160 MEMORIAL. OF JOSEPH HENRY. 



From these observations the conclusion was drawn, that rigidity 

 differs from liquidity more in a polarity which prevents slipping 

 of the molecules, than in a difference of the attractive for6e with 

 which the molecules are held^ together; or that it is more in accord- 

 ance with the phenomena of cohesion, to suppose that in the case of 

 a liquid, instead of the attraction of the molecules being neutralized 

 by heat, the effect of this agent is merely to neutralize the polarity 

 of the molecules, so as to give them perfect freedom of motion 

 around any imaginable axis. In illustration of this subject the 

 comparative tenacity of pure water in which soap had been dis- 

 solved, was measured by the usual method of ascertaining the 

 weight required to detach from the surface of each the same plate 

 of wood, suspended from the beam of a balance, under the same 

 condition of temperature and pressure. It was found by this 

 experiment that the tenacity of pure water was greater than that 

 of soap and water. This novel result is in accordance with the 

 supposition that the mingling of tlie soap and the water interferes 

 with the perfect mobility of the molecules, while at the same time 

 it diminishes the attraction. 



XV. A series of experiments was also made on the tenacity of 

 soap-water in films. For this purpose sheets of soap-water films 

 were stretched upon rings, and the attempt made to obtain the 

 tenacity of these by placing on them pellets of cotton until they 

 were ruptured. The thickness of these films was roughly estimated 

 by Newton's scale of the colors of thin plates, and from the results 

 the conclusion was arrived at that the attractive force of the mole- 

 cules of water, for those of water, is approximately equal to those 

 of ice for those of ice, and that the difference in this case, of the 

 solidity and liquidity, is due to the want of mobility in the latter, 

 which prevented the slipping of the molecules on each other. It 

 is this extreme mobility of the molecules of water that prevents 

 the formation of permanent bubbles of it, and not a want of 

 attraction. 



The roundness of drops of water is not due to the attraction 

 of the whole mass, but merely to the action of the surface, which 

 in all cases of curvature is endowed with an intense contractile 

 power. 



