REPORT ON CAPILLARY ATTRACTION. 283 



By molecular action M. Poisson understands the excess of 

 the repulsion above the attraction between the molecules. This 

 force he supposes to be different for different points of the two 

 molecules. Its mean value he c?iW.s \he principal iorce, and the 

 variation from this normal value according as different points of 

 the molecules are directed towards each other, the secondary 

 force. This latter plays an important part in solids, giving rise 

 to their rigidity and resistance to the lateral motion of their 

 molecules. Its absence from fluids is the occasion of the per- 

 fect mobility of their particles. The characteristic of fluids that 

 distinguishes them from solids is thus stated : If a point be 

 taken anywhere in the interior of a fluid mass, and a straight 

 line of insensible length but a great multiple of the mean inter- 

 val between the molecules, be drawn in any direction from that 

 point, the mean interval between the molecules that lie on the 

 line is constant, though the particles may be irregularly dis- 

 posed along it. The constancy of this mean interval is consi- 

 dered to be owing to the absence of the secondary force above 

 mentioned, by reason of which the molecules can readily take 

 positions that satisfy this condition*. 



Setting out with these principles M. Poisson arrives at equa- 

 tions relative to the pressure in the interior of a fluid mass, 

 which are the same as those usually obtained on the supposition 

 of equality of pressure in all directions from any given point. 

 The reasoning by which these equations are deduced is not 

 immediately founded on any observed fact, and as it conducts to 

 the same equations as those deducible from the known law of 

 equal pressure, it may be said to account theoretically for the 

 existence of this law. This is an instance of mathematics ap- 

 plied in the manner spoken of at the commencement of the 

 Report. The bases of the reasoning are hypotheses ; and it 

 leads to the explanation of a known fact. It would not be right 

 to conclude from this one explanation that the hypotheses must 

 necessarily be true. They can be considered to be satisfactorily 

 established, only when they have been successfully employed in 

 accounting for all the facts that are known to depend on the in- 

 timate constitution of fluids, and when they are found to require 

 for this purpose no modification. 



After calculating, on the above-mentioned hypotheses, the 



• Dr. Young has also speculated on the interesting subject of the immediate 

 cause oi fluidity. He remarks in his Lecture on Cohesion, {Lectures, vol. i. 

 p. 620,) that "the apparent weakness of the cohesion of fluids is entirely owing 

 to the mobility of their particles." It would be perhaps more correct to say, 

 that a weak cohesive power is a condition necessary to the existence of a great 

 degree of mobility. 



