver more than 
1000 molecules thick, should be pervious to water, and thus allow of contact with the 
glass. In order, however, to entirely meet this objection, experiments were made 
with considerably thicker layers. 
The second objection is that the silver might be deposited so irregularly that the 
choking effect might mask the quickening due to slip. It had been found, in a series 
of preliminary experiments, that if a tube was used whose diameter was much less than 
a millimetre, it was exceedingly difficult to get a uniform silver deposit, and the time 
of flow for the silvered tube was always much greater than for the unsilvered. To 
prevent this the tube had to be silvered in a vertical position, and various details, only 
to be learnt by experience, attended to. The time of flow for the silvered tube could, 
however, never be brought below that for the plain one, although as greater care was 
taken in the silvering it continually approximated to it, and this is conclusive against 
the existence of an effect at all comparable with that given by Helmholtz for polished 
gold, or with that which he deduced from Girard’s experiments on copper tubes. As 
the choking effect was naturally greater for small tubes, a series of experiments was 
next made on some of rather greater diameter. 
4 c 2 
