COAL-TAR AND WATER-GAS TAR CREOSOTES. 
31 
and those which would have been obtained 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- 
mom- 
Cor- 
mom- 
rected 
eter, 
Differ- 
rected 
eter, 
Differ- 
temper- 
A. W. 
ence. 
temper- 
A. W. 
ence. 
ature. 
P. A., 
uncor- 
rected. 
ature. 
P. A., 
uncor- 
rected. 
° a 
° a 
° C. 
° a 
° C. 
C. 
ISO 
178.8 
1.2 
265 
260.2 
4.8 
205 
203.0 
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 
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 different 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 fixed scale J. Above this split movable prism is 
mounted a spyglass, F, which in held is a fixed position in respect to 
