CHEMISTRY 223 



It must be noted that carbon atoms to 

 which four different groups are attached are 

 asymmetric, that is to say, they can exist in 

 two forms which resemble each other as the 

 right hand resembles the left (Pasteur, LeBel, 

 van't Hoff). This characteristic results in 

 further increase in the number and variety of 

 organic compounds. It is therefore neces- 

 sary, in writing the formula, to represent the 

 form of the molecule as it exists in space (in 

 three dimensions), and this is actually accom- 

 plished by imagining the three-dimensional 

 formula to be projected upon the paper so that 

 when the hydrogen atom is written to the right 

 of the carbon atom one asymmetric form of 

 the latter is designated, when the hydrogen 

 atom appears to the left, the other. 



It has long been known that when glucose 

 is dissolved in water its optical activity, as 

 the power of a substance to rotate the plane 

 of polarization of light is loosely termed, 

 changes slowly for some time before reaching 

 a constant value. Recently it has been shown 

 that this phenomenon probably depends upon 

 the existence in solution of three different forms 

 of glucose, which pass freely into one another 

 and ultimately attain a state of equilibrium. 1 



1 This subject has been fully discussed by Hudson, Journal 

 of the American Chemical Society, XXXII, 889, 1910. 



