171 



disc, and the great inclination and eccentricity of its orbit, may also 

 be considered as a true asteroid. 



Dr. Herschel concludes by observing, that the specific difference 

 between planets and asteroids appears now, by the addition of a third 

 individual of the latter species, to be more fully established ; and that 

 circumstance, he thinks, has added more to the ornament of our 

 system, than the discover)' of another planet could have done. 



An Essay on the Cohesion of Fluids. By Thomas Young, M.D. For. 

 Sec. R.S. Read December 20, 1804. [Phil. Trans. 1805, p. 65.] 



Dr. Young's principal objects in this paper are to reduce the phe- 

 nomena of the capillary action of fluids to the general law of an 

 equable tension of their surfaces ; to investigate the properties of the 

 curves resulting from this law ; to determine the magnitude of the 

 apparent adhesion of solids to fluids, and the cohesion of moistened 

 solids ; and to show in what manner the law itself is probably de- 

 rived from the fundamental properties of matter. 



Dr. Young observes, that a fluid which is not capable of wetting 

 a given solid, forms with it an angle of contact which is constant in 

 all circumstances ; that the curvature of the surface of a fluid, or the 

 sum of the curvatures, where the curvature is double, is always pro- 

 portional to the elevation or depression with respect to the general 

 surface, and that the curve itself admits, in all cases, an approximate 

 delineation by mechanical means, but is not always capable of being 

 easily subjected to calculation. When, however, the curvature is 

 simple, the direction of the surface, at any given height, admits a 

 correct determination. Hence the elevation of a fluid in contact with 

 a given surface, whether vertical, horizontal, or inclined, is deduced 

 from its ascent between plates, or in a tube, of the same substance ; 

 and the result is shown to agree with experiments. Thus, taking 

 -ffth of an inch for the diameter of a tube, in which water rises to 

 the height of an inch, it is inferred that the apparent adhesion of 

 water, to a square inch of any horizontal surface capable of being 

 wetted by it, must be 50^ grains, which is only half a grain more 

 than the result of Taylor's experiments. The adhesion of alcohol, 

 and of sulphuric acid, as measured by Achard, are also found to 

 agree with the ascent of those fluids in capillary tubes. Lord Charles 

 Cavendish's table of the depression of mercury in barometer tubes, 

 is compared with the same principles by means of diagrams con- 

 structed for each particular case ; and the apparent adhesion of the 

 surface of mercury to glass, as well as the depth of a portion of mer- 

 cury spread on a plate of glass, is deduced from these measures, and 

 is shown to agree with experiments. The observations of Morveau, 

 on the attraction of the different metals to mercury, are also dis- 

 cussed ; and some remarks are made on the magnitude of drops of 

 various substances. 



The hydrostatic actions of these elevations and depressions of 

 fluids are such as to afford a ready explanation of the attractions 



