318 
MR. W. CROOKES ON THE ATOMIC WEIGHT OF THALLIUM. 
where t is the temperature of the air, b the barometric pressure, v the pressure of the 
vapour (of water) present in the air, both expressed in millimetres of mercury at 0°, 
5 the height above the mean level of the sea, \ the latitude. My laboratory at 
Mornington Road, Regent’s Park, where the atomic weight of thallium was determined, 
is in lat. 51° 32' 6", at a height of 35-05 metres above the mean level of the sea. The 
expression consequently becomes 
(-99999289815) (1-0005802549). 
As a litre is the volume of 1000 grammes of water at its maximum density, the division 
of this expression by 1000 gives the ratio of the density of air to the maximum density 
of water. By the addition of the logarithm of Z> — 0-378a in millimetres to logA, we 
obtain the logarithm of the ratio of the density of air at t° to the maximum density of 
water. 
Table A. — Calculated for Mornington Road, Regent’s Park. 
t. 
10+log A ( . 
Diff. 
t. 
10+log A,. 
Diff. 
15 
4-207868 
0 
4-231055 
1585 
16 
4-206366 
1502 
1 
4-229470 
1579 
17 
4-204868 
1498 
2 
4-227890 
1574 
18 
4-203376 
1492 
o 
O 
4-226317 
1567 
19 
4-201889 
1487 
4 
4-224750 
1563 
20 
4-200406 
1483 
5 
4-223187 
1556 
21 
4-198925 
1477 
6 
4-221631 
1551 
22 
4-197458 
1471 
7 
4-220980 
1545 
23 
4-195990 
1468 
8 
4-218535 
1540 
24 
4-194528 
1462 
9 
4-216995 
1535 
25 
4-193071 
1457 
10 
4-215460 
1529 
26 
4-191618 
1452 
11 
4-213931 
1524 
27 
4-190171 
1447 
12 
4-212407 
1518 
28 
4-188728 
1443 
13 
4-210889 
1513 
29 
4-187290 
1438 
14 
4-209376 
* 1508 
30 
4-185857 
1433 
15 
4-207868 
