450 SCIENCE PROGRESS 



lose its power of rotating the plane of polarisation, though it has 

 lost its crj'stalline character. In the case of these substances, 

 therefore, the influence exerted on the plane of polarisation is 

 not due to the same cause as the rotatory power of the quartz 

 crystal ; it is rather to be sought within the molecule itself. 



It has been found that these optically active substances exist 

 in two forms, one of which rotates the plane of polarisation to 

 the right, while the other isomer rotates it through an equal 

 angle to the left. This has led to the adoption of the current 

 nomenclature, wherein the name of the right-rotating substance 

 has the prefix dextro attached to it, while the left-rotating variety 

 is termed the Icevo isomer. Thus we speak of dextro camphor 

 and laevo camphor, or simply rt'-camphor and /-camphor. Now, 

 obviousl}", if we mix equal quantities of the dextro and laevo 

 isomer together, the resulting mixture will have no influence 

 upon the plane of polarisation ; for the action of the right- 

 rotating substance will be counteracted by the equal and 

 opposite effect of the left-rotating bod}^ and the mixture will 

 be inactive by external compensation. Under certain conditions 

 it is found that the dextro and laevo molecules combine together 

 to form an actual inactive compound, which is termed a racemic 

 substance (usually abbreviated to r or dl). The dextro and laevo 

 bodies are usually referred to as optical antipodes. 



The first successful step in the study of these substances was 

 made by Pasteur^ in i860, when he took up the question of the 

 tartaric acids. He was able to show how to isolate the dextro 

 and laevo forms from the racemic compound, discovering no 

 fewer than three separate methods of attaining this object. He 

 showed, further, that while racemic tartaric acid crystallizes 

 in holohedral form, the dextro and laevo isomers show hemi- 

 hedrism, the dextro cr3'stal being the mirror image of the la^vo 

 crystal. A consideration of the general properties of the three 

 substances led Pasteur to the conclusion that in the dextro and 

 laevo forms the atoms might be grouped in spirals or situated at 

 the corners of a tetrahedron. He was unable, in the elementary 

 state of structural chemistry at that time, to do more than put 

 this forward as a conjecture, and it remained for two later 

 investigators to reap the credit of establishing the correctness 

 of this view. 



* Pasteur, Recherches sur la dissyvimetrie inoUculaire des prodiiits organiques 

 naturels^ Paris (i860). 



