ME. W. CROOKES OK THE ATOMIC WEIGHT OF THALLIUM. 
281 
obtained in atmospheres balancing 30 inches of mercury and 5 inches of mercury respec- 
tively will give a more accurate result ; but the best weighings whereby the absolute 
weight of a substance may be calculated are undoubtedly one in air at ordinary pressure 
and temperature, and one in a highly rarefied atmosphere, — it cannot be said in vacuo , 
owing to the difficulty of working under such a difference of pressure between the 
atmosphere of the balance and that surrounding it. 
The Balances. 
Two balances were used. That which I shall call the «z>-balance was made by Messrs. 
Keisslek and Neu expressly for this work, and will clearly indicate a difference of O'OOOl 
of a grain when loaded with 1000 grains in each pan*. It is always kept in a dry room 
of tolerably uniform temperature, away from draught, artificial heat, or chemical vapours, 
and was (in the most accurate experiments) only used when no fire had been in the room 
for at least twelve hours. 
The second balance, which I shall call the vacuum- balance, is almost a duplicate of 
the first, of 14-inch beam, with agate knife-edges and planes, made by Oertling. It is 
enclosed in a cast-iron case connected with an air-pump, and so arranged that I can 
readily weigh any substance in air of any desired density, the rarefaction being measured 
by a barometer-gauge. The accompanying diagram (Plate XIV.) shows the method of 
the connexions. The upper and lower portions of the iron case are connected by flanges 
and bolts; while to ensure that the joint shall be air-tight, there is cemented to each 
flange a band of thick unvulcanized india-rubber, a lead wire being laid between the 
two pieces of india-rubber. By this means, and by causing the arm by which the riders 
are adjusted and the key liberating the pans and beam to work in a double-packed 
stuffing-box, a nearly perfect vacuum can be maintained. The openings in the metal 
work, through which observations are made, are fitted with clear stout plate glass ; 
that to the left of the centre of the case, for the introduction of the apparatus &c., 
is closed with an iron door, clamped and fitted with washers. The apparatus, when 
attached to the air-pump and exhausted to 25 inches of mercury, seldom allows the 
column of mercury to sink at a greater rate than O’Ol inch in an hour. A plug of gold- 
leaf is inserted in the tube connecting the barometer-gauge with the vacuum-chamber, 
in order to absorb any mercury vapour that might otherwise be carried over. 
* M. Stas employed four balances. One of them when loaded with 1000 grammes turns with of a milli- 
gramme ; another when loaded with 5000 or 6000 grammes turns with 1 milligramme, and with 2000 or 3000 
grammes in each pan turns with Ar or A a milligramme. The third balance loaded with 500 grammes turns 
with of a milligramme ; the fourth laden 25 grammes turns to A of a milligramme. Reducing these weights 
to grains, we find that — 
Ko. 1 loaded with 15,432 grains turns with 0-0077 grain. 
o 
99 ** 
99 
92,592 
99 
0-0154 
„ 2 
99 
46,296 
99 
0-0060 
„ 3 
99 
7,716 
99 
0-0030 
„ 4 
99 
386 
99 
0-0005 
99 
