2O Biological Chemistry. 



The pressure in the apparatus is indicated by a manometer, 

 m, and the very low pressures (under 1 mm.) by a 

 MacLeod gauge (p, o, n).* It is not, of course, always 

 necessary to fractionate under such low pressures. Ordi- 

 narily, fractionation under 10-12 mm. pressure is sufficient, 

 and in this case the usual laboratory Bunsen water-pump 

 can be employed. The apparatus can then be simplified, 

 in that the pressure can be determined by the ordinary 

 manometer, and a vapour-trap cooled with liquid air is 

 unnecessary. The boiling point of a liquid will vary with 

 the pressure under which distillation takes place ; this con- 

 stant, therefore, should never be cited unless the pressure 

 is given at the same time. When a substance is distilled 

 under ordinary atmospheric pressure, the height of the 

 barometer, as read at the time, should also be noted. 



Certain substances, which are solid at ordinary tem- 

 peratures, can also be purified by distillation, and these 

 generally melt before boiling. Other solid substances can 

 be vaporized without melting (such as iodine), and puri- 

 fied in this way. This is known as the process of subli- 

 mation ; in this process the vapours are allowed to come 

 into contact with a cooled surface, and the substance is 

 condensed in a crystalline form. 



A more complete separation can often be effected by 

 distilling through a long column or still head, examples of 

 which are illustrated (Fig. 5). When this is used the vapour 

 of the less volatile substance is being continually condensed 

 and falling back into the distillation flask, whilst the 

 more volatile product passes over. 



Certain substances will not distil without decomposition 

 even under the lowest pressures. Various resinous pro- 

 ducts come under this category, and they are often not to 



* For description of this gauge reference should be made to text-books on 

 Physics. 



