^5^ 



ON THE COHESION OP FLUIDS. 



the elevation of a fluid in contact with a ho- 

 rizontal surface, tiie ordinate may be deter- 

 mined from the weight required to produce 

 A separation ; and the appropriate rectangle 

 may be found in this manner also, the angle 

 of contact being properly considered, in this 

 as well as in tlie former case. It will ap- 

 pear that these experiments by no means ex- 

 hibit an immediate measure of the mutual 

 attraction of the solid and fluid, as some 

 authors have supposed. 



Sir Isaac Newton asserts, in his Queries, 

 that water ascends between two plates of glass 

 at the distance of one hundredth of an inch, 

 to the height of about one inch ; the product 

 of the distance and the height being about 

 .01 ; but this appears to be much too little. 

 In the best experiment of Musschenbroek, 

 with a tube, half of the product was .0196; 

 in several of VVeitbrecht, apparently very ac- 

 curate, .0214. In Monge's experiments on 

 plates, the product was 2.6 or 2.7 lines, or 

 about .0210. Mr. Atwood says, that for tubes, 

 the product is .0530, half of which is .0265. 

 Until more accurate experiments shall have 

 been made, we may be contented to assume 

 .02 for the rectangle appropriate to water, 

 and .04 for the product of the height in a 

 tube by its bore. Hence, when the curve 

 becomes infinite, its greatest ordinate is .2, 

 and the height of the vertical portion, or the 

 height of ascent against a single vertical plane, 

 • 14, or nearly one seventh of an inch. 



Now when the horizontal surface of a so- 

 lid is raised from a vessel of water, the sur- 

 face of the water is formed into a lintcaria, 

 to which the solid is a tangent at its highest 

 point, and if the solid be still further raised, 

 the water will separate: the surface of the 

 water, being horizontal at the point of con- 

 tact, cannot add to the weight tending to 



depress the solid, ■which is therefore simply 

 the hydrostatic pressure o^ a column of wa- 

 ter equal in height to the elevation, in this 

 case one fifth of an inch, and standing on 

 the given surface. The weight of such a co- 

 lumn will be 50^ grains for each square 

 inch; and in Taylor's well known experi- 

 ment, the weight required was 50 grains. 

 But, when the solid employed is small, the 

 curvature of the horizontal section of the wa- 

 ter, which is convex externally, will tend to 

 counteract the vertical curvature,, and to di- 

 minish the height of separation ; thus, if a 

 disc of an inch in diameter were employed, 

 the curvature in this direction would perhaps 

 betquivalent to the pressure of about one 

 hundredth of an inch, and might reduce the 

 height from .2 to about .19, and the weight 

 in the same proportion. There is, however, 

 as great a diversity in the results of diflferenl 

 experiments on the force required to elevate 

 a solid from the surface of a fluid, as in those 

 of the experiments on capillary tubes ; and 

 indeed the sources of error appear to be here 

 more numerous. Mr. Achard found that a 

 disc of glass, l^- inch French in diameter, 

 required, at 69° Fahrenheit, a weight of 91 

 French. grains to raise it from the surface of 

 water; this is only 37 English grains for 

 each square inch ; at 44^° the force was -^ 

 greater, or 39^ grains ; the difference being 

 yjT^ for each degree of Fahrenheit. It might 

 be inferred, from these experiments, that 

 the height of ascent in a tube of a given 

 bore, which varies in the duplicate ratio 

 of the height of adhesion, is diminished 

 about -rr^ for every degree of Fahrenheit 

 that the temperature is raised above 50°; 

 there was, however, probably some consider- 

 able source of error in Achard's experiments, 

 for I find that this diminution does not ex- 



