302 



SCIENCE 



[N. S. Vol. XXVI. No. 662 



ment borrowed from physics were taken 

 away from the chemistry of to-day, but 

 little would be left of the science. Chem- 

 istry would then become a purely qualita- 

 tive observational study; she could pene- 

 trate but superficially into the hidden 

 world. Therefore it would not be an ex- 

 treme statement to call all quantitative 

 chemistry physical chemistry, with the 

 understanding that by physical chemistry 

 in this sense is meant the application of 

 physical methods of research to the study 

 of chemical problems. 



Indiscriminate measurement will lead 

 nowhere, however. The results of the 

 numerical determination of chemical phe- 

 nomena are by no means all of equal im- 

 portance. They may be divided into two 

 classes: the first class comprises those 

 which are variable and accidental, depend- 

 ing upon the relatively unimportant condi- 

 tions of the special case, such as the 

 analytical composition of a piece of 

 granite; and the second class comprises 

 those which are invariable and general, 

 recurring almost or quite unchanged under 

 widely varying conditions. Such results 

 as the latter may be called "physicoehem- 

 ieal constants." They claim our im- 

 mediate attention. 



A "physicochemical constant" is a 

 numerical magnitude expressing one of the 

 numerous apparently permanent quantita- 

 tive relations of mass or energy which seem 

 to be essentially associated with the ele- 

 mentary substances, or chemical elements, 

 and their compounds; it is a fundamental 

 fact, a unique number which touches very 

 closely the ultimate structure of material. 

 As examples, the atomic weights stand out 

 strikingly. Whether or not these quanti- 

 ties, representing the relative weights in 

 which elementary substances combine with 

 one another, are to be referred to the 

 weights of hypothetical atoms, they are 



certainly concerned in determining the 

 composition of every compound substance 

 in the heavens above, on the earth beneath, 

 or in the waters under the earth. Every 

 proteid in each muscle of our body, every 

 drop of liquid in the ocean, every stone on 

 the mountain top bears within itself the 

 stamp of the influence of this profoundly 

 significant and impressive series of four- 

 score numbers. 



The heat evolved during any chemical 

 combination typifies a different kind of 

 physicochemical constant. Coal on burn- 

 ing sets free a quantity of heat which man- 

 kind uses in exceedingly divers ways, 

 deriving therefrom the major part of the 

 energy of manufactures and transportation 

 as well as that needed to warm his habita- 

 tions. The evolution of quantities of heat 

 in this and other chemical reactions indi- 

 cates a decrease in the total energy of the 

 substances during the reaction involved; 

 therefore from the point of view of the 

 chemical philosopher, as well as from that 

 of the practical engineer, these figures also 

 are of great importance. 



Many other exajiiples of other types of 

 constants might be cited, such as densities, 

 compressibilities, or electrochemical equiva- 

 lents ; all are not of equal significance, but 

 each in its way is fundamental. These 

 properties although undoubtedly some- 

 what connected with one another, can not 

 yet be safely predicted ; each must be ascer- 

 tained for itself. Thus a colossal task is 

 involved in their accurate determination. 



How nearly has this task been com- 

 pleted? The comparative study of the 

 existing accumulation of experimental data 

 concerning chemical phenomena affords 

 reason for congratulation that so much has 

 been done within a single century; but it 

 also reveals the fact that much remains to 

 be done. For in spite of the fact that 

 physical measurements are the basis of all 



