1910] on Problems in Capillarity 789 



obtained : from whence we deduce that the rate of diminution of 

 pressure is proportional to the pressure. 



The actual quantities dealt with in a typical instance are as 

 follows. 50 mm. of the alcohol manometer scale are equivalent to 

 1 mm. water pressure : — 



Observations of pressure with time were : 



P (mm. HoO) 4-4 3-15 2-4 1-31 082 0-44 

 t (minutes) J 1 2 3 4 



Volume of enclosed space = 30 c.c. 



Area of membrane = 0*049 sq. cm. (0*25 cm. diameter). 

 Fall of pressure in first ^ minute = 1'25 mm. water = 0-00012 atmosphere. 

 .-. Volume of air passed in first h minute = 30 x 0-00012 = 0-0036 c.c. 



= 10-45 cc. per day. 



.-. Volume passed through 1 sq. cm. in 1 day = — = 209 c.c. 



0-049 



The mean water pressure during experiment was 3-8 mm., or 

 0*00037 atmosphere. 



On the assumption that the rate of diffusion remains proportional 

 to the pressure up to one atmosphere, then the extrapolated value of 

 the rate per sq. cm. per day at atmospheric pressure wotild be 

 505 litres. 



A similar determination from the fall of pressure in the second 

 whole minute gives a value of 930 c.c. per day per sq. cm. at a mean 

 pressure of 1-86 mm. of water. If this be extrapolated to one 

 atmosphere, the rate per sq. cm. becomes 520 litres. 



Compared with this, the rubber membrane O'OOl cm. thick had 

 a rate of 2 c.c. of air per sq. cm. per day. The mean thickness of 

 the black collodion film is • 000003 cm. or y-J^oths of the rubber. 

 The rate of air transit through the collodion at one atmosphere 

 appears thus to be 200,000 times that of rubber. The square of the 

 ratio of the thicknesses is roughly of the same order as the inverse 

 ratio of the diifusion rates. 



The corresponding extrapolated values at one atmosphere were 592 

 for a single black film, 30 for the double film (whose colour appeared 

 very pale golden), and just under 1 for the triple film (reflecting a 

 brown-magenta colour). The extraordinary diminution of gas per- 

 colation thus observed is very remarkable. 



Collodion Films : Microscopic Examixatiox. 



A microscopic examination seems to throw some light on this 

 question of gas transit. A regular distribution of minute bubbles is 

 revealed, which occur largely in colonies composed of one or two 

 larger bu])bles with ten or twenty very small ones surrounding them 

 like small patches of froth. These patches seldom exceed • 025 mm. 



