THE MOLECULAR THEORY. 465 



ters not how little oxygen is taken, provided only that the proper pro- 

 portion of hydrogen is supplied. Then let us conceive the least possi- 

 ble portion of oxygen. Let the mind wrestle with the conception and 

 reduce the volume of this gas until it is fixed at the smallest that can 

 take part in a chemical action. Then conceive it combined with a 

 volume of hydrogen twice as great. We may contemplate the infini- 

 tesimal droplet of water so produced, but to conceive a droplet any 

 smaller is impossible, since this one contains no more than the least 

 possible portions of its elements. To break this droplet of water 

 would be to reduce it to fragments of hydrogen and oxygen. Again, 

 then, do we find our minds in the presence of particles which can not 

 be divided except at the sacrifice of their identity in other words, in 

 the presence of molecules. 



But if molecules exist, the second question at once arises, Are they 

 so closely packed as to constitute a continuous mass, or are they sepa- 

 rated by intervening spaces ? 



A second class of phenomena directs our judgment here. We might 

 detail the mathematical investigation by Cauchy, a third of a century 

 ago, by which he demonstrated the impossibility of the dispersion of 

 light in a substance whose minutest parts are absolutely homogeneous. 

 It was proved that dispersion happens only on condition that "two 

 contiguous portions of the medium, whose dimensions are moderately 

 small fractions of a wave-length of light, are dissimilar." The molecules 

 with intervening spaces would realize such dissimilarity. 



But, confining ourselves to the experimental side of the problem, 

 we find a variety of familiar phenomena ready to bear witness to this 

 structure. Among them are porosity, expansion and change of physi- 

 cal condition, and the diffusion of vapors. 



In regard to porosity, an old and homely experiment will give us a 

 starting-point. We take a tall and narrow glass jar and fill it so com- 

 pletely with alcohol that the addition of a single drop will endanger 

 an overflow. The jar appears to be full of a perfectly homogeneous 

 liquor. But if a sheet of cotton wool, whose fibers have been previ- 

 ously well loosened, be at hand, fragment after fragment may, with 

 care, be slowly introduced, without causing the overflow of a single 

 drop, until the jar appears to be filled with moistened cotton instead 

 of with alcohol. We have before us the surprising appearance of two 

 bodies filling the same space at the same moment. Surely, however, 

 we are not at liberty to adopt this explanation. For what should we 

 call that which has no power to exclude another from the space which 

 it occupies ? To call it matter is to obliterate all distinction between 

 matter and space. We are impelled to seek another explanation, and 

 we find one more acceptable in the hypothesis that neither of the two 

 bodies wholly fills the space which it appears to occupy that spaces, 

 too minute for even microscopic vision to detect, intervene between 

 the ultimate particles of both, and to such an extent that these mate- 



TOL. XV. 30 



