i6 



Biological Chemistry. 



of a capillary tube (which may be sealed at either one 

 or both ends). The capillary tube is then fastened to a 

 thermometer by means of an india-rubber band, and the 

 lower ends of the two are then immersed in a liquid such 

 as a high boiling paraffin or sulphuric 

 acid contained in a beaker or small 

 bulb of about 50 c.c. capacity (see 

 illustration, Fig. 2). The liquid is, 

 then slowly warmed and stirred, whilst 

 the substance in the capillary tube, 

 which should be close to the mercury 

 reservoir of the thermometer, is care-, 

 fully watched. As soon as it melts, 

 the temperature indicated by the ther- 

 mometer is noted. If the liquid is not 

 too rapidly heated, a pure substance 

 should melt within the interval indi- 

 cated by one degree of the ther- 

 mometer. 



Although the majority of the 

 simpler organic substances possess a 

 definite melting point, there are some 

 which do not melt, or of which the melting point is not a 

 very characteristic constant. In these cases, it is usually 

 possible to prepare by a simple process a derivative which 

 can be more readily characterized. 



Fig. 2. 



(b) LIQUID SUBSTANCES. 



Just as the melting point is the chief criterion of 

 purity employed for solid substances, the boiling point is 

 the constant which usually serves to characterize liquids. 



To determine this a liquid is placed in a flask (see illus- 

 tration, Fig. 3) with a side tube (" fractionating " flask), and 



