1904.] Cotton ly Water and by Water Vapour. 251 



observed, it must certainly occur, and that the air which adheres to 

 each fibre and fills all interstices maintains that separation of water 

 and cotton surfaces which is necessary for distillation to occur from 

 one to the other. 



Two questions remain to be discussed, viz., the probable fate of the 

 vapour after it has been condensed on the surface of the cotton, and 

 the bearing of the conclusions here arrived at on the nature of the 

 Pouillet effect in general. 



As to the first question there seem to be three possible views. One 

 of these the view that it becomes chemically combined to form 

 definite hydrates of cellulose may be quickly dismissed, as there are 

 no facts to support it and as the evidence points to the absence of any 

 definite limit to the action. The ease with which the action is reversed 

 (for the cotton parts with practically all its moisture if kept for a day 

 at the ordinary temperature over sulphuric acid) is not by itself 

 evidence against the chemical theory, but it may be mentioned here. 

 At the opposite extreme is the view that the condensed moisture forms 

 and remains as a liquid film on the surface of the solid. If it is so, the 

 film must in time become a substantial one, as the following calculation 

 shows. 



The average diameter of cotton fibres varies, according to the 

 quality, between 64 x 10~ 5 and 84 x 10~ 5 inch.* The mean of these 

 may be taken as the most probable average diameter in the present 

 case, viz., 74 x 10~ 5 inch, or 188 x 10~ 5 cm. Actual measurement of fibres 

 under the microscope led to practically the same result but were not 

 numerous enough to give a perfectly trustworthy average value. The 

 density of cellulose may be taken as 1*525 (Clarke's Specific Gravity 

 Table). The total surface of any given weight (w) of cotton may be 

 calculated on the assumption that the fibres are uniform cylinders of 

 such length that the ends are negligible, in which case the area is 

 4^/1 *525 x 188 x 10~ 5 = 1395w sq. cm. If m grammes of water be 

 deposited on this surface as a film, the thickness of the film is 

 w/1395w sq. cm. In Experiment 20 of Series V (Table V) w was 

 0'90 gramme and m was shown to have attained the value 0*188 gramme 

 at the end of 12 hours. In this case, therefore, the thickness of the 

 film would be about 15 x 10~ 5 cm. Now Dr. G. J. Parks f concluded 

 from similar calculations that in all such cases "the thickness of the 

 surface film varies from" 10 x 10~ 6 to 80xlO~ 6 cm. according to the 

 substances used and the conditions of temperature and pressure, and for 

 the water film on glass in saturated vapour at 15 C. the thickness is 

 about 13'4x 10~ 6 cm." Hence the film on cotton in the above case 

 was considerably outside Dr. Parks's superior limit, and it had by no 



* Thorpe's ' Diet, of Techn. Chem.,' vol. 1, p. 613. 



f " On the Thickness of the Liquid Film formed by Condensation at the Surface 

 of a Solid," ' Phil. Mag.,' May, 1903, p. 517. 



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