Contact- Angle of Liquids and Solids. 167 



The bubble was blown from a glass pipette. By means of the 

 levelling-screws of the tank the bubble could readily be placed 

 central under the lens. Its horizontal section appeared always 

 to be strictly circular. The curvature of the lens' surface was 

 such that bubbles with diameters ranging from 25 millim. to 

 150 millim. could be examined. In all the experiments the 

 liquids seemed to readily and completely wet the lens, and the 

 bubbles formed moved freely and without distortion for slight 

 changes of the levelling-screws. 



The tank stood on a heavy slate slab mounted on a levelling- 

 table. To the slab was cemented a plane plate of plate glass, 

 which was set horizontal by means of the levelling-screws of 

 the table. Upon this plate slid freely a smaller plate of glass 

 carrying a pair of Y's, in which rested a microscope. By 

 reversals and adjustments of the Y's, the axis of the micro- 

 scope was placed horizontal. The microscope carried a 4-inch 

 object-glass, and was furnished with an eyepiece-micrometer. 

 The arrangement was, in fact, a simple microscope-catheto- 

 meter*. With it the measurements of the distance q were 

 made. At about two or three metres from the tank, and at 

 the same level as the bubble, the collimator of a spectroscope 

 was set up with the slit horizontal, and illuminated by a gas- 

 flame or by an incandescent electric lamp. An image of the 

 slit was formed by reflexion at the surface of the bubble, 

 along the line in which that surface was met by the greatest 

 horizontal section. The position of this image was deter- 

 mined by the eyepiece-micrometer. Slight variations in the 

 height of the slit made no measurable changes in the apparent 

 position of the image. To enable the position of the bottom 

 of the bubble to be determined, use was made of a fine glass 

 index or pointer. This was carried in a clamp attached to 

 the tank, and could be moved vertically by means of a fine 

 screw until its point came in contact with the bottom of the 

 bubble. The position of the point of contact could be very 

 exactly determined by the eyepiece-micrometer from the 

 apparent contact, at that point, of the pointer and of its image 

 rejected from the surface of the bubble. The distance from 

 the image of the slit to the point of contact of the pointer, 

 measured by the eye-piece micrometer, is the distance q. 



The measurements of the distance k were made by a 

 method which 1 have elsewhere described f. A vertical mi- 

 croscope mounted on an arm carried on the sliding piece 

 of a cathetometer was placed so as to be as nearly as possible 

 over the centre of the lens and of the bubble. On the face 



* Quincke I' ;_'_-. Ann. cv. p. I (1858). 



t Amer. Journ. Science, xxxi. p. 189 (1886). 



