PROGRESS IN NINETEENTH CENTURY 235 



double decomposition. In most cases, although not invariably, the 

 phenomena are ionic, and the molecules are first broken down. In 

 the third place, the uniform heat of neutralization between acids 

 and bases was explained by showing that in all cases it represented 

 one and the same change, namely, the union of hydrogen and hy- 

 droxyl ions to form water --a conclusion which gave a significant 

 datum to thermo-chemistry. In brief, many distinct lines of physico- 

 chemical research converge in the kinetic theory of solutions - - a 

 theory whose development has hardly more than begun. Like most 

 successful theories, its importance may at first be exaggerated; we 

 have not yet the perspective which shall enable us to judge it truly ; 

 in all probability, it is but one phase of some larger law; but, not- 

 withstanding all difficulties and all objections, it is a stride forward, 

 and will bring us to new truth. 



We now reach a point where it is difficult to disentangle the many 

 threads of investigation, and to determine their relations to one 

 another and to the past. Current work is more or less confusing, for 

 it is too near our eyes, and its ultimate significance is not easily 

 apprehended. The theory of solutions, the law of mass-action enun- 

 ciated by Guldberg and Waage, and the phase rule of Willard Gibbs 

 interact in so many ways, and are so rapidly developing, that I for 

 one dare not attempt to predict what the outcome shall be. Chem- 

 istry is becoming more and more a mathematical science, and so is 

 gaining in precision; but mathematical reasoning leads to correct 

 conclusions only when its premises are secure. The data must be 

 verified and reverified before we can certainly determine their mean- 

 ing, and in the enthusiasm of new investigation this necessary duty- 

 is often deferred. The pioneer leaves much undone behind him, and 

 patient laborers are needed to follow in his lead. The first glimpse 

 of truth is rarely the whole truth, for that is best gained by what 

 we may call the method of successive approximations. 



If prophecy is difficult, retrospection is easy; we may therefore 

 retrace our steps and see what road we have followed. Boyle, Priest- 

 ley, Scheele, and Lavoisier prepared the way for Dalton, and his 

 atomic theory, the first quantitative theory of its kind, has been for 

 a century the key to all chemistry. All of the great advances in our 

 science have hinged directly upon Dalton's conception, and his atomic 

 weights, as developed by Berzelius and Cannizzaro, are now seen to 

 be fundamental constants, with whose aid the physical relations of 

 different substances are easiest interpreted. The periodic law is based 

 upon the atomic weights, valence is an atomic function, in stereochem- 

 istry we have a hint of atomic form, isomerism is intelligible only 

 upon the assumption of variable atomic position, and the structure 

 of a molecule depends upon atomic groupings. The ions of physical 

 chemistry and the molecules of thermodynamics are either atoms 



