24 PRACTICAL ORGANIC AND BIO-CHEMISTRY 



A melting-point tube consists of a capillary of thin glass about 

 I mm. in diameter, $-6 cm. long and closed at one end. It is 

 made by heating near one end a dry piece of glass tubing of about 

 I cm. bore in a blow-pipe flame until it is red-hot and soft, removing 

 it from the flame and pulling it out carefully just when the glass begins 

 to harden. A long capillary tube is thus made. Several more such 

 lengths can be made from the glass tube if the capillary so made be 

 broken off about 2-3 inches from the remainder of the glass tube. 

 The long capillaries are cut into short lengths of about 5-6 cm. by 

 scratching at these distances with a file and breaking by bending. 

 Short lengths and lengths of too small bore must be rejected. One 

 end of each capillary is sealed by holding it in a small flame. The 

 tubes so made are preserved in a corked dry test tube. 



The powdered substance is introduced by scooping up solid with 

 the open end and making it fall to the other end by gently tapping 

 the closed end on the bench. This process is repeated until sufficient 

 of the substance to occupy a lengh of 2-5 mm. in the capillary has 

 been introduced and shaken down, or pushed down with a fine wire, 

 so as to form a compact and continuous layer. 



The filled melting-point tube is attached to the thermometer so that 

 the substance is on a level with the bulb. If a bath of water or 

 paraffin wax be used the attachment is made with a strip of rubber cut 

 from a length of rubber tubing. If a bath of sulphuric acid be used 

 the attachment is made by adhesion. The thermometer is wetted with 

 acid and if held horizontally the acid runs along it. The melting-point 

 tube is wetted with acid by drawing it along the thermometer and it 

 will adhere when the surfaces of contact are wet and will not fall off on 

 putting it carefully into the bath. 



The pure specimens of benzoic acid, succinic acid and urea may be 

 used for determination of the melting-point. 



Corrected Melting-point Determinations. 



Just as in the case of boiling-point determinations a correction should be 

 made for the thread of mercury outside the bath. The small thermometers 

 of 50 range may be used and attached by platinum wire to the ordinary ther- 

 mometer. A corrected thermometer is generally used which has been cali- 

 brated against the small ones. 



