178 GREAT MEN OF SCIENCE 



composition of the substance, e.g. water. These final 

 particles, from which all matter is built up, are called by 

 Dalton 'atoms.' He speaks not only of atoms of the funda- 

 mental substances, but also of atoms of water, for example 

 where we to-day speak of molecules, in order to distinguish 

 groups of atoms from single atoms. Such an atom (mole- 

 cule) of water might in the simplest case consist of one atom 

 of hydrogen and one atom of oxygen (this is Dalton's as- 

 sumption), and in this case the known composition of water 

 would already give us the relationship by weight of these two 

 atoms (very nearly i : 8 according to our present know- 

 ledge, I : 7 by the figures known to Dalton). But instead of 

 one atom, there might also be two, three, and so on, of each 

 fundamental substance in each molecule; any multiples of 

 whole numbers, but no intermediate stages, since the atoms 

 are supposed to be alike and unchangeable; this gives the 

 explanation of the law of multiple proportions. The possi- 

 bility of multiples did not prevent the weights of the atoms 

 being determined; they only made it more difficult, inas- 

 much as many compounds of the same fundamental sub- 

 stance had to be investigated, in order to exclude as far as 

 possible errors due to multiples. This is the line taken by 

 Dalton with the existing knowledge of the quantitative com- 

 position of various compounds, and he accordingly constructs 

 for the first time a table of atomic weights, in which that of 

 hydrogen is already taken as unity. 



This became the foundation of the whole further develop- 

 ment of chemistry, and indeed, of the whole further develop- 

 ment of all our knowledge of matter. It was at that time 

 only an hypothesis (supposition), but an hypothesis founded 

 on a very great deal of quantitative knowledge, and one also 

 capable of further quantitative tests. It has stood these tests 

 in the fullest possible manner, inasmuch as it has always led - 

 in cases which to-day are innumerable - to correct conclu- 

 sions. It has thus gradually become a theory, that is to say a 



