ON CAPILLARY ACTION. / 



Mr. I^aplace's second method of considering tlie effects Laplace's se- 



„ .„ , ... J cond method 



of capillary action, though not wholly new, is ingenious and iaceiuoi.s, but 



satisfactory: but it requires the assistance of the same hypo- requues the 



. '' , ^ . K,._ sanje hypothe- 



thesis respecting repulsion, as is necessary to Ijis first theory, sis of repulsion 



The attraction of a capillary tube A B (Fig. 7) on the co- 

 lumn C consists of two equal parts, one of which is de- 

 rived from the action of the part,,D E F G on the upper 

 portion of the fluid at C, the other from that of the end of 

 the tube at H 1 upon the portion below at K ; and these 

 two forces are opposed by the attraction of L M, the part of 

 the jffuid forming a continuation of the solid, which draws 

 the column downwards in the same manner as each of the * 



other forces draws it upwards : so that the weight of the fluid - 

 elevated must be proportional to the excess of twice the den- - 

 sity of the solid above that of the fluid. Supposing the 

 fluid to be elevated in a verj'^ narrow space of a given 

 breadth, the half of this breadth being the radius, the se- 

 cant of the angle of contact will become equal to the radius 

 of curvature of the surface, which is always inversely as the 

 height of the elevated column ; hence the cosine of the an- 

 gle of Contact will be directly as the height, that is, as the 

 difference between the density of the fluid and twice that of 

 t|ie solid, the whole density of the fluid being represented 

 by the radius ; and this determination agrees precisely with 

 the former. 



Mr. Laplace has very justly observed, that where two Laplace's ob- 



floatiniJ: bodies are surrounded by an elevation and a depres- servation on 



1-1 1 • 1 • 1 I • 1 ■ -HI unequal eleva- 



sion which are unequal in height, their repulsion will become tions and de- 



a maximum at a certain distance, and upon a still nearer pressions of a 

 approach will be changed into an attraction. When the floatuig bodies 

 distance is very small, the height of the fluid elevated be-J"st. 

 tween the bodies is the mean of the heights to which it 

 would be raised between two similar portions of the respect- 

 ive substances, and hence the magnitude of the force may 



be readily determined. Dr. Youncr seems to have consider- J^^^^"°^ consi- 



^ p dered by Dr. 



ed only the case of an equal depression and elevation. Young. 



As an illustration of the combined effects of the forces of Combineii ef- 



cohesion and repulsion in the constitution of natural bodies, ^^'^^'^ of cohe- 

 T1111-- 1^. . oi . sion and repul- 



1 shall subjoin a short investigation of the magnitude of the sion in the con. 



attractive power which retains the particles in solids and stuutionofna- 



fluids 



