Identification of Organic Compounds. I 3 



of the constitution of its degradation products, it is often 

 possible to obtain an approximately correct idea of the 

 chemical architecture of a complex substance. With such 

 an approximation, in the present state of development of 

 chemical technique, the investigator must often remain 

 content. 



From the foregoing remarks it is obvious that it is 

 essential to study, in the first instance, the methods by 

 means of which the simpler organic compounds can be 

 isolated in a state of purity and their chemical composi- 

 tion and constitution determined. 



The Preparation of Pure Products and the Criteria of Purity. 



(a) SOLID SUBSTANCES. 



The more complex substances which are found in living 

 objects are, for the most part, amorphous that is to say, 

 they possess no definite crystalline structures. They can 

 also be heated until they char and decompose without 

 showing any definite " melting point." In these respects 

 they differ markedly from the simpler derived products. 

 The latter can generally be dissolved in some liquid heated 

 to its boiling point, from which, on cooling, they will 

 separate in a crystalline form, provided that the degree of 

 saturation in the hot liquid is sufficiently great. This 

 process of " crystallization " is very often employed for the 

 purpose of purification, for if the substance originally dis- 

 solved in the hot solvent is not perfectly pure, the con- 

 taminating substances will often remain in the "mother 

 liquors" on cooling, and the chief product will separate 

 out in a purer state in crystalline form when the hot 

 solution is cooled. Even after this process the crystals 

 may not be those of a perfectly pure substance ; in this 

 case a second recrystallization will result in the production 



