ARCHITECTURE OF MOLECULES 213 



otherwise not to be accomplished. These are to be found 

 especially in connection with laboratory processes employed in 

 the study of organic compounds. The remarkable characters 

 and properties of enzymes will be described in a later chapter. 



CHAPTER XII 



ARCHITECTURE OF MOLECULES 

 STEREO-CHEMISTRY AND THE NATURE OF VALENCY 



WE may begin by recalling the fact that Dalton and several of 

 the early promoters of his atomic theory were led to consider, 

 though without giving the subject much attention, the question 

 of the arrangement which chemically combined atoms assume 

 in space of three dimensions. If a detached molecule could be 

 seen, what would it look like ? There is a great deal of evidence, 

 some of which is indicated in a preceding chapter, that each 

 atom retains its independence, so that there is a certain rough 

 analogy between the bricks in a wall and the atoms in a mole- 

 cule. Dalton, referring to the diagrammatic representations of 

 atoms in his Chemical Philosophy, Part I (1808), says : 



'' The combinations consist in the juxtaposition of two or 

 more of these (atoms) ; when three or more particles of elastic 

 fluids are combined together in one it is to be supposed that the 

 particles of the same kind repel each other and therefore take 

 their stations accordingly." 



Dalton also gives diagrams showing the arrangements which 

 he supposed might exist in a number of different compounds, 

 including a substance so complex as alum. 



Wollaston about the same time recognised that it would be 

 necessary " to acquire a geometrical conception of their relative 

 arrangements in all the three dimensions of solid extension." 



It was nearly fifty years before the germ of the doctrine of 

 valency was recognised by Frankland, but it has been a subject 

 of constant enquiry, experiment, and discussion down to the 

 present day. 



In the chapter on Electrolysis a brief indication was given of 

 one, and perhaps the most important method, of measuring this 

 property, though for obvious reasons it cannot be applied in all 



