496 MEMORIAL OF JOSEPH HENRY. 



the copper plate with zinc chloride, when the absorbed silver made 

 its appearance, having penetrated to a slight distance into the copper. 



In 1844 Henry is again at work in molecular physics, investigat- 

 ing the nature of the forces acting in liquid films. This investi- 

 gation was duly valued by Plateau, who has given us his beautiful 

 researches into the conditions of equilibrium of polyhedra with 

 surfaces formed of films of water, and Plateau chided Henry for 

 having neglected to investigate further into phenomena which he 

 was the first to discover. Of Henry's work in this direction there 

 only remains the record of a scanty verbal communication which he 

 he made to the American Philosophical Society in 1844. From 

 this I make following abstract: "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 repulsion of the increased 

 quantity of heat; the liquid being supposed to retain a small por- 

 tion 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 phe- 

 nomena 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,) ex- 

 hibited 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 ruptured ; and so on' until the plate is 

 entirely separated; the whole effect being similar to tearing the 

 water apart atom by atom. 



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

 correct idea of the magnitude of the molecular attraction might be 

 obtained by studying the tenacity of a more viscid liquid than 

 water. For this purpose he had recourse to soap-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 



