In the meanwhile, the gradual reception of the undulatory theory of light necessitated the assumption of the existence of an ‘ether’ filling all space.  But whether this ether was to be regarded as a strictly material and continuous substance was an undecided point, and hence the revived atomism, escaped strangling in its birth.  For it is clear, that if the ether is admitted to be a continuous material substance, Democritic atomism is at an end and Cartesian continuity takes its place.

[Sidenote:  The real value of hypothesis; it predicates the existence of units of matter.]

The real value of the new atomic hypothesis, however, did 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 interatomic 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 permanent individuality.  Just as a man is the unit of sociology, without reference 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.[F]

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 temperature, but that equal masses of different bodies require different quantities.  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.  Thus a definite relation was established between the hypothetical units and heat.  The phenomena of electrolytic decomposition showed that there was a like close relation between these units and electricity.  The quantity of electricity generated by the combination of any two units is sufficient to separate any other two which are susceptible of such decomposition.  The phenomena of isomorphism showed a relation between the units and crystalline forms; certain units are thus able to replace others in a crystalline body without altering its form, and others are not.

Again, the laws of the effect of pressure and heat on gaseous bodies, the fact that they combine in definite proportions by volume, and that such proportion bears a simple relation to their combining weights, all harmonised with the Daltonian hypothesis, and led to the bold speculation known as the law of Avogadro—­that all gaseous bodies, under the same physical conditions, contain the same number of units.  In the form in which it was first enunciated, this hypothesis was incorrect—­perhaps it is not exactly true in any form; but it is hardly too much to say that