MOLECULAR PHYSICS. 83 



cular movements in liquids which give rise to liquid waves. When 

 unchecked, the vertical displacement of a part of a liquid at the 

 surface results in travelling annular elevations and depressions. 

 But here the wave is the sum of the motions of the individual 

 molecules in orbits, which lie in vertical planes in the direction of 

 propagation. Amplitude and wave length have now both an effect 

 upon wave rate, the second directly influencing it and the first 

 indirectly, because in a boundless liquid the amplitude is partly 

 convertible into length. In confined spaces, the reflected waves 

 may so meet the advancing ones that nodes ensue and stationary 

 waves are established which are constant in wave length. Whatever 

 be the amplitude of such waves, their periods are the same, and the 

 rate of wave progression varies as the square root of the wave length 

 and the wave period is, in the case of a circular vessel in which 

 there is one nodal ring, the same as that of a pendulum whose length 

 is equal to the radius of the vessel. 



In the cases of intermolecular motion between unlike bodies or 

 mixtures, we may, for the sake of classification, consider matter in 

 its three forms, solid, liquid, and gaseous. Between solids and 

 solids molecular motion ensues, in the process of cementation of 

 carbonation of iron, where the one element penetrates the other 

 without either being liquefied. Here, indeed, there may be 

 various stages of carbonation, reaching from the carbon on one 

 side to the iron on the other, and any intermediate molecule may 

 be receiving carbon from the one side and giving it to the other. 

 Yet. mineralogy furnishes many unquestionable examples of the 

 penetration of one kind of solid matter through another without 

 fusion or solution. The true molecular penetration of a liquid 

 into a solid is also not infrequent, witness the absorption of water 

 by gelatin, a phenomenon quite distinct from the absorption of a 

 liquid into a porous body, which is a case of capillarity. The 

 entrance into a solid of a vapour or gas furnishes the cases of 

 occlusion, in which iron, palladium, and other metals absorb many 

 times their own volume of hydrogen. To occlusion may probably 



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