4 PROCEEDINGS OP THE AMERICAN ACADEMY. 



In solids one must consider also the directive agency which manifests 

 itself in crystalline form and optical structure. In a few cases the 

 " crystallogenic force" seems to be rather directive than attractive; in 

 other cases it seems to have both properties, for considerable diminution 

 in volume may occur. The presence of the crystal-making force compli- 

 cates the phenomena and is a considerable stumbling-block in the way 

 of the study of the internal tension of solids. 



In view of these facts, it seemed to me possible that the study of com- 

 pression as manifested by atomic volume under different circumstances, as 

 well as of atomic compressibility, might afford some light as to the 

 affinities at work. The attempt, while only just begun, has not been 

 wholly unsuccessful. 



Evidently the liquid is the most suitable state in which to study the 

 effects of molecular and atomic compressibility. It is most suitable 

 because the irregularities in the behavior of liquids are very great, indi- 

 cating various internal stresses, and because they are nevertheless not at 

 the mercy of the directive crystal-making tendency which superposes its own 

 influence upon that of cohesion. The great difficulty in the subject lies 

 in the fact that the total compressibility of a substance is usually made 

 up of a number of parts ; the molecular compressibility might be due 

 partly to a diminishing of the so-called " free-space" between the mole- 

 cules, as well as to a diminishing of the distance between the atomic 

 centres. In words free from hypothesis, we may say that the compressi- 

 bility may be made up of a chemical and a physical compressibility. 

 When one comes to compute from compressibility the probable affinities, 

 one is still more at a loss, — for each affinity is a mutual affair, concern- 

 ing two specific substances. The immense number of variables thus 

 introduced has discouraged most investigators, and I can find little if any 

 hint of the significance of chemical compressibility in the literature 

 familiar to me.* 



In a case of this kind, one naturally seeks at first cases as simple as 

 possible. A study of the volume changes which take place on mixing 

 liquids reveals at first no apparent regularity. In some cases an expan- 

 sion occurs, but more usually a contraction ; sometimes heat is evolved, 

 and at other times heat is absorbed. One law may, I think, be detected 

 in the midst of the confusion, namely : Similar liquids exhibit less 

 change of volume on mixing than dissimilar ones do. That is, where the 



* The considerations of NordenskjiJld are too seriously complicated by uncer- 

 tain assumptions to liave much value. (See Ostwald's Lehrbuch, I. 850 (1891), for 

 these and similar considerations.) 



