SECT. XIV. CAPILLARY ATTBACTIOK 113 



platinum, while drawing the particles of the two gases towards its 

 surface by its great cohesive attraction, "brings them so near to 

 one another that they come within the sphere of their mutual 

 affinity, and a chemical combination takes place. Dr. Faraday 

 attributes the eifect in part also to a diminution in the elasticity of 

 the gaseous particles on their sides adjacent to the platinum, and 

 to their perfect mixture or association, as well as to the positive 

 action of the metal in condensing them against its surface by its 

 attractive force. The particles when chemically united run off 

 the surface of the metal in the form of water by their gravitation, 

 or pass away as aqueous vapour and make way for others. 



The oscillations of the atmosphere, and the changes in its 

 temperature, are measured by variations in the heights of the 

 barometer and thermometer. But the actual length of the liquid 

 columns depends not only upon the force of gravitation, but upon 

 the cohesive force or reciprocal attraction between the molecules 

 of the liquid and those of the tube containing it. This peculiar 

 action of the cohesive force is called capillary attraction or capil- 

 larity. If a glass tube of extremely fine bore, such as a small 

 thermometer tube, be plunged into a cup of water or spirit of 

 wine, the liquid will immediately rise in the tube above the level 

 of that in the cup ; and the surface k of the little column thus 

 suspended will be a hollow hemisphere, whose diameter is the 

 interior diameter of the tube. If the same tube be plunged into 

 a cupful of mercury, the liquid will also rise in the tube, but it 

 will never attain the level of that in the cup, and its surface will 

 be a hemisphere whose diameter is also the diameter of the tube 

 (N. 172). The elevation or depression of the same liquid in 

 different tubes of the same matter is in the inverse ratio of their 

 internal diameters (N. 173), and altogether independent of their 

 thickness ; whence it follows that the molecular action is insen- 

 sible at sensible distances, and that it is only the thinnest possible 

 film of the interior surface of the tubes that exerts a sensible 

 action on the liquid. So much indeed is this the case, that, 

 when tubes of the same bore are completely wetted with water 

 throughout their whole extent, mercury will rise to the same 

 height in all of them, whatever be their thickness or density, 

 because the minute coating of moisture is sufficient to remove 

 the internal column of mercury beyond the sphere of attraction 



