432 SCIENTIFIC THOUGHT. 



the image in a mirror from the original. Was it possible 

 any longer to suppress the conviction that the smallest 

 particles of matter, in forming chemical compounds, do 

 so not only in definite proportions of weight, but also in 

 definite geometrical distances and positions ? 



About the middle of the century the atomic view of 

 matter had thus received considerable modifications. Or- 

 iginally suggested only to explain, describe, or symbolise 

 the fact that different substances combine in fixed, and 

 especially in fixed multiple proportions, it had to be 

 modified by a recognition of the fact that in gases at least 

 a distinction exists between particles which are closely 

 knit together as it were, geometrically inseparable and 

 such as can move away from each other. The latter 

 explain the increase of volume under increasing tempera- 

 ture or decreasing pressure. Geometrical distance came 

 in as the means of distinguishing the molecule from the 

 atom. And lastly, about 1850, the phenomena of right- 

 and left-handedness,^ discovered by Pasteur, suggested the 

 33. idea of geometrical position as well as of distance. The 



Atom and 



molecule, atoui had become a molecule, with a definite geometrical 

 arrangement. 



It took, however, a full generation before, in the second 

 half of the century, these different suggestions for a modi- 

 fication of the atomic view became clear, before philo- 

 sophers took seriously the opinion that molecules and 

 atoms existed in reality, and were not merely a convenient 

 symbolism, as many great chemists during the first half 

 of the century were inclined to think. This change in 

 the habit of chemical thought has no doubt been greatly 



1 Called by Lord Kelvin " chirality." 



