EXAMPLES OF THE EXPLANATION OF LAWS. 339 



which a given element combines with one substance, and that in which it 

 combines with others. The great step made by Dalton consisted in per- 

 ceiving that a unit of weight might be established for each substance, 

 such that by supposing the substance to enter into all its combinations in 

 the ratio either of that unit, or of some low multiple of that unit, all the 

 different proportions, previously expressed by percentages, were found to 

 result. Thus 1 being assumed as the unit of hydrogen, if 8 were then 

 taken as that of oxygen, the combination of one unit of hydrogen with one 

 unit of oxygen would produce the exact proportion of weight between the 

 two substances which is known to exist in water ; the combination of one 

 unit of hydrogen with two units of oxygen would produce the proportion 

 which exists in the other compound of the same two elements, called perox- 

 ide of hydrogen ; and the combinations of hydrogen and of oxygen with 

 all other substances, would correspond with the supposition that those ele- 

 ments enter into combination by single units, or twos, or threes, of the 

 numbers assigned to them, 1 and 8, and the other substances by ones or 

 twos or threes of other determinate numbers proper to each. The result 

 is that a table of the equivalent numbers, or, as they are called, atomic 

 weights, of all the elementary substances, comprises in itself, and scientific- 

 ally explains, all the proportions in -which any substance, elementary or 

 compound, is found capable of entering into chemical combination with 

 any other substance whatever. 



§ 2. Some interesting cases of the explanation of old uniformities by 

 newly ascertained laws are afforded by the researches of Professor Gra- 

 ham. That eminent chemist was the first who drew attention to the dis- 

 tinction which may be made of all substances into two classes, termed by 

 him crystalloids and colloids ; or rather, of all states of matter into the 

 crystalloid and the colloidal states, for many substances are capable of ex- 

 isting in either. When in the colloidal state, their sensible properties are 

 very different from those of the same substance when crystallized, or when 

 in a state easily susceptible of ci-ystallization. Colloid substances pass with 

 extreme diflSculty and slowness into the crystalline state, and are extreme- 

 ly inert in all the ordinary chemical relations. Substances in the colloid 

 state are almost always, when combined with watei'^ more or less viscous or 

 gelatinous. The most prominent examples of the state are certain animal 

 and vegetable substances, particularly gelatine, albumen, starch, the gums, 

 caramel, tannin, and some others. Among substances not of organic origin, 

 the most notable instances are hydrated silicic acid, and hydrated alumina, 

 with other metallic peroxides of the aluminous class. 



Now it is found, that while colloidal substances are easily penetrated by 

 water, and by the solutions of crystalloid substances, they are very little 

 penetrable by one another : which enabled Professor Graham to introduce 

 a highly effective process (termed dialysis) for separating the crystalloid 

 substances contained in any liquid mixture, by passing them through a 

 thin septum of colloidal matter, which does not suffer any thing colloidal 

 to pass, or suffers it only in very minute quantity. This property of col- 

 loids enabled Mr. Graham to account for a number of special results of 

 observation, not previously explained. 



For instance, " while soluble crystalloids are always highly sapid, soluble 

 colloids are singularly insipid," as might be expected ; for, as the sentient 

 extremities of the nerves of the palate "are probably protected by a col- 

 loidal membrane," impermeable to other colloids, a colloid, when tasted, 



