12 



densation ; and the burners should be surrounded by asbestos shields 

 to keep off drafts of air. The water in G also is allowed to drip slowly, 

 at the rate of about one drop per second, to insure plenty of liquid on 

 the brass collars. 



At the beginning of the distillation it is necessary to watch the still 

 rather carefully and occasionally to relieve the steam pressure through 

 the valve J, until the dephlegmator is evenly heated and the rate of 

 reflow uniform in each section. 



It is customary, in ordinary fractional distillation, to change the 

 receiver at regular temperature intervals, which have been previously 

 determined. In this method, however, the receiver is not changed 

 according to prescribed temperature intervals, but according to the 

 nature of the distillation. If the oil contains a large proportion of one 

 substance, the temperature will remain nearly constant while that 

 portion of the oil is being distilled. During this part of the distilla- 

 tion but few fractions need be taken. When, however, the oil is com- 

 plex and the thermometer is rising rather rapidly, it is better to take 

 more frequent and smaller fractions. Each time the cylinders are 

 changed the temperature is recorded. Usually from ten to fifteen 

 fractions are collected during one distillation. The cylinder with its 

 contents is weighed and the weight of the dry cylinder subtracted. 

 This gives the weight of the total distillate, which consists of both oil 

 and water. Since by this method no greater accuracy than to 10 mg. 

 is essential, the volume of the water in cubic centimeters as read on 

 the cylinder can be assumed to be numerically equal to the weight of 

 the water in grams. Subtracting this from the weight of the total 

 distillate gives the weight of the turpentine in the fraction. By such 

 a procedure results are obtained from which a table can be made 

 giving for each fraction the steam temperature and the weight of oil 

 it contains. 



The odor and color of each fraction are observed and recorded at 

 once. The specific gravity is determined, according to accepted 

 methods, with a 10 c. c. pycnometer. The determinations are made at 

 ordinary temperatures and corrected to give the value at 15 C. If 

 the temperature at which the determinations are made is above 15, 

 then the correction factor 0.00083 is added for every degree of dif- 

 ference. If the temperature at which the determinations are made 

 is below 15, then 0.00083 is subtracted for every degree of difference. 

 This correction factor was obtained experimentally by using a very 

 pure wood turpentine. The index of refraction is read by means of an 

 Abbe refractometer compensated to the D line of the spectrum, and 

 the values corrected to 15 C. by means of factors. The factor 0.00049 

 is applied to all observed values below 1.4750, and is added for each 

 additional degree of temperature above 15 C. and subtracted for 

 each below. For oils having an index of refraction above 1.4750 

 the correction factor 0.00044 is similarly used. These values were 



[Cir. 152] 



