ANALYSIS, REFINING, AND COMPOSITION. 



13 



Specific gravity. The specific gravity can be determined accu- 

 rately enough by means of a Westphal balance, making the deter- 

 minations at room temperature and correcting to 15 C. by using the 

 factor 0.00083 for every degree centigrade of difference from this 

 standard temperature. The correction for temperature is large enough 

 so that especial care must be taken to make the temperature readings 

 accurate. In order to make specific gravity determinations with the 

 Westphal balance on as small a frac- 

 tion as is often desirable, it is neces- 

 sary to have a smaller cell than that 

 usually furnished with the instrument. 

 This can readily be made by properly 

 shortening a test tube with a diameter 

 only slightly greater than the bob of the 

 balance and supporting it in a flat cork. 

 In this way the gravity of a fraction 

 of only 18 grams (3.6 per cent) can 

 readily be determined on a balance with 

 a bob having a displacement of 5 c. c. 



Index of refraction. The index of 

 refraction can be determined with suffi- 

 cient accuracy by means of an Abbe re- 

 fractometer, taking the readings at 

 room temperature' and correcting by 

 means of the factor 0.00047 for every 

 degree difference from the standard 

 temperature of 15 C. It has been 

 found that this factor 0.00047 is more 

 nearly correct for the general run of 

 turpentines than 0.00049 (as recom- 

 mended in Circular No. 152) and that 

 the use of different temperature correc- 

 tion factors for different values of the 



index of refraction is not necessary in order to obtain the degree of 

 accuracy required for the work. 



RECORDING DATA. 



A typical data sheet showing the original and corrected values as 

 first recorded is shown in Table 1. For the purpose of making the 

 interpretation of these results simpler and easier the final corrected 

 values for the boiling point, specific gravity, and index of refraction 

 of the different fractions are plotted against the total percentage 

 weight of these fractions. The curves resulting from plotting the 

 data given in Table 1 are shown in figure 2. In other distillations 



160 165 170 175 160 185 190 195 

 O -86 487 8 .89 9O .91 .92 .93 



O U68 U70 W72 1-414 L476 U18 L46O 



FIG. 2. Typical curves showing 

 bciling points, specific gravi- 

 ties, and indices of refraction 

 of a gum turpentine. 



