498 MEMORIAL OF JOSEPH HENRY. 



" Many experiments were made to determine the amount of this 

 force, by blowing a bubble on the larger end of a glass tube in the 

 form of a letter U, and partially filled with water; the contractile 

 force of the bubble, transmitted through the enclosed air, forced 

 down the water in the larger leg of the tube and caused it to rise in 

 the smaller. The difference of level observed by means of a micro- 

 scope gave the force in grains per square inch, derived from the 

 known pressure of a given height of water. The thickness of the 

 film of soap-water which formed the envelope of the bubble was 

 estimated as before, by the color exhibited just before bursting. The 

 results of these experiments agree with those of weighing the bubble, 

 in giving a great intensity to the molecular attraction of the liquid; 

 equal at least to several hundred pounds to the square inch. Sev- 

 eral other methods were employed to measure the tenacity of the 

 film, the general results of which were the same ; the numerical 

 detail of them are reserved however until the experiments can be 

 repeated with a more delicate balance. 



"The comparative cohesion of pure water and soap- water was 

 determined by the weight necessary to detach the same plate from 

 each ; and in all cases the pure water was found to exhibit nearly 

 double the tenacity of soap- water. The want of permanency in the 

 bubble of pure water is therefore not due to feeble attraction, but to 

 the perfect mobility of the molecules, which causes the equilibrium, 

 as in the case of the arch, without friction of parts, to be destroyed 

 by the slightest extraneous force." 



Another of Henry's investigations in molecular physics, having 

 important practical bearings, should be more generally known than 

 it is. Among his other duties as chairman of the United States 

 Light-House Board was the testing of the various physical properties 

 of the oils submitted to the Government for purchase. Fluidity 

 was one of these properties of which it seemed most difficult to get 

 reliable comparative tests. Henry discarded all the crude instru- 

 ments and methods which give results in which the different degrees 

 of fluidity of the oils are masked by their various powers of adhesion 

 to the surface over which they flow during the process of testing. 

 Henry very ingeniously applied the theorem of Torricelli, which 

 shows that equal quantities of all liquids supposing them to be all 



