Manchester Memoirs, Vol. xhii. {igo'^), No. W. 15 



constitution fell into their proper places under the new 

 generalisation, for that was broad enough to comprehend 

 them all. All constitutional chemistry depends upon this 

 property of the atoms, and any other adequate foundation 

 for it would be difficult to find. 



I have said that the discovery of valency explained 

 the phenomena of isomerism. Indeed, it enabled chemists 

 to foresee the existence of new isomers, and it established 

 the conditions under which such compounds could exist. 

 And yet, in one direction at least, its power was limited, 

 and substances were found which the theory could not 

 interpret. Tartaric acid, for example, exists in two 

 modifications, differing in crystalline form and in their 

 action upon polarised light. One acid was dextro-, the 

 other laevorotatory, while a mixture of the two in equal 

 proportions was neutral to the polarised beam, and gave 

 no rotation at all. Their crystals exhibited a similar 

 difference in the arrangement of certain planes, one set 

 being right-handed, the other left-handed ; and each 

 crystal resembled its isomer like a reflection in a mirror, 

 alike, but reversed. For a long time this physical 

 isomerism, as it was called, remained inexplicable, for 

 the rules of valency gave to both molecules the same 

 structure, and offered no hint as to the cause of difference. 

 Structural formulae, however, said nothing of the arrange- 

 ment of the atoms in tridimensional space, and it was 

 soon suspected that the root of the difficulty was here. 

 The mere linking of the atoms with one another could be 

 represented in a single plane, but that was obviously an 

 imperfect symbolism. 



In 1874 van'tHoffand Le Bel, working independently 

 of each other, suggested a solution of the problem. One 

 simple assumption was enough ; merely that the quadriva- 

 lent carbon atom was essentially a tetrahedron, or, more 



