AND STATICS UNDER THE INFLUENCE OF LIGHT. 
343 
and then again of pitch. This thermopile was very sensitive to variations of tem¬ 
perature, and does not interrupt any essential portion of the light on its passage 
through the quartz vessel to the back plate. The second iron-nickel thermocouple 
was wound outside the tube as near as possible to the first couple in the glass 
cylinder, the two iron-and-nickel wires being drawn up round the cylinder, and then 
wound up round the capillary tube. These wires were first covered with shellac and 
then with pitch varnish. The thermopile assumed the temperature of the air or gas, 
owing to its extreme thinness, with very great rapidity. Subsequently a thin, glass 
bulb of similar volume was used instead of the quartz vessel. 
The capillary tubes used were of a very narrow diameter. The capillary tube E 
of the manometer was necessarily somewhat wider, so that the mercury might 
move easily, and rapidly assume equal levels in both arms of the U-tube. A length 
of 155‘7 centims. of the capillary of one of the manometers contained 9'476 grams of 
mercury = 0'701 eentim. The total volume of the capillary tubes in the manometer 
over the mercury during the experiments (20 to 50 centims , &c.) is very small in 
comparison with the volume of the cylinder of the quartz vessel, which is over 
40 cub. centims., amounting to only a few tenths of 1 per cent. The manometer 
contained a layer of concentrated sulphuric acid (about 10 centims. long), enough to 
protect the mercury from the chlorine. The concentrated sulphuric acid freed the 
capillary tube from any specks of dust, thus enabling the mercury to move in it much 
more easily. The acid must be heated in vacuo after it is brought into the capillary 
to remove air and sulphurous acid. Only after the sulphuric acid has been heated in 
vacuo can a very high vacuum be obtained which will remain constant for any length 
of time. In filling the capillary tubes with the sulphuric acid and mercury, the part 
of the apparatus between (22) and (19'), which can be separated from the other parts, 
is kept almost in a horizontal position, the open tube of the manometer being con¬ 
nected with a tube dipping into a small beaker of mercury covered by concentrated 
sulphuric acid. Tap (22) being closed, tube (19') is held over the mercury in the 
concentrated sulphuric acid till a few centims. of sulphuric acid rise in the tube, and 
then dipped into the mercury, allowing it to follow the sulphuric acid. If the mercury 
column is broken by sulphuric acid, the column is driven out from the capillary by 
forcing air into the vessel at (19') until most of the acid is expelled from the capillary, 
when the tube is refilled. The capillary U-tube of the manometer was calibrated in 
the usual way. Since concentrated sulphuric acid adheres to the glass, the sulphuric 
acid column is shorter when the mercury is low and longer when it is high, especially 
if the mercury is moving rapidly up and down. During the experiment upon the 
velocity of reaction the movement of the mercury is so slow that uniform results 
are obtained ; but a reading must be made of the levels of both mercury and 
sulphuric acid. 
The density of the sulphuric acid used was easily found from the heights of the 
sulphuric acid and of the mercury in both arms of the manometer when exposed to 
