PHILOSOPHICAL SOCIETY OP WASHINGTON. 277 



whether a i)iece of silver-plated copper heated to the melting 

 point of silver would show any absorption of that metal, he 

 learned that it was a common experience under such circumstances 

 to find the silver disappear; but that this had always been at- 

 tributed to a volatilization of the silver, or in the workman's 

 phrase, — to its being "burnt off." At Henry's request the expe- 

 riment was tried : the heated end of a silver-plated piece of 

 copper exhibited on cooling and cleaning, a copper surface, the 

 other end remaining unchanged. Henry next had the copper 

 surface slightly dissolved off by immersion for a few minutes in a 

 solution of muriate of zinc, when as he had anticipated, the 

 silver was again exposed, having penetrated to but a very short 

 and tolerably uniform distance below the original surface.* 



In 1844, he made some important observations on the cohesion 

 of liquids. Notwithstanding that Dr. Young early in the century 

 maintained that " the immediate cause of solidity as distinguished 

 from liquidity is the lateral aclheaion of the particles to each 

 other," and had shown that " the resistance of ice to extension 

 or compression is found by experiment to differ very little from 

 that of water contained in a vessel, "f all the most popular text- 

 books on physics continued to teach that the cohesion of the 

 liquid state is intermediate between that of the solid and the 

 gaseous states. | It seemed therefore desirable to test the ques- 

 tion by some more direct means than the resistance of liquids 

 contained in closed vessels ; and for this purpose Henry em- 

 ployed the classical soap-bubble. " The effect of dissolving the 

 soap in the water is not as might at first appear, to increase the 

 molecular attraction, but to diminish the mobility of the mole- 

 cules." In fact the actual tenacity of pure water is greater than 

 that of soap-water. 



The first sot of experiments was directed to determine "the 

 quantity of water which adhered to a bubble just before it burst." 

 The second set of experiments w^as devised to measure the con- 

 tractile force of a soap-bubble blown on the wider end of a U 

 shaped glass tube half filled with water, by the barometric column 

 sustained in the narrower stem of the tube ; the difference of 

 level being carefully observed by means of a microscope. The 

 thickness of the soap-bubble film at its top was estimated by the 



* Proceed. Am. Phil. Soc. June 20, 184.5, vol, iv. p. 177. 

 f Young's Lectures on Nat. Philos. Lect. 50, vol. i. p. (j27. 

 X " If we attempt to draw up from the surface of water a circular disk 

 of metal say of an inch in diameter, we shall see that the water will 



merely indicates the force of attraction of a single film of atoms around 

 the perpendicular surface, and not of the whole column elevated.' 

 (^Agricultural Report for 1857, p. 427.— Paper on lMt-t^orology ) 



