14 BULLETIN 229, U. S. DEPARTMENT OF AGRICULTURE. 



ing 75 per cent rosin and 25 per cent turpentine, the turpentine in the 

 absence of water will not begin to boil at 313° F., but at about 363° F., 

 owing to the presence of the rosin. Rosin does not begin to decom- 

 pose perceptibly until a temperature of about 392° F. is reached, but 

 after the turpentine in the gum begins to distill off at 363° F., the tem- 

 perature of the gum rises rapidly. For this reason, only a portion of 

 the turpentine will be obtained before the decomposition of the rosin 

 begins. In fact, it would be impossible to obtain all the turpentine in 

 the gum, and that secured would be yellow, while at such high tem- 

 peratures the rosin would also be quite dark. Practice has shown 

 that the best quality of turpentine and rosin is obtained at a tempera- 

 ture of 302° F., which calls for the use of water in the distillation. 



A liquid begins to boil when the pressure of its vapors is equal to 

 or slightly exceeds the pressure of the atmosphere. Thus, water boils 

 at 212° F. and turpentine at 313° F. Turpentine and water are non- 

 miscible liquids, and according to physical law will distill together 

 when the sum of their vapor pressures equals the vapor pressure 

 of the atmosphere. Theoretically a turpentine, with a constant 

 boiling point of 313° F., and water will distill together at a tempera- 

 ture of 203° F., the proportion of water and turpentine in the distillate 

 remaining practically constant until one of the liquids is exhausted. 

 Owing, however, to the complex nature of ordinary turpentine with its 

 wide range of boiling points, turpentine and water will begin to 

 distill at about 203° F., and the temperature will rise finally to about 

 212° F. The distillate will at first contain about 60 per cent of tur- 

 pentine by volume, the turpentine content of the distillate gradually 

 decreasing to practically zero. 



In a mixture of gum (containing 75 per cent rosin and 25 per cent 

 turpentine) and water, distillation will not begin at 203° F., as in the 

 case of pure turpentine and water, but at 207° F. As the gum grows 

 poorer in turpentine the temperature rises until 212° F. is reached. 

 At this temperature all the turpentine will have distilled over. 



In actual practice all the turpentine does not distill at 212° F. 

 when water is added, owing to the physical difficulty of bringing 

 more than the surface of the gum in contact with the water. On this 

 account the gum must be heated to a temperature that will make it 

 readily fluid and produce convection currents in lieu of stirring. If 

 live steam were introduced into the mass all the turpentine could be 

 removed below 212° F. 



COMMERCIAL METHODS OF COLLECTING CRUDE GUM. 



BOX SYSTEM. 



Cutting the boxes. — The first operation in turpentining by the box 

 method consists in cutting a cavity (fig. 3) into the base of the tree 

 for holding the crude gum. This cavity, called the "box" is cut 



