* 
RESEARCHES ON THE VOLATILE HYDROCARBONS. 163 
If the bulb be in the -vapor, the occurrence of either of these disturbing influences 
would then affect the principal mass of the mercury in the thermometer ; while, on 
the contrary, if the bulb were in the liquid, only the small quantity of mercury in the 
stem of the thermometer would be subjected to these influences; the liquid then 
serving as a regulator, and reducing the error from these sources to a minimum. 
Fluctuations from currents of cold air are comparatively slight, and more easily pre- 
vented than those from overheating the vapor. The latter is the more likely to 
occur the lower the boiling-point of the liquid, or when the quantity of liquid in the 
retort is small. I have, however, observed from this cause an elevation of 3°—4° in 
distilling a body boiling as high as 98° C., without an unnecessarily large flame. But 
the liquid in this instance was pretty low in the retort. 
In the case of liquids boiling below the common temperature, it seems indispensable 
that the bulb of the thermometer should be placed in the liquid. As evidence of this 
I will here state the results of observations made while occupied in fractioning some 
exceedingly volatile products from American petroleum. 
Experiment 1.— The liquid operated upon boiled at so low a temperature that the 
distillation was effected by the heat of the surrounding atmosphere. The distillation 
was conducted in a flask, and the bulb of the thermometer placed in the vapor. The 
flask was attached to my condensing apparatus, including the “ refrigerator, , Fig. 2.”* 
The temperature of the condensing-worm contained in the “elevated bath, aa, Fig. 2,”* 
and also that of the “first receiver, #, Fig. 2,"* was 11°.5. The temperature of the 
“cold bath, #, Fig. 2,”* was 11°. The condenser in “the refrigerator, 8,’ and the 
“second receiver,’ were cooled in a mixture of ice and salt. With the liquid boiling 
steadily from several points on the bottom of the flask, and the condensed product 
from the distillation running well from the refrigerator into the “second receiver,” 
not a drop was condensed in any of the apparatus intervening between the flask and 
the “second receiver,’ although this part of the apparatus was cooled, as already 
stated, to about 11°. The temperature of the vapor in the flask at this time was 
18°.5, or only 2°.5 below the temperature of the laboratory. These observations show 
that the liquid was boiling at a temperature considerably below that indicated by the 
thermometer in the vapor. Additional evidence of this was furnished by the fact 
that, during the distillation, the exterior of the flask, from the bottom to about one 
quarter of an inch above the surface of the liquid, was thickly covered with water 
condensed from the atmosphere, resembling heavy dew; while above, the sides of the 
* See Memoir “On Process of Fractional Condensation,” Memoirs of the American Academy, 1864. 
