70 SCIENCE IN THE LAST HALF CENTURY. 



unalterable by any known means (except in so far as they might be 

 made to change their state from solid to fluid, or vice versa), unless 

 they were brought into contact with other kiuds of matter, and that 

 the properties of these several kinds of matter were always the same, 

 whatever their origin. All other bodies were found to cousist of two 

 or more of these, which thus took the place of the four " elements " of 

 the ancient philosophers. Further, it was proved that in forming 

 chemical compounds, bodies always unite in a definite proportion by 

 weight, or in simple multiples of that proportion, and that, if any one 

 body were taken as a standard, every other could have a number as- 

 signed to it as its proportional combining weight. It was on this foun- 

 dation of fact that Dalton based his re-establishment of the old atomic 

 hypothesis on a new empirical foundation. It is obvious that if ele- 

 mentary matter consists of indestructible and indivisible particles, 

 each of which constantly preserves the same weight relatively to all 

 the others, compounds formed by the aggregation of two, three, four, or 

 more such particles must exemplify' the rule of combination in definite 

 proportions deduced from observation. 



In the meanwhile, the gradual reception of the undulatory theory of 

 light necessitated the assumption of the existence of an " tether" filling 

 all space. But whether this aether was to be regarded as a strictly ma- 

 terial and continuous substance was an undecided point, and hence the 

 revived atomism escaped strangling in its birth. For it is clear that 

 if the sether is admitted to be a continuous material substance, Demo- 

 critic atomism is at an end, and Cartesian continuity takes its place. 



The real value of the new atomic hypothesis, however, dul not lie in the 

 two points which Democritus and his followers would have considered 

 essential, namely, the indivisibility of the "atoms" and the presence 

 of an inter-atomic vacuum; but in the assumption that, to the extent to 

 which our means of analysis take us, material bodies consist of definite 

 minute masses, each of which, so far as physical and chemical processes 

 of division go, may be regarded as a unit — having a practically perman- 

 ent individuality. Just as a man is the unit of sociology, without refer- 

 ence to the actual fact of his divisibility, so such a minute mass is the 

 unit of physico-chemical science — that smallest material particle which 

 under any given circumstances acts as a whole.* 



The doctrine of specific heat originated in the eighteenth century. 

 It means that the same mass of a body, under the same circumstances, 

 always requires the same quantity of heat to raise it to a given temper- 

 ature, but that equal masses of different bodies require dilierent quan- 

 tities. Ultimately, it was found that the quantities of heat required to 

 raise equal masses of the more perfect gases through equal ranges of 

 temperature were inversely proportional to their combining weights. 



*" Molecule" would be the more appropriate name for sucli a particle. Unfortu- 

 nately cli emists employ this term in a special sense as a name for an ae;gregation o«f 

 their smallest particles, for which they retain the designation of "atoms." 



