HYPOTHESIS. . 213 



the following attractions : (1) that of cohesion, by which each 

 individual micella is an aggregate of molecules ; (2) that which 

 tends to bring adjacent micellae together ; (3) that of adhe- 

 sion, by which the surfaces of the micellae retain their films of 

 water. 



kinds to combine chemically, each preserving its integrity in the compound. . . . 

 Any antecedent improbability on theoretical grounds is far more than out- 

 weighed by the evidence of a large number of compounds whose constitution 

 is most simply explained on the hypothesis of molecular combination. For 

 example, in the crystalline salts it is impossible to doubt that the water 

 exists as such, not as a part of the salt molecule, but combined with it as a 

 whole. So also there are a number of double salts whose constitution is most 

 simply explained on a similar hypothesis" (Cooke's Chemical Philosophy, 

 1882, p. 137). 



The word micella is applied by Nageli to those aggregates of molecules 

 which (like crystals) can increase or diminish in size without changing their 

 chemical nature. The micella is assumed to be much larger than the pleon. 

 "The internal structure of the micella is crystalline, while the exterior may 

 assume any shape. " The micellae unite to form micellar aggregates ; of such 

 the crystalline protein granules afford a good example. Thus, according to 

 Nageli, five terms must be recognized, the atom, the molecule, the pleon, the 

 micella, and the micellar aggregate. Pfeffer applies a general term, Tagma, to 

 all aggregates of molecules, thus bringing under one head the pleon, micella, 

 and micellar aggregate ; and he applies the name Syntagma to all bodies made 

 up of tagmata. The subject will be again referred to under "Osmosis." 



To make clearer the conception of a micella, it may be well to examine 

 briefly two terms in common use ; namely, atom and molecule. 



When a solid body, for instance a crystal of sodic chloride (common salt), 

 is mechanically separated into the smallest possible fragments, each particle 

 still possesses all the properties of salt. Beyond this mechanical limit of sepa- 

 ration the process of subdivision may be carried still further by solution : 

 the minutest fragments of the salt can be broken up and diffused through the 

 solvent, and yet not lose their essential character as salt ; in fact, they can be 

 again recovered without change from the solution. But it is impossible to go 

 beyond this latter limit of separation without altering the essential properties 

 of the substance. In other words, by this subdivision the physical limit has 

 been reached ; namely, the molecule. 



A molecule is understood to be the smallest amount of any substance 

 which can exist as such in the free state. Hence the molecule is the physical 

 unit. 



If, however, the salt is subdivided by chemical means, for instance, by the 

 action of strong sulphuric acid, its identity is destroyed, and its component 

 parts enter into new relations, and cannot be restored to their original relations 

 except by an exceedingly complicated process. In other words, the physical 

 limit has been overpassed and the chemical limit reached ; namely, the atom. 



Atom is generally defined as ' ' the smallest amount of a given substance 

 which can exist in combination," or "the smallest mass of nn element that 

 exists in any molecule." The atom is the chemical unit. 



Atoms are variously combined to form molecules : molecules are variously 

 aggregated to form masses. 



