PROGRESS IN NINETEENTH CENTURY 223 



It was in October, 1803, that John Dalton published the beginning 

 of his famous atomic theory, but it was not until five years later that 

 he gave it completely to the world. Merely as a speculation, the idea 

 of atoms was as old as philosophy; but in its scientific form it was 

 something entirely new. Under it, the law of definite proportions 

 became necessary and significant; the law of multiple proportions, 

 which had been partially anticipated by others, was made complete ; 

 and these considerations alone would have justified the provisional 

 acceptance of the doctrine. It unified the known or suspected laws of 

 chemical combination and gave them philosophic validity. It incited 

 chemists to verify the evidence in its favor, and so led to new dis- 

 coveries; in short, it fulfilled all the conditions of a good scientific 

 theory. Its chief peculiarity, however, its prime difference from all 

 preceding atomism, remains to be stated. Dalton discovered that 

 to every element a single definite number could be assigned, and that 

 these numbers or their multiples governed the formation of all com- 

 pounds. Oxygen, for instance, unites with other elements in the 

 proportion of eight parts by weight or some multiple thereof; never 

 in other ratios. These combining numbers, under Dalton's theory, 

 became the relative weights of the atoms; and atomism, hitherto a 

 qualitative notion only, received a quantitative expression. With the 

 help of these atomic weights, or combining numbers, as some anti- 

 theorists preferred to call them, the composition of any substance 

 could be represented by a simple formula; and chemical calculations, 

 which had been empirical and arbitrary, became systematic and easy. 

 In short, Dalton had discovered a new class of constants, the funda- 

 mental numbers of quantitative chemistry, whose significance has 

 steadily increased and is probably not even yet completely appre- 

 ciated. To this point I shall recur later. 



The decade following Dalton's unique discovery was chiefly 

 characterized by two lines of research, the study of inorganic com- 

 pounds, and the investigation of their physical relations. Davy, 

 by decomposing the alkalies and earths, gave precision and definite- 

 ness to the conception of a chemical element, while Gay Lussac and 

 Avogadro discovered the laws which connected the volume relations 

 of gases with their chemical composition. To Avogadro we owe the 

 discrimination between atoms and molecules a distinction which 

 physics, unaided by chemical evidence, could probably not have 

 reached, and which even now is often overlooked by physicists. Max- 

 well, for example, in his article upon atoms in the Encyclopaedia 

 Britannica, deals with molecules throughout, and fails to mention 

 Dalton's work at all. To Maxwell the physical arguments were clear, 

 the chemical relations were not adequately appreciated. 



In 1819 Dulong and Petit discovered the law connecting the 

 specific heat of a solid element with its atomic weight, but apart 



