REVIEWS 323 



a depth of about one-half the thickness of the bark and at a height 

 from the ground of from 2 to 2^ feet. Within from 4 to 6 days the 

 wound is "freshened" by shaving off a very thin layer of bark from 

 the old blaze down to within an inch of its lower border, and also about 

 one inch in height of new bark on the upper edge of the original wound. 

 The hardened gum is scraped from the wound with a dull knife before 

 each retapping operation, the tappers and scrapers working in pairs. 

 The average yield of gum per tree per year is about 2^4 pounds. In 

 terms of our turpentine "crop" (8,000 to 10,000 trees), the yield would 

 be from 18,000 to 22,500 pounds of gum per season — a far less quantity 

 than our turpentine pines yield. The total yield of turpentine from this 

 crude gum-oleo-resin is estimated at 7 per cent, or about 157 to 197 

 gallons for 8,000 to 10,000 trees, the remaining yield consisting of 

 nearly equal proportions of resin and gum, which are separable by 

 treatment with solvents. 



It is not apparent, except for the greater comfort of the chipper, why 

 the first blazing should not be begun at about 6 inches above the ground, 

 which would afiford a much greater length of workable trunk surface 

 and a greater total yield per tree. The tapping is begun in November 

 and terminated within 5 or 6 months — a period which corresponds in 

 length with our pine turpentining season. Trials showed that it is not 

 profitable to tap trees under 30 inches in diameter. 



The total number of years Boswellia serrata can be worked is not 

 stated. Tapped continuously, as is the native custom in some forests, 

 probably the period would be about 5 or 6 years. It. is recommended, 

 however, that each year's tapping be followed by 2 years' rest, fixing 

 the rotation at 3 years. The effect of tapping on the vitality of the 

 trees, it is said, need cause no serious apprehension. The physical ap- 

 pearance of a trunk after complete working and healing of the wounds 

 presents a conspicuously gnarled and lumpy surface. Whether or not 

 the intention is to rework such trees is not stated. Owing, however, to 

 the exceedingly uneven surface of these trunks, it would seem very 

 difficult, if not entirely impracticable, to again work the trees. 



The three commercially valuable constituents of Boswellia gum-oleo- 

 resin are gum, rosin, and turpentine. The gum is obtained bv treating 

 the mass with a solvent, while the turpentine is extracted by steam dis- 

 tillation, the residual product being rosin. It is of interest to note in 

 this connection that 89 per cent of the turpentine from Boswellia ser- 

 rata is distilled at temperatures of from 153° to 160° C, while 85 per 

 cent of American turpentine distils between 155° and 163° C, and 85 



