certain Dilute Aqueous Solutions. 375 



prong of a vibrating fork, and which clipped into the liquid 

 to be experimented upon. Then, " in order to see the waves 

 well, the light was made intermittent in a period equal to 

 that of the waves, and Foucault's optical method, for rendering 

 visible small departures from truth in plane or spherical 

 reflecting-surfaces, was employed/' The liquid was contained 

 in a shallow porcelain tray, and its surface was cleaned by a 

 flexible brass hoop wider than the depth of the liquid. This 

 was placed in the liquid so as to include the plate that 

 generated the waves, but otherwise in as contracted a position 

 as possible. It was then expanded to its maximum extent. 

 This gave a fresh surface as clean as the body of the liquid. 

 The mean of his results for water is T = 74 dynes per centi- 

 metre at 18° C, and this is probably correct to about one per 

 cent. 



Description of Apparatus. 



I shall now describe the apparatus used in this work. 

 It is simply a natural development of Lord Rayleigh's 

 although it contains many new features. 



The arrangement of apparatus is represented diagram- 

 matically in fig. 1. S is a sixteen-candle-power incan- 

 descent electric lamp ; S 2 consists of two narrow strips of 

 thin copper, one of which is fastened to each prong of the 

 large tuning-fork I\, and each has near its upper end a small 

 hole so placed that when the fork is at rest these holes shall 

 lie one directly behind the other, in a horizontal line. F 2 is 

 the fork which generates the ripples by means of the plate of 

 glass (P) 2"5 by 9*7 cm. Li and L 2 are two small double- 

 convex lenses. D is the vessel for holding the liquid to be 

 experimented upon ; it is a porcelain developing-tray 2"3 by 

 29 by 36 centimetres. E is a dividing-engine furnished with 

 a screw whose pitch is 1/0328 millim. M l and M 2 are silver- 

 on-glass mirrors. T is a telescope. To the carriage of the 

 dividing-engine are rigidly fastened a wooden arm ( B) and a 

 brass tube (A) ; the tube is parallel to the wooden arm and 

 is fastened to it by means of V. To the end of B is fastened 

 the mirror M 2 by means of brass mountings that allow it to 

 be turned and clamped in any position desired. Through the 

 tube A passes a brass rod (R) to which is fastened the mirror 

 M^ This allows Mx to be rotated about the axis of the tube, 

 and it is clamped in the necessaiy position by means of the 

 screws I and K. W is an iron weight hung from a rod 

 fastened to the carriage, and is intended to counterbalance 

 the rod, mirrors, etc. 



