OF ARTS AND SCIENCES. 149 



X. 



CONTRIBUTIONS FROM THE CHEMICAL LABORATORY OF 

 HARVARD COLLEGE.— J. P. Cooke, Director. 



THE RELATIVE VALUES OF THE ATOMIC WEIGHTS 

 OF HYDROGEN AND OXYGEN. 



Br Josiah Parsons Cooke 



AND 



Theodore William Richards. 



Presented June 15, 1887. 



Introduction. 



Since the application by Dalton of the atomic theory to explain the 

 definiteness of the combining proportions of the elementary substances 

 of chemistry, these proportions have been generally regarded as the 

 ratios of the weights of the atoms, and the values assigned to each 

 element have been generally called atomic weights. 



The conception was early suggested and advocated by Dr. Prout, 

 an eminent physician of London during the first half of this century, 

 that the elementary atoms were all aggregates of the atom of hydro- 

 gen, the lightest atom known. If this were true, it would of course 

 follow that the atomic weights of the elements would all be multiples 

 of the atomic weight of hydrogen ; so that, if the weight of the atom 

 of hydrogen were selected as the unit of the system, all other atomic 

 weights must he multiples of this unit, and therefore whole numbers. 



The facts known at the time (1815) were not inconsistent with this 

 view ; but as the methods of chemical analysis were improved, and the 

 combining proportions determined with greater accuracy, marked dis- 

 crepancies from Prout's hypothesis appeared. Still, so great was the 

 hold which the conception had taken upon chemical students, that for 

 a long time the nearest whole numbers to the combining proportions 

 observed were accredited as the true value of the atomic weights, rather 

 than the actual mean of the experimental results ; and this practice is 

 still followed in many standard publications, notably the " Jahresbericht 



