m THE PRESENCE OP DUST-FREE AIR AND OTHER GASES. 
281 
Calculation of from the Ohsei'vaiions of Pi'cssure. 
To obtain the ratio of the final to the initial volume we have, when the gas present 
obeys Boyle’s law, 
= Pj/Po, 
where Pj is the pressure exerted by the gas alone before expansion, and P.^ is its 
pressure after expansion, when the temperature has risen to its former value. 
Now 
P^ = -f B — 77 — 10, 
and 
Po = 7?2 + B — 77 — ?/', 
where p^, p^ are the pressures measured by the mercury gauge, before and after 
expansion, as already described, B is the atmospheric pressure, 77 is the maximum 
vapour pressure at the temperature of experiment, and lo is the pressure required to 
balance the weight of the piston. 
Results obtained ivith Air in the smcdl Apparatus. 
The same phenomena are observed as in the larger apparatus, as well as others to 
be described later. 
After the removal of “dust” by repeated expansio7i, no condensation takes place 
within the moist air, unless exceeds a certain limit. With greater expansions 
rain-like condensation results. As will be seen from the following table, measure¬ 
ments of this critical value of vfv-^ made with the two machines give identical results, 
although the larger one contained twenty times as great a volume of air as the smaller. 
The expansion, therefore, appears to be sufficiently rapid to prevent the walls having 
any sensible effect. 
Expansion required for Bain-like Condensation in Air. 
Pressures all given in millims. of mercury. Correction for piston weight 
= 1 millim. 
Date. 
B. 
f C. 
fv 
Pi- 
Pr 
Ps- 
PkP2=ri)/vi. 
1 
1 1 
February 13 . 
77.5 
18-5 
16 
651 
1409 
372 
1130 
1-247 
2 
„ 14 . . 
775 
17-5 
15 
685 
1444 
386 
1145 
1-261 
3 
„ 17 . . 
776 
15-5 
13 
685 
1447 
392 
11.54 
1-254 
4 
22 
764 
165 
14 
602 
1.351 
330 
1079 
1-252 
5 
„ 22 . . 
764 
16’5 
14 
506 
1255 
2.54 
1003 
1-251 
G 
March 2 . . . 
7,50 
18-5 
16 
654 
1387 
377 
1110 
1-250 
]\Iean 
1-252 
2 O 
MDCCCXCVir. — A. 
