390 ENZYMES 



column, so much of the water is removed that the distillate 

 contains about 80 to 95 per cent of alcohol.* 



No amount of fractional distillation without dehydrating 

 agents will produce alcohol containing less than 4-43 per cent 

 by weight of water, since such alcohol gives a constant boiling 

 mixture. 



Alcohol containing 0-5 per cent or less of water is, in 

 commerce, known as absolute alcohol, although in a scientific 

 laboratory the term is only correctly applied to alcohol which 

 is quite free from moisture ; such alcohol can only be obtained 

 by careful fractionation from freshly burnt quicklime, f If 

 the alcohol is dehydrated over quicklime to which a little 

 barium oxide has been added, complete dehydration is marked 

 by the formation of a yellow colour due to the production of 

 barium ethylate, which can only be formed in the absence of 

 any trace of moisture. 



A delicate test for the detection of traces of moisture in 

 alcohol consists in adding a few drops of the sample to a 

 solution of liquid paraffin in anhydrous chloroform ; if there 

 is any moisture present, a turbidity will be at once produced. 



OXIDASES. 



The oxidases are enzymes which have the power of oxi- 

 dizing various aromatic compounds and chromogens, which 

 action is frequently indicated by a change in colour. This 

 change in colour in vegetable tissues on exposure to air is an 

 everyday phenomenon ; the exposed surfaces of a bitten apple, 

 especially cider varieties, will rapidly turn brown ; similarly 

 the fruit-body of Boletus quickly assumes a prussian-blue colour 

 on being broken. The darkening in the colour of raw rubber 

 is also due to an oxidase which is associated with the protein 

 of the coagulated latex. J 



These changes are often of considerable economic import- 

 ance ; thus the discoloration of sap wood markedly depreciates 



* The residue remaining after distillation contains, in addition to the solid 

 unfermentable materials, a certain amount of other soluble products of fermenta- 

 tion, such as glycerol and succinic acid ; it is used as a cattle food. 



f Occasionally the last traces of moisture are removed by treating the alcohol 

 with sodium wire. 



JSpence : " Biochem. Journ.," 1908, 3, 165, 351. 



