July 24, 1913] 



NATURE 



5-15 



green When the bright yellowish-green is extin- 

 guished a grey "neutral tint" is produced which is 

 extremely sensitive to small rotations of the plane of 

 polarisation, and was at one time used very largely in 

 polarimeters illuminated with white light 



When monochromatic light is used— as for instance, 

 when a green screen is placed in front of the mercury 

 arc— the light can be extinguished completely even 

 after it has passed through a very long column of 

 quartz. Using green light purified by a spectroscope 

 and rods of quartz cut from a crystal of extraordinary 

 beauty I have obtained a perfectly sharp extinction 

 with a column of quartz half a metre in length, giving 

 an actual rotation of 12,789-20° + o-oi°. I have also 

 been making experiments with the same material to 

 determine accurately what rotation is produced by 

 quartz in light of dill, rent wave-lengths, not only in 

 the visible spectrum, but also in the infra-red and 

 ultra-violet regions; but as the work is still incom- 

 plete I will not attempt to describe it, but pass on at 

 once'to other ways in which rotatory polarisation may 

 be produced. . . 



Three years after his discovery of rotatory polansa- 

 tion in quartz Biot was astonished to find that the 

 same property was possessed by certain liquids tur- 

 pentine and "laurel-oil rotating the plane of polarisa- 

 tion to the left, and oil of lemon and camphor (dis- 

 solved in alcohol) rotating it to the right. 



In the case of quartz, Biot had attributed the rota- 

 tion of the Plane of polarisation to the crystalline 

 structure of "the material. The correctness of this 

 view was proved when it was shown that rotatory 

 polarisation no longer took place when the crystalline 

 structure of quartz was destroyed by melting it or by 

 dissolving it in alkali. In the case of liquids this 

 explanation was no longer possible. Rotatory polar- 

 isation must here be attributed to some lack of sym- 

 metry in the structure of the molecule rather than ot 

 the crystal. It is in such cases that the polanmetcr 

 has proved its supreme value in the investigation of 

 molecular structure. In this connection it will be 

 sufficient if I refer to the classical researches ot 

 Pasteur van't Hoff, and le Bel, and to the brilliant 

 contemporary work of Pope, Kipling, Smiles, and 

 Mills in our own country, and of Meisenheimer and 

 Werner on the Continent. In each of these inves- 

 tigations the development of "optical activity" has 

 been accepted as a conclusive proof of molecular asym- 

 metry and no firmer basis for theories of molecular 

 structure has vet been found than that which rests 

 upon the use 'of the polarimeter to detect rotatory 

 polarisation. 



C. — Mutarotatiou. 

 In 1846, thirty years after Biot had discovered that 

 rotatory polarisation might occur in liquids as well as 

 in crystals, a remarkable discovery was made by the 

 French chemist Dubrunfaut in reference to the rota- 

 tory power of aqueous solutions of grape-sugar or 

 glu'cose Dubrunfaut found that by using freshly 

 prepared solutions of the sugar he could observe a 

 transient rotatory power which was twice as great as 

 that observed in solutions which had been prepared 

 a few hours previously. To this remarkable pheno- 

 menon he gave the name Birotation. 



The same phenomenon, which is now generally 

 known as mutarotation, has since been observed in 

 the case of nearlv all the "reducing" sugars. Many- 

 explanations were given to account for so mysterious 

 a change, but nothing in the way of proof could, as 

 a rule, be offered in support of these suggestions. 

 In 1890, however, Emil Fischer discovered that similar 

 changes of rotatory power occurred when gluconic 

 lactone was dissolved in water and thus partial. y 

 hvdrolysed to gluconic acid— 

 NO. 2282. VOL. Qll 



C 6 H,„O +H,O — C^O,. 



Gluconic lactone Gluconic acid. 



He therefore suggested that a similar explanation 



might be given of the mutarotation of glucose, thus— 



C 6 H 1= 6 +HX>.=C,H,,0 T . 



Glucose. Glucose hydrate. 



Mutarotation oj Nitrocamphor. 

 In 1806 a happy accident led to the discovery that 

 very marked changes of rotatory power occur in 

 freshly prepared solutions of nitrocamphor. but un- 

 like ttv. case of glucose, these changes could be 

 observed in a large range of solvents. The change 

 varied greatly in the numerical values involved, but 

 was always in the same direction-from left towards 



n The cause of the mutarotation was not difficult to 

 discover It could not be due to hydration, nor indeed 

 to anv direct chemical action of the solvent, but must 

 be attributed to some change of structure in the mole- 

 cule of the nitrocamphor itself. In view of the fact 

 that the nitro-compound is able to simulate the pro- 

 perties of an acid, giving rise to strongly dextro- 

 rotatory salts, there could be little doubt that the 

 change of rotatory power was caused by a partial 

 conversion of the nitrocamphor into its acidic form- 

 a conversion which can be rendered^ complete by the 

 addition of alkali. This view was immediately con- 

 firmed by the discovery of a dextrorotatory anhydride, 

 which could be prepared from nitrocamphor merely 

 bv evaporating its solutions on a water-bath. 

 'This interconversion of isomeric compounds which 

 we have called dynamic isomerism, could also be used 

 to explain the mutarotation of glucose of which two 

 isomeric forms are known; but there is good reason 

 to believe that the hydrolysis suggested by Fischer 

 is also an important factor when aqueous solutions 

 of the sugar are under consideration. 



In the case of ir-bromonitrocamphor two isomeric 

 forms of the substance can actually be isolated, thus 

 affording direct evidence that the mutarotation ob- 

 served in the case of this compound is due to a 

 reversible isomeric change. 



Form of the Curves. 

 In most cases the change of rotatory power pro- 

 ceeds according to a very simple law, he rate o 

 change being directly proportional to the distance still 

 remaining to be traversed. 



But I have recently found a number of cases in 

 which the curves are far more complex. In such 

 instances it is necessary to assume a series of suc- 

 cessive isomeric changes; but this assumption pre- 

 sents no difficulty, as the substances in question can 

 all be formulated in at least five different ways. 

 Acceleration by Catalysts. 

 The mutarotation of glucose is accelerated to a 

 moderate extent by acids and very largely indeed b> 

 alkalis. Similar observations have been made in the 

 case of nitrocamphor. Piperidine added to a solution 

 of nitrocamphor in benzene produces a remarkable 

 acceleration which can be detected even at a concen- 

 tration of N/ 10,000,000, i.e. 1 part in 100 million or 

 1 centigram per ton. Aniline is 100,000 times less 

 active. 



Arrest of Isomeric Change. 

 The fortunate selection of chloroform as one of a 

 series of solvents led to the discovery of one of the 

 most important facts that have come to light in the 

 course of fifteen years' work on mutarotation. In 

 the very earliest stages of the work it was found that 



