420 CHEMICAL DISCOVERY AND INVENTION 



subject dates back to the period, now sixty years ago, when 

 chemical constitution began to be understood, and of course 

 the facts and deductions from them are recounted in all the 

 principal textbooks of organic chemistry. But the steps for- 

 ward which resulted from Emil Fischer's work twenty years ago 

 represent one of the great triumphs of synthetical chemistry, of 

 which the first stages were indicated in a recent chapter (p. 334), 

 and of which a further development will have to be reported 

 further on. 



The first step to be taken in the study of such a class of com- 

 pounds as the sugars is to discover a method by which the 

 several compounds may be discriminated and recognised even in 

 the presence of one another. Nearly all these compounds are 

 characterised by rotating the plane of polarisation of a ray of 

 polarised light, and this serves as an indication that the molecule 

 contains one or more atoms of carbon in the condition which has 

 been described as asymmetric (p. 215). This alone would be 

 insufficient, but Fischer was successful in finding a chemical 

 reagent, phenyl-hydrazine, with which all the saccharoses unite 

 and with an excess of it interact to produce a characteristic com- 

 pound, called an osazone, by which each sugar may be separated, 

 purified, and identified. 



The first formed product of the union of glucose, for example, 

 with phenylhydrazine C 6 H 5 NH'NH 2 , is a soluble compound 

 called a hydrazone which is represented as follows : 



CH 2 -OH 



CH-OH 



CH-OH 



CH-OH 



I 

 CH-OH 



CH : N-NHC 6 H 5 



water being formed at the same time. 



In the presence of a larger quantity of the reagent a yellow, 

 almost insoluble precipitate of the osazone is thrown down, and 

 this may be collected, examined under the microscope, and its 



