256 MEASUREMENT OF HIGH TEMPERATURES. [bull. 54 
nometer B B' B' ' ; for not only will such air be moist, but the mercury 
hurled out of B' B' by bubbling displaces the lower meniscus to a seri 
ously low level some times. During the experiments the lower meniscm 
remains constant in level. Hence the upper meniscus need be read oubj 
for the detection of slight variations, and the fact that the lower menis 
cus is finally hid by the surrounding mercury is no disadvantage. 
Nomenclature. — The results were computed from equation (5), on pag( 
254. The data in the table are similarly designated, and their full sig 
nification is as follows : 
P is the pressure of the gas at influx, i. e., when it enters the plati 
num capillary. P therefore is the zero height of the mercury columr 
in the manometer plus the zero height of the barometer. Correction h 
to be made for capillary depression of the upper meniscus of the ma: 
nometer. Correction is also to be made for optic displacement of the 
lower meniscus of the manometer when seen through the walls of the 
reservoir B (Fig. 44). As the mean effects produced by these twc 
errors were in a*n opposite sense, and nearly the same in magnitude, ] 
did not apply them. Moreover they have no effect on F {6"). 
p is the pressure of the gas at efflux. Hence it is equal to the zerc 
height of the barometer increased by the mercury value of the dept'fc 
of the efflux tubes below the level of water in the trough. Perhaps J 
ought also to have been increased by the capillary reaction effect oj 
water at the point of efflux. But as I could not estimate this, I used 
large values of P in order to secure as steady and rapid a flow as per 
missible. 
t is the time which corresponds to the volume of gas V Q measured 
after transpiration. It is usually advisable to make / nearly constanl 
and measure the variation of V . The time errors made in opening and 
closing the stop-cock K are not larger than one-fifth of a second. Henct 
t can be measured sharply. 
V Q is the volume of dry air escaping at the normal pressure 76 cn 
and temperature 0° C. V (page 248) being the true variable, some pains 
must be taken in correcting it. In the first place, inasmuch as V is meas- 
ured over water, it must be reduced to dryness, to zero centigrade, and 
to 76 cm . This is conveniently done by the aid of Landolt and Boern- 
stein's tables, it being remembered that Fis under atmospheric pressure 
minus the mercury value of the residuary column of water in the bu- 
rette llll. Finally V must be corrected for the solubility of the gas, 
which in case of air may be neglected, but in case of hydrogen is as 
large as 2 per cent. Formula (5), however, calls for V at '# and p. 
Hence, if Fis reduced at once to 0° and 76 cm by tables, it is convenient 
to transform formula (5) into 
J 1 
l.^C" ~L16 76 V Q l" 1 + ±Q\H J l " J l + «0" 
+ W B 
(910) 
