COAL-TAR AND WATER-GAS TAR CREOSOTES, 



31 



and those which would have been obtamed under the same conditions 

 by a thermometer complying with the specifications adopted by the 

 American Wood Preservers' Association in 1912 are shown in Table 13. 



Table 13. — Comparison of temperature readings of corrected thermometers and standard 

 thermometers of the American Wood Preservers' Association, uncorrected. 





Stand- 







Stand- 







ard ther- 







ard ther- 





Cor- 



raom- 





Cor- 



mom- 





rected 



eter, 

 A. W. 



Differ- 



rected 



eter, 



Differ- 



temper- 



ence. 



temper- 



A. W. 



ence. 



ature. 



P. A., 

 uncor- 

 rected. 





ature. 



P. A., 



uncor- 

 rected. 





° a 



° a 



° C. 



° a 



° C. 



° a 



180 



178.8 



1.2 



265 



260.2 



4.8 



205 



203. 



2.0 



275 



269.7 



5.3 



215 



212. 7 



2.3 



285 



279.1 



5.9 



225 



222.2 



2.8 



295 



288.4 



6.6 



235 



231.8 



3.2 



305 



297.7 



7.3 



245 



241.3 



3.7 



320 



311.7 



8.3 



255 



250.8 



4.2 



I 







The fractions taken in this work are those given in Tables 12 and 

 13 in the first column. Two hundred and fifty grams of oil were used 

 for a distillation, and the percentage weight of each fraction was 

 determined to the nearest one-tenth of 1 per cent. 



Index of refraction test.— The nature of the refraction test may be 

 briefly described. When a ray of light passes from one medium to 

 another of difi^erent density, it is bent out of its course or refracted. 

 A familiar example of this refraction is shown by the appearance of 

 piling just at the water line. Both above and below the water line 

 the pile appears straight, but that portion of the pile under water 

 appears to be nearer than the portion above. This is because of the 

 difference in the refraction of light in air and in water. The refrac- 

 tion of light varies with every pair of media through which the light 

 passes; but, if some medium is taken as a standard, then all measure- 

 ments may be referred to it. The standard adopted is air. The 

 refraction of light is measured by the angle through which the beam 

 is bent in passing from air into the other medium. This measure- 

 ment may be made by degrees and minutes or, for convenience, in 

 what is known as the index of refraction. The index of refraction is 

 the sine of the angle of incidence divided by the sine of the angle of 

 refraction. The most convenient instrument for measuring this 

 property is known as the Abbe refractometer, a view of which is 

 shown in figure 12. It consists essentially of a split prism, AB, sur- 

 rounded by heating chambers, all of which are mounted on a movable 

 carriage, which in turn is connected to a lever carrying the reading 

 glass L, over the fix;ed scale J. Above this split movable prism is 

 mounted a spyglass, F, which in held is a fixed position in respect to 



