208 LECTURES. 



solid bodies exhibited by the same. Transmission of small impulses 

 through a very long rod of wood. 



Atoms set in motion by the smallest force. 



(88.) In connexion with the subject of the constitution of matter, 

 the following extract from a paper by the author, puldished in the 

 Proceedings of the Am. Phil. Soc, may be given. 



" The passage of a body from a solid to a liquid state is generally 

 attributed to the neutralization of the attraction of cohesion by the re- 

 pulsion of the increased quantity of heat ; the liquid being supposed 

 to retain a small portion of its original attraction, which is shown by 

 the force necessary to separate a surface of water from water, in the 

 well known experiment of a plate suspended from a scale beam over a 

 vessel of the liquid. It is, however, more in accordance with all the 

 phenomena of cohesion to suppose, instead of the attraction of the 

 liquid being neutralized by the heat, that the effect of this agent is 

 merely to neutralize the polarity of the molecules so as to give them 

 perfect freedom of motion around every imaginable axis. The small 

 amount of cohesion (52 grains to the square inch) exhibited in the 

 foregoing experiment, is due, according to the theory of capillarity of 

 Young and Poisson, to the tension of the exterior film of the surface 

 of water drawn up by the elevation of the plate. This film gives way 

 first, and the strain is thrown on an inner film, which, in turn, is rap- 

 tured ; and so on until the plate is entirely separated ; the whole effect 

 being similar to that of tearing the water apart atom by atom. 



"Reflecting on this subject, the author has thought that a more 

 correct idea of the magnitude of the molecular attraction might be ob- 

 tained by studying the tenacity of a more viscid liquid than water. 

 For this purpose he had recourse to soaj) water, and attempted to 

 measure the tenacity of this liquid by means of weighing the quantity 

 of water which adhered to a bubble of this substance just before it 

 burst, and by determining the thickness of the film from an observa- 

 tion of the color it exhibited in comparison with Newton's scale of 

 thin plates. Although experiments of tliis kind could only furnish 

 apj)roximate results, yet they showed that the molecular attraction of 

 water for water, instead of being only about 52 grains to the square 

 inch, is really several hundred pounds, and is probably equal to that 

 of the attraction of ice for ice. The effect of dissolving the soap in the 

 water, is not, as might at first appear, to increase the molecular at- 

 traction, but to diminish the mobility of the molecules, and thus ren- 

 der the liquid more viscid. 



"According to the theory of Young and Poisson, many of the phe- 

 nomena of liquid cohesion, and all those of capillarity, are due to a 

 contractile force existing at the free surface of the liquid, and which 

 tends in all cases to urge the liquid in the direction of the radius of 

 curvature towards the centre, with a force inversely as this radius. 



The fact of the existence of this force is derived from a consideration 

 of the molecular constitution of matter. The molecules within the 

 mass of a liquid and at a distance from the surface, can be moved 

 freely in all directions among each other, because they are acted on 

 by equal forces on all sides. Not so with the molecules very near the 

 surface,, these are separated by the preponderance of repulsion, and 



