PHILOSOPHICAL SOCIETY OP WASHINGTON. 271 



whether a piece 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 adhesion 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 

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

 molecular atti'action, but to climinish the mobility of the mole- 

 cules." In fact the actual tenacity of pure v/ater is greater than 

 that of soap-water. 



The first set of experiments was directed to determine "the 

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

 The second set of experiments was 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 Sue. JiDie 20, 1S4.5, vol. iv. p. 177. 



f Young's Lectures on Nat. Philos. Lt^ct. 50, vol. i. p. 627. 



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 

 adhere and be supported several lines above the general surfate. This 

 experiment which is frequently given in elementary books as a measure 

 of the feeble attraction of water for itself, is improperly interpreted. It 

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

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

 {Agricultural Beportiov 1857, p. 427. —Paper on Meteorology.) 



51 



