THE HOYAL AETILLEliY INSTITUTION. 
55 
height from the surface of the mercury left in the bowl as the column 
in an ordinary barometer stands at the same time. 
The process of taking the density of a sample of powder, is as 
follows:—The air is first exhausted from the tube and globe, and the 
mercury allowed to fill them. The upper and lower cocks of the globe 
are then closed, the nozzle is screwed off, and the globe then un¬ 
screwed from the tube and weighed. This weight—the weight of the 
globe full of mercury, which will of course vary with the specific gravity 
of the mercury, which again varies with the temperature—is registered. 
The globe is then emptied, and a definite weight of the powder 
(generally 100 grammes), is introduced into it. It is then attached to 
the barometer-tube, the air is exhausted, and the mercury allowed 
to enter and to rise in the tube as before. The stop-cocks at each end 
of the globe are provided with diaphragms—the lower one of wire gauze, 
the upper one of chamois leather—which allow a free passage for the 
mercury, but prevent any particle of powder being carried out of the 
globe. As soon as the mercury has risen to the proper height, the 
stop-cocks are again closed, the nozzle unscrewed, and the globe taken 
off and again weighed. This second weight—that of the globe full of 
mercury, plus the weight of the powder, and less the weight of the 
volume of mercury displaced by it—is also registered. 
The density of the powder is then calculated by the following 
formula:— 
D x 100 
(P'-P) + 100 5 
specific gravity of mercury at the time of experiment, 
weight of globe, full of mercury and powder, 
weight of globe, full of mercury alone, 
weight of powder employed. 
The specific gravity of mercury is 
At 50° Tahr. ...... 
60° . 
70° . 
80° . 
90° . 
An actual example worked out is subjoined. 
Grammes. 
Weight of globe, full of mercury . 4188 = P' 
„ „ mercury and powder ... 3434 = P 
Difference. 704 
Temperature at time of experiment 60°; consequent specific gravity 
of mercury 13*56 = D. Then 
D x 100 = 1356*00 
(P'-P) + 100 = 804. 
804) 1356 (1*686 = Density required. 
13*57 
13*56 
13*54 
13*53 
13*52 
Density = 
where D = 
P = 
P' = 
100 = 
