CHEMICAL SCIENCE. 173 



ent elementary substances have the same relation to each other as the 

 subordinate atomic volumes-of the same substances. But further, these* 

 more and less mobile or light and heavy forms of matter have a singu- 

 lar relation connected with equality of volume. Equal volumes of two 

 of them can coalesce together, unite their movement, and form a new 

 atomic group, retaining the whole, the half, or some simple proportion 

 of the original movement and consequent volume. This is chemical 

 combination. It is directly an affair of volume, and only indirectly 

 connected with weight. Combining weights are different, because the 

 densities, atomic and molecular, are different. The volume of com- 

 bination is uniform, but the fluids measured vary in density. This 

 fixed combining measure the metron of simple substances weighs 

 1 for hydrogen, 16 for oxygen, and so on with the other " elements." 

 To the preceding statements, respecting atomic and molecular mobility, 

 it remains to be added, that the hypothesis admits of another expression. 

 As in the theory of light we have the alternative hypothesis of emission 

 and undulation, so in molecular mobility the motion may be assumed 

 to reside either in separate atoms and molecules, or in a fluid medium 

 caused to undulate. A special rate of vibration or pulsation originally 

 imparted to a portion of the fluid medium enlivens that portion of mat- 

 ter with an individual existence, and constitutes it a distinct substance 

 or element. With respect to the different states of gas, liquid and 

 solid, it may be observed that there is no real incompatibility with each 

 other in these physical conditions. They are often found together in 

 the same substance. The liquid and the solid conditions supervene 

 upon the gaseous condition rather than supersede it. Gay-Lussac 

 made the remarkable observation, that the vapors emitted by ice and 

 watef, both at Cent., are of exactly equal tension. The passage 

 from the liquid to the solid state is not made apparent in the volatility 

 of water. The liquid and solid conditions do not appear as the ex- 

 tinction or suppression of the gaseous condition, but something super- 

 added to that condition. The three conditions (or constitution) prob- 

 ably always coexist in every liquid or solid substance, but one 

 predominates over the others. In the general properties of matter we 

 have, indeed, to include still further (1.) the remarkable loss of elas- 

 ticity in vapors under great pressure, which is distinguished by Mr. 

 Faraday as the Caignard-Latour state, after the name of its discoverer, 

 and is now undergoing an investigation by Dr. Andrews, which may 

 be expected to throw much light upon its nature ; (2.) the colloidal 

 condition or constitution, which intervenes between the liquid and 

 crystalline states, extending into both, and affecting probably all kinds 

 of solid and liquid matter in a greater or less degree. The predomi- 

 nance of a certain physical state in a substance appears to be a distinc- 

 tion of a kind with those distinctions recognized in natural history as 

 being produced by unequal development. Liquefaction or solidifica- 

 tion may not, therefore, involve the suppression of either the atomic or 

 the molecular movement, but only the restriction of its range. The 

 hypothesis of atomic movement has been elsewhere assumed, irrespect- 

 ive of the gaseous condition, and is applied by Dr. Williamson to the 

 elucidation of a remarkable class of chemical reactions which have their 

 seat in a mixed liquid. Lastly, molecular or diffusive mobility has an 

 obvious bearing upon the communication of heat to gases, by contact 



15* 



