220 ANNUAL EEPORT SMITHSONIAN INSTITUTION, 1916. 



and partly graphical. From the nature of the problem, which in- 

 volves many unknown variables, perfect mathematical exactness is 

 not to be expected. Nevertheless, little by little, one may hope to 

 trace the conflicting tendencies and ascribe them to a few common 

 causes. 



With the help of these methods the tentative conclusion has been 

 reached that the space occupied by the atom and molecule in solids 

 and liquids is highly significant. The actual atomic bulk or volume 

 is diminished but slightly by moderate mechanical pressures and 

 by cooling even to the absolute zero ; but it is very greatly affected, 

 apparently, by the mutual attractions of the atoms, called cohesion 

 and chemical affinity. Usually the less volatile a substance (that is 

 to say, the more firmly it is held together by cohesion) the gi^eater 

 is its density and the less is its compressibility, other things being 

 equal. Greater cohesion is associated with greater compactness. 

 Likewise, the existence of powerful chemical affinity between ele- 

 ments forming a compound is usually associated with great decrease 

 in volume during the act of combination, and consequent increase in 

 the density of the product in relation to the average density of the 

 constituents. Thus, we can hardly escape the inference that both 

 cohesion and affinity, by i)ulling the atoms together with enormous 

 pressiu-e, actually exert a compressing effect upon the atoms, or at 

 least upon the space which they demand for their occupation. The 

 result of each of these compressing agencies is found to be greater 

 the greater the compressibility of the substances concerned — a new 

 evidence of the reasonableness of the inference. Not always are 

 these effects easily traced, because the situation is often complicated, 

 and the several effects are superposed. Nevertheless, enough evi- 

 dence has been obtained to leave but little doubt, at least in 

 my mind, as to the manner of working of the essential agencies 

 concerned. 



But we need not dwell upon this tentative hypothesis. Many 

 more data and much more thought are necessary to establish it in an 

 impregnable position, although no important inconsistency has thus 

 far been pointed out in it. At present it may be looked upon as 

 valuable because it, like other hypotheses of this type, has stimu- 

 lated thought and experiment concerning the fundamental facts 

 with which it deals. 



As the years go on, the recent contributions to the study of atomic 

 weights and volumes and other properties will be sifted and tested ; 

 and such contributions as may stand the test of time will take their 

 places among the multifarious array of accepted chemical facts, 

 laws, and interpretations accumulated by many workers all over the 

 world. 



