490 



EEPORT 1880. 



tangential stiffness into the oil-sheet, which prevented the oil taking up the tan* 

 gential motion of the water beneath. Several other phenomena were also men- 

 tioned. The author hopes shortly to publish a fuU account of the investigation. 



6. Experiments on thin Films of Water, with regard to their absorption of 

 Eadiant Heat. By the Hon. F. A. R. Russell. 



The experiments, the general results of which are given below, were made 

 with the object of ascertaining the diathermancy of water in very thin films, and 

 these experiments afforded incidentally an opportunity of observing the behaviour 

 of films subject to varying conditions. 



The arrangement of instruments was similar to that illustrated at p. 383 of 

 Prof. Tyndall's ' Heat as a Mode of Motion.' The instruments used were : a dead- 

 beat mirror galvanometer and scale, a thermopile, and a screen. The soap film 

 was carried by a piece of a cork sole perforated by a hole slightly larger than the 

 hole in the screen, about 1 j inches in diameter. The sources of heat were (1) a 

 copper or iron ball heated from behind by a small gas flame ; (2) a common gas 

 flame from a Bunsen burner, and (3) a hydrogen flame in air. 



The film was mostly made from a solution of about half a drachm of shavings 

 of Castile soap, dissolved 5 to 15 minutes in about 5 cubic inches of water, at 60° 

 Fahrenheit. 



The film soon after being placed perpendicularly at the orifice hi the screen 

 exhibited coloured bauds, which descended in regular succession until the last 

 band appeared, which contained a bright blue line. The descent of the bands 

 continued at a slackened rate tiU the grey, and finally the black, occupied a portion 

 of the upper half of the film, which half was alone subject to experiment. A 

 condition more or less of equilibrium then prevailed, the tension of the black 

 portion counteracting the force of gravity. A light yellow or bronze was always 

 the last colour to appear, and preceded the white or grey, which again was 

 succeeded by black. When there was any black in the film, the bursting of the 

 film was marked by a slight click or snapping sound. The best films lasted 

 frequently between 10 and 30 minutes, and sometimes the black portion alone 

 was under observation 15 or 20 minutes. 



The following table shows the absorption per cent, for each of the three sources 

 of heat, and the thickness of the film, as derived from a table in Watts's ' Dictionary 

 of Chemistry,' giving Newton's thicknesses of thin films of air, water, and glass. A 

 table in Cooke's 'New Chemistry' gives the thicknesses of soap-films as consider- 

 ably greater than those stated in Newton's table. The ' light film ' of Cooke 

 corresponds to my ' grey,' and his ' grey ' to my ' fine grey.' Newton's ' white ' 

 corresponds to my ' grey.' The refractive index of the solution used by me was 

 1"34 and 1'35, a little higher than that of pure water. 



' The absorption in this case is deduced from that of a film containing a portion 

 of grey, the absorption of the grey being known. 



