3J^ 



INDUCTION. 



to the elementary properties of the 

 tissues into which they are anatomi- 

 cally decomposed. 



Another striking instance is af- 

 forded by Dalton's generalisation, 

 commonly known as the atomic theory. 

 It had been known from the very 

 commencement of accurate chemical 

 observation, that any two bodies com- 

 bine chemically with one another in 

 only a certain number of proportions ; 

 but those proportions were in each case 

 expressed by a percentage — so many 

 parts (by weight) of each ingredient, 

 in lOO of the compound, (say 35 and 

 a fraction of one element, 64 and a 

 fraction of the other :) in which mode 

 of statement no relation was per- 

 ceived between the proportion in 

 which a given element combines with 

 one substance, and that in which it 

 combines with others. The great 

 step made by Dalton consisted in 

 perceiving, that a unit of weight 

 might be established for each sub- 

 stance, such that by supposing the 

 substance to enter into all its com- 

 binations 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 i being 

 assumed as the unit of hydrogen, if 8 

 were then taken as that of oxygen, 

 the combination of one unit of hydro- 

 gen with one unit of oxygen would 

 produce the exact proportion of weight 

 between the two substances which is 

 known to exist in water ; the com- 

 bination of one unit of hydrogen vi^ith 

 two units of oxygen would produce 

 the proportion which exists in the 

 other compound of the same two ele- 

 ments, called peroxide of hydrogen ; 

 and the combinations of hydrogen 

 and of oxygen with all other sub- 

 stances would correspond with the 

 supposition that those elements enter 

 into combination by single units, or 

 twos, or threes, of the numbers as- 

 signed to them, I 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 i 



of the equivalent numbers, or, A6 

 they are called, atomic weights, of all 

 the elementary substances, comprises 

 in itself, and scientifically explains, 

 all the proportions in which any sub- 

 stance, elementary or compound, is 

 found capable of entering into chemi- 

 cal combination with any other sub- 

 stance whatever. 



§ 2. Some interesting cases of the 

 explanation of old uniformities by 

 newly ascertained laws are afforded by 

 the researches of Professor Graham. 

 That eminent chemist was the first 

 who drew attention to the distinction 

 which may be made of all substances 

 into two classes, termed by him crys- 

 talloids and colloids ; or rather, of all 

 states of matter into the crystalloid 

 and the colloidal states, for many 

 substances are capable of existing in 

 either. When in the colloidal state, 

 their sensible properties are very dif- 

 ferent from those of the same sub- 

 stance when crystallised, or when in 

 a state easily susceptible of crystal- 

 lisation. Colloid substances pass with 

 extreme difficulty and slowness into 

 the crystalline state, and are ex- 

 tremely inert in all the ordinary 

 chemical relations. Substances in 

 the colloid state are almost always, 

 when combined with water, more or 

 less viscous or gelatinous. The most 

 prominent examples of the state are 

 certain animal and vegetable sub- 

 stances, particularly gelatine, albu- 

 men, starch, the gums, caramel, tan- 

 nin, and some others. Among sub- 

 stances 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 crys- 

 talloid substances, they are very little 

 penetrable by one another : which 

 enabled Professor Graham to intro- 

 duce a highly effective process (termed 

 dialysis) for separating the crystalloid 

 substances contained in any liquid 



