246 ANNUAL EEPOET SMITHSONIAN INSTITUTION, 1908. 



experimental material is set forth in the table of atomic weights. 

 What quantitative facts can the man of science draw forth from its 

 figures, such as comj)osition, specific heat, vapor density, lowering 

 of the freezing point, and the like of innumerable substances. What 

 can he not forecast, by the aid of general analogy, in the way of 

 physical and chemical properties of many other kinds when he calls 

 to his aid that happy artifice, the periodic classification. Tlie very 

 fact that such an abundance of material has been brought together 

 necessarily increases many times its importance after it has once been 

 successfully classified. 



The theory of the constitution of organic combinations furnishes 

 a good example of this fact. In a short time, wlien the organic com- 

 pounds gathered together in the new edition of Beilstein's hand- 

 book which this society has under preparation are published, we 

 shall, I have been told by the editor, be confronted by more than a 

 hundred thousand structural formulae. It is by the aid of this theory 

 of organic constitution that a great part of these combinations have 

 been worked out, and it is only by its aid that it has been possible to 

 describe and classify this stupendous amount of material. And if 

 one finally considers all the information the initiated can read out 

 of the structural formulas and considers the mass of experimental 

 data that often had to be obtained to establish even one of them, he 

 realizes that in the quantity of experimental facts logically construed 

 and classified, the doctrine of the constitution of organic combina- 

 tions stands at the head of all theories that the human mind has 

 conceived. 



Another effect of the development of chemistry from the theoretical 

 point of view is that already in a great many cases theoretical and 

 experimental work can hardly be distinguished, so impregnated with 

 theory have most of the branches of chemistry become. Consequentl}^ 

 it is my task to-day to give not a general view of the whole of theo- 

 retical chemistry, but merely of that part which may be called 

 physical. 



In the first part of this sketch the relations between physical prop- 

 erties and chemical constitution will be touched on, a subject which 

 formed the principal field of research for physico-chemical work in 

 the latter half of the period we are considering. The description of 

 this work will be very much facilitated by an adherence to a system- 

 atic mode of classification, according to which the properties of sub- 

 stances are divided into three groups. 



There is, first, the measurement of those accessible quantities which 

 render possible the immediate deduction of the values for the molecu- 

 lar weights, and which may be briefly designated as molar properties. 

 Among these, as Avogadro has shown, the vapor density deserves a 



