246 SCIENCE PROGRESS 



their arrangement and distances apart, were the same in the 

 two salts. 



Matters were in this condition when, in 1 848, Pasteur, then 

 a young man of twenty-six, made the first of his long series of 

 brilliant discoveries. Having just completed his studies at the 

 Iscole Normale, he decided to re-examine crystallographically 

 tartaric acid and its salts in order to perfect himself in crystal 

 measurement, a subject which much attracted him. In this 

 re-examination he noticed that hemihedral facets, although 

 not very obvious, and consequently unrecognised by previous 

 observers, were present on the crystals both of tartaric acid and 

 of its salts. 



Pasteur, being acquainted with Herschel's proof of the 

 connection between the hemihedry and the optical activity of 

 quartz, became convinced that a similar relationship must 

 exist between the hemihedry of the tartrates and their optical 

 activity. He reasoned that, although Mitscherlich had failed 

 to observe hemihedral facets on crystals of ammonium sodium 

 tartrate, he was probably correct in the statement that they 

 did not exist on the crystals of the inactive ammonium sodium 

 racemate. 



To his surprise, however, he found hemihedral facets also 

 on the crystals obtained by allowing a solution of ammonium 

 sodium racemate to evaporate slowly at the ordinary tempera- 

 ture. On examining both the tartrate and racemate crystals 

 more closely, however, he found that there was a difference 

 between them, for, whilst in the tartrate the hemihedral facets 

 were identically disposed, in the racemate the disposition was 

 of two kinds, conventionally termed right-handed and left- 

 handed respectively. Faced by this unexpected result, he 

 carefully picked out the right-handed crystals from the left 

 and examined a solution of each in the polariscope. He then 

 found that the solution of the right-handed crystals was dextro- 

 rotatory and that of the left-handed laevo-rotatory, whilst a 

 solution of equal weights of each of the two kinds was inactive 

 to polarised light in consequence of the compensating equal 

 but opposite activities of the two forms. 



From these active salts he prepared the two corresponding 

 active tartaric acids. The laevo variety of the acid had up to 

 that time been unknown. These varieties were not distin- 

 guishable chemically, and crystallised in similar forms with 



