THE ATOMIC VIEW OF NATURE. 441 



cists were now alike compelled to venture on some more 

 definite hypothesis, descriptive of the great variety of 

 constitution which the molecules of chemically distinct 

 substances exhibit. These molecules show in their com- 

 bining numbers, and in their physical properties, great 

 fixity, excluding apparently all gradual transitions. The 

 manner in which they enter into, and again separate out 

 of, combinations and compounds, always regaining and 

 showing their original characteristics, forced more and 

 more upon natural philosophers the conviction that com- 

 pounds were merely geometrical arrangements of indi- 40. 



Geometrical 



vidually independent atoms, and that these atoms must arrangement 



of atoms. 



possess geometrically different forms and figures, enabling 

 them, without loss of their individuality, to enter into 

 varying configurations. 



The conception of the molecule as a system of atoms, 

 geometrically arranged, had gradually grown from vague 

 suggestions in the minds of physicists as well as chemists 

 i.e., of students of the quantitative as well as of those 

 of the qualitative properties of substances. To the former 

 it was especially the forms of crystals, to the latter the 

 different degrees of saturation of chemical substances, 

 that suggested a geometrical arrangement of atoms as 

 the constitution of the smallest particles or molecules 

 of different substances. 



Ever since the study of the regular forms of minerals . 



Crystallo- 



or of artificially prepared crystals was reduced to an graphy. 

 exact science by the labours of Haiiy, at the end of 

 the last century, 1 the forms of these regular shapes 

 have been valued by investigators, for two distinct rea- 



1 See above, chapter i. p. 116. 



