RliPORT ON CAPILLARY ATTRACTION. 277 



formula involving the angles of contact proper to the fluids when 

 separately contained in the tube. 



When water and mercury are put in the same cylindrical glass 

 tube, and the water completely wets the tube, the mercury may 

 be considered to be contained in an aqueous tube, the action of 

 which on the mercury being small, the angle of contact is nearly 

 180° instead of 136°'8, which, according to Laplace, is that be- 

 tween glass and mercury. This result, which is also deduced 

 from the above-mentioned formula, is confirmed by the observa- 

 tions of Gay-Lvissac. Another deduction from the theory also 

 receives confirmation from the same eminent experimenter, viz. 

 that mercury is less depressed in a capillary tube when its upper 

 surface is covered with a small portion of water, than when 

 covered by alcohol. For the capillary action of water on itself 

 exceeds that of alcohol on itself, and would therefore be likely 

 to have a greater action than alcohol on mcrcur^^ 



Various other interesting results, which it would be long to 

 enumerate here, are readily deduced by the method of consider- 

 ing capillary effects exhibited by the author in his Supple- 

 ment. This method, in some applications, leads to results 

 more rapidly than that of the Treatise ; wliile at the same 

 time the latter has advantages peculiar to itself in all questions 

 relating to the surfaces of fluid inclosed in capillary spaces, or 

 subject in any way to capillary action. Three such qviestions, 

 which had been either omitted or partially treated in the first 

 work, are handled at considerable length in the Supplement ; 

 and it will be proper now to advert to their solutions. These 

 problems, which for the most part had previously engaged the 

 attention of Dr. Young, are : (1.) The apparent attraction and 

 repulsion of small bodies swimming on the surfaces of fluids. 

 (2.) The adhesion of discs to the surfaces of fluids. (3.) The 

 figure of a large drop of mercury, and the depression of mercury 

 in a glass tube of a large diameter. 



(1.) By considering generally the capillary action between 

 two vertical and parallel planes of different matters plunged with 

 the lower extremities in tlie same fluid, the following theorem 

 is obtained : Whatever be the substances of which the planes 

 are formed, the tendency of each towards the other is equal to 

 the weight of a fluid prism whose height is the difi"erence of ele- 

 vation of the extreme points of contact of the fluid on the oppo- 

 site sides of the plane, whose depth is half the sum of these 

 elevations, and breadth, that of the planes. The elevation is to 

 be taken negatively when it changes into a depression ; and if 

 the product of these three dimensions be negative, the apparent 

 attraction of the planes becomes a repulsion. These tendencies 



