5o6 POPULAR SCIENCE MONTHLY. 



or the molecule be made up of two or more atoms, as in the case 

 of most of the elements, it is undisputed, not only that the uniting 

 parts are likes to each other, but that the molecules which they 

 form are also likes to each other. That which, moreover, is espe- 

 cially obvious in the case of the elements, where the molecule is 

 always made up of the same kind of atoms, is none the less true 

 of compounds ; for, however superficially unlike the atoms or 

 molecules may be that enter into combination to form such com- 

 pounds, and whether the matter formed be what we know as in- 

 organic or what we know as organic whether it be extremely 

 simple, as in the case of water, or highly complex, as in the case 

 of albumin the resultant system is in every instance a system 

 whose unit parts all possess the same general character. 



The power of material systems to dissociate unlikes is best 

 seen when the dissociating stress is exerted by matter in the liquid 

 or gaseous state in a form, that is to say, in which the power to 

 dissociate is at its maximum and the resistance to dissociation at 

 a minimum. If we thrust a ball of wood into a volume of water, 

 the entering mass is immediately expelled to the surface, and it 

 is thus expelled for the reason that it possesses less density than 

 water, and is therefore an unlike : the act of dissociation, more- 

 over, is manifestly an act of the water system ; for, if we remove 

 the water, our wooden ball will descend freely through the space 

 previously occupied by the fluid. So a balloon ascends through 

 air because, being less dense than air, it appears in the air sys- 

 tem as an unlike, and is by that system expelled to those regions 

 of the atmosphere which resemble it in density : in this case, as 

 well, the act of dissociation is an act of the air system, for, if the 

 air could be removed, the balloon would not rise at all. 



Unlikes, sufficiently free to move, often repel one another when 

 forced into association. Zinc and lead, or zinc and bismuth, may 

 be melted together, but they separate more or less completely 

 during the process. Chloroform and water may be mechanically 

 mixed, but when they are no longer shaken the two fluids part 

 into distinct layers. If, upon oil placed in a receptacle, water be 

 poured, the water will displace the oil ; while, if mercury be 

 added, it will displace both oil and water ; the final result of the 

 experiment three layers occupying positions in the order of their 

 density being reached by acts involving both association and 

 dissociation association of the like parts of each layer, and dis- 

 sociation of the unlike layers. 



How likes are associated and unlikes dissociated may also be 



shown in most of the phenomena of cohesion, and especially in 



those wherein particles of matter assume the spherical or globular 



form. Where the medium is not apparent, as in the case of the 



sun, moon, and planets, the association of likes is alone displayed ; 



