Stereoisomerism. 175 



of treatment, undergo what is known as " racemization " 

 that is, they are converted into the racemic form, and 

 the expression " racemic " is now applied not only to the 

 tartaric acids, as in its original sense, but to all the ex- 

 ternally compensated substances which can be resolved 

 into enantiomorphous forms. The methods of accomplish- 

 ing this process are considered in the next section. The 

 chemical properties of all isomerides are the same, all act 

 as dihydroxysuccinic acid. When substances contain 

 more than two asymmetric carbon atoms, the number of pos- 

 sible stereoisomerides increases. Whereas, as already stated, 

 two isomerides only are possible when one asymmetric 

 carbon atom is in the molecule, four can exist in substances 

 containing two asymmetric carbon atoms, eight when 

 three such atoms are present, and no less than sixteen 

 when four are present. A very important example of the 

 last description of isomerism is afforded by the sugars, the 

 chemistry of which will be discussed in some detail later. 



The Resolution of Raceme Forms into their Enantio- 

 nwrphs. Various methods exist for the resolution of 

 optically inactive forms. (N.B. Where the inactivity is 

 due to external compensation only ; as already stated, in- 

 ternally compensated forms cannot be resolved.) Only the 

 more important of these need be considered. 



A. Resolution by Crystallization. This method has 

 been already mentioned in the historical introduction to 

 this section, and was employed by Pasteur, who showed 

 that from solutions of the ammonium sodium salt of 

 racemic acid two kinds of crystals were formed, which 

 could be separated from one another mechanically. This 

 is, of course, a very tedious process, and the two kinds of 

 crystals (of the sodium ammonium salts of the d- and l- 

 acids) must be carefully sorted and separated from one 

 another by hand. The crystallization, where a separation 



