284 
MR. W. CROOKES ON THE ATOMIC WEIGHT OP THALLIUM. 
4 Cours de Physique,’ gives the most accurate results with the least expenditure of time. 
The weights are placed in the left-hand pan, and the object to be weighed in the right. 
At the last three consecutive oscillations of the pointer along the divided scale the 
division reached by the pointer is recorded, and 
a-\-c+2b 
4 ~ 
the reading of the scale when the balance attains equilibrium. It is better to allow 
the first oscillation to occur unnoticed, and to record only the next three consecutive 
oscillations, while the first can be employed to check the result if required. 
Finally, the balance-case contains a jar of pure oil of vitriol exposing a large surface, 
and another of caustic potash. The air is admitted through long U-shaped tubes, one 
filled with chloride of calcium, and the other with platinized asbestos. 
For each weighing all necessary observations of the barometer and thermometer were 
made, as will be found noted in the fourth and fifth sections of this memoir. 
I would here ask whether chemists in their analytic analyses sufficiently allow for 
barometric variation. The temperature at the time of weighing is generally recorded : 
chemists have known the influence of pressure on the boiling-point, and its effect upon 
gases, yet they appear to neglect reference to the barometer when weighing solids, 
forgetting that they are weighing in a gas which itself possesses weight. Weighings 
are repeated after some operation, such as expelling moisture, at intervals sufficiently 
long to admit of considerable variation in atmospheric pressure, and the increase or 
decrease of a few milligrammes in weight is considered to determine the gain or loss of 
certain constituents. It remains to be seen whether a neglect of variation in barometric 
pressure would not account for these minute differences. 
An approximation to the true weight of bodies, that is their weight in vacuo , may be 
obtained by the following formula, when their specific gravity and weight in air of 
760 millims. pressure of mercury at 0°C. is known. 
Let W — weight in grains ; then the weight of air displaced is 
W 
sp. gr. 
X 0-00122. 
This weight plus the value of the weights in vacuo balancing the substance is its true 
weight in vacuo. 
Let the weight of 800 grains of water in 200 grains of glass be required from 
two assistants, the one weighing against brass, the other against platinum weights, 
neglecting (1) the weight of air displaced and (2) its variation in weight from baro- 
metric disturbances. 
(1) The true value of 800 grains of water weighed in air (bar 760 millims., 0° C.) 
= 800 '9 7 6 grains. 
