50 BACTERIOLOGICAL AND ENZYME CHEMISTRY 



difficultly crystallisable syrups ; they can be combined with 

 a substance known as phenyl hydrazine to form well-defined 

 crystalline compounds. 



The crystalline compound is pure when it has a constant 

 melting-point ; that is, if the melting-point of the substance 

 is determined and it is redissolved and recrystallised, and 

 the melting-point of the crystals again determined, the two 

 melting-point determinations should be the same. 



To purify a substance by distillation, it can be distilled 

 either at the ordinary atmospheric pressure, or under reduced 

 pressure, so long as the temperature of the vapour remains 

 constant; if a rise of the thermometer is observed during 

 distillation, it means that some substance other than the 

 lower boiling substance is being distilled over. By repeating 

 the distillation of the portions distilled over between various 

 limits of temperature, a distillate is finally obtained having a 

 constant boiling-point ; such a process is known as fractional 

 distillation. The separation of the products of petroleum by 

 distillation on the large scale is a good instance of this process. 

 It is characteristic of a pure compound that it has a constant 

 boiling-point. 



It may not be superfluous here to emphasise the fact that to 

 the chemist a substance can only be considered to be a definite 

 chemical entity when it satisfies one of three conditions : 



1. It has a definite crystalline form, 

 or 



2. It has a constant melting-point, 

 or 



3. It has a constant boiling-point. 



Many of the substances met with in the chemistry of 

 vital processes, more especially the derivatives of albumin, 

 do not satisfy these conditions. Such substances can be 

 differentiated one from another by their general chemical and 

 physical properties, and by the products of their decom- 



