336 GREAT MEN OF SCIENCE 



as familiar to us in Faraday's work. Bunsen thus withdrew 

 at a period of uncertainty from the quarrel about 'hypotheses 

 which are changeable, and are often changed.'^ 



By means of the combining weights, which were free from 

 hypothesis, he was just as well able to express in formulae the 

 actually observed composition by weight of all compounds, 

 as by using the atomic weights, the true values of which 

 could only be discovered beyond doubt later, after Avo- 

 gadro's law had been rendered certain by Clausius' proof 

 of the kinetic theory of gases. When this matter had been 

 cleared up, Bunsen refused to make a further change, and 

 adhered to combining weights. This, however, prevented 

 him from penetrating into the manner in which the molecules 

 of compounds are built up from their atoms> since this 

 requires the use of formulae founded upon true atomic and 

 molecular weights. This advance, which represented an 

 important step forward, was left to his younger contem- 

 poraries and successors to accomplish. In effecting it they 

 were able to rely upon Avogadro's law, which was already 

 proved to be true, and gives the correct molecular weight. It 

 then appeared that we must ascribe to the various atoms 

 various values expressing their power of combination with 

 other atoms, according as, for example, they are able to com- 

 bine with one, two, three or more hydrogen atoms, to form a 

 molecule; this value for the combining power also holds good 

 in general as regards the combination of the various atoms 

 among themselves. It is thus possible to ascribe to each 

 atom, in proportion to its combining power - its 'valency' - 

 a corresponding number of valency positions, and to assume 

 that when a molecule is formed, atoms are linked together 

 because their valencies 'saturate' one another in pairs. 



These hypotheses agreed with Davy's and Berzelius' 

 ideas concerning the electrical nature of the forces holding 



^ Bunsen thus expressed in his lectures his decided distaste for hypo- 

 theses which are often put forward as finished theories. 



