LETTER OF J. HENRY To REV. S. B. HOP. 159 



acid of* the soil, and being thus removed, its place is supplied 

 by a diffusion from within, and so on; it is not improbable that a 

 large portion of the alloy may be removed in progress of time, 

 and the purity of the coin !»• considerably increased. It is known 

 to the jeweler that articles of copper plated with gold lose their 

 brilliancy after awhile, and that this can be restored by boiling 

 them in ammonia. This effect is probably produced by the 

 ammonia acting on the copper and dissolving off its surface so as to 

 expose the gold, which by diffusion had penetrated into the body 

 of the metal. 



The slow diffusion of one metal into another at ordinary tem- 

 peratures would naturally require a long time to produce a per- 

 ceptible 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 phenome- 

 non called segregation — such as the formation of nodules of flint 

 in masses of carbonate of lime, or in other words, to the expla- 

 nation of the manner in which the molecular action, which is 

 insensible at perceptible distances, may produce results which would 

 appear, at first sight, to be the effect of attraction acting at a 

 distance. 



XIV. Another series of experiments had reference to the con- 

 stitution 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 Cap- 

 tain Stockton, by the explosion of which several prominent 

 members of the United States Government were killed at Wash- 

 ington. 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 slip- 

 ping, and hence the rupture. 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 

 of the metal sustained its connection longer than the interior. 



