166 BIOGEAPHICAL MEMOIE OF JOSEPH HENEY. 



soil, and, being tlins removed, its place is supplied by a diffusion from 

 within, and so on ; it is not improbable that a large portion of tbe alloy 

 may be removed in progress of time, and the purity of the coin be con- 

 siderably increased. It is known to the jeweler that articles of copper 

 plated with gold lose their brilliancy after a while, and that this can be 

 restored by iDoiling them in ammonia. This effect is probably i)roduced 

 by the ammonia acting on the copper and dissolving off its surface so 

 as to expose the gold, which by diffusion had jienetrated into the body 

 of the metal. 



The slow diffusion of one metal into another at ordinary temperatures, 

 would naturally require a long time to produce a perceptible effect, since 

 it is probably only produced by the minute vibrations of the particles 

 due to variations of temperature. 



The same principle is applied to the explanation of the phenomenon 

 called segregation — such as the formation of nodules of ffiut in masses 

 of carbonate of lime ; or, in other words, to the explanation of the man- 

 ner in which the molecular action, which is insensible at perceptible dis- 

 tances, may produce results which woidd appear, at first sight, to be the 

 effect of attraction acting at a distance. 



XIV. Another series of experiments had reference to the constitution 

 of matter in regard to its state of liquidity and solidity, and they had 

 their origin in the examination of the condition of the metal of the large 

 gun constructed under the direction of Captain Stockton, by the explo- 

 sion of which several prominent members of the United States Govern- 

 ment werie killed at Washington. It was observed in testing the bars 

 of iron made from this gun, that they varied much in tensile strength in 

 different parts, and that, in breaking these bars, the solution of the con- 

 tinuity took place first in the interior. This phenomenon was attributed . 

 to the more ready mobility of the outer molecules of the bars, the inner 

 ones being surrounded by matter incapable of slipi^ing, and hence the 

 rui)ture. A similar effect is produced in a piece of thick copper wire, 

 each end, when broken, exhibiting, at the point of rupture, a cup-shaped 

 surface, showing that the exterior ot the metal sustained its connection 

 longer than the interior. From these observations the conclusion was 

 drawn,, that rigidity differs from liquidity more in a j)olarity which pre- 

 vents slipping of the molecules, than in a difference of the attractive 

 force with which the molecules are held together ; or that it is more in 

 accordance with the phenomena of cohesion to sui)pose 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 sul)ject, the comparative tenacity 

 of pure water and water in which soap had been dissolved 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 tckiacity of pure water was 

 greater than that of soap and water. This novel result is in accordance 

 with the supposition that the mingling of the soap and the water inter- 

 feres 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 in 

 films. For this purj^ose 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 Xewton's scale of the colors of thin 



