292 A CENTURY OF SCIENCE 



the periodic law, and so on, and suitable changes were 

 made in the chemical formulas. 



Berzelius used 100 parts of oxygen as the basis of his 

 atomic weights, a practice which was generally followed 

 for several decades. Dalton, however, had originally 

 used hydrogen as unity as the basis, and this plan finally 

 came into use everywhere, as it seemed to be more log- 

 ical and convenient, because hydrogen has the smallest 

 atomic weight, and also because the atomic weights of a 

 number of common elements appeared to be exact multi- 

 ples of that of hydrogen, thus giving simpler numbers for 

 use in calculations. 



Within a few years a slight change has been made by 

 the adoption of oxygen as exactly 16 as the basis, which 

 gives hydrogen the value of 1-008. 



As early as 1815, Prout, an English physician, had 

 advanced the view that hydrogen is the primordial sub- 

 stance of all the elements, and consequently that the 

 atomic weights are all exact multiples of that of hydro- 

 gen. This hypothesis has been one of the incentives to 

 investigations upon atomic weights, for it has been found 

 that these constants in the cases of a considerable num- 

 ber of the elements are very close to whole numbers 

 when based upon hydrogen as unity, or even still closer 

 when based upon oxygen as 16. 



With our present knowledge Prout 's hypothesis may 

 be regarded as disproved for nearly all the elements 

 whose atomic weights have been accurately determined, 

 but the close or even exact agreement with it in a few 

 cases is still worthy of consideration. There is an inter- 

 esting letter from Berzelius to B. Silliman, Jr., in the 

 Journal (48, 369, 1845) in which Berzelius considers the 

 theory entirely disproved. 



For a long time entire reliance was placed upon the 

 atomic weights obtained by Berzelius, but it came to be 

 observed that the calculation of carbon from carbon diox- 

 ide appeared to give high results in certain cases, so that 

 doubt arose as to the accuracy of Berzelius 's work. Con- 

 sequently in 1840 Dumas, assisted by his pupil Stas, made 

 a new determination of the atomic weight of carbon, and 

 found that the number obtained by Berzelius, 12-12, was 

 slightly too large. Subsequently Dumas determined 



