21.--OXYGEN BY SECOND METHOD.

STANDARD OF PRESSURE AND TEMPERATURE.

The globe a, Fig. 11, whose capacity was about sixteen litres, contained the hydrogen which was used to reduce to the standard temperature and pressure the

oxygen which was to be weighed. It was supported in the copper cylinder bb, having a water-tight cover provided with a tube c. This globe was connected by a somewhat flexible glass tube d to a differential manometer, of which part is shown in more detail in Fig. 11a. The tubes, of 24 millimetres internal diameter, contain platinum points which are in nearly the same horizontal plane. It is not necessary that they be at exactly the same level, but it is important that they preserve an invariable relation to each other. The gauge was therefore firmly secured by brass brackets, a, Fig. 12, to a brass tube which was solidly attached to a brick wall. The tube and the gauge touched nothing else during the whole series of experiments, except that they were partly immersed in water. The branches of the manometer were some eighty-five centimetres long; a tube b was fused into the bend and provided with a stopcock c, by which the volume of the mercury in the manometer could be regulated. At d, Fig. 11, was a tube leading to the Toepler pump and to a supply of pure dry hydrogen. When the globe had been exhausted

୮

b

a

to a pressure of a few millionths of an atmosphere, hydrogen was admitted to about the atmospheric pressure, and this connecting tube was fused off, so that the hydrogen was now of unalterable mass. The cylinder containing the globe was then deeply covered with melting ice, from which the tube carrying the manometer was protected, so that it should not be disturbed. The manometer was also covered with powdered ice, and examined through an opening. The open branch of the manometer was connected to the syphon barometer and also to a contrivance for micrometrically regulating the pressure of the air in this connecting tube, seen at e, Fig. 11. The two levels of the mercury in the syphon barometer were in the same vertical, and were surrounded with water kept well stirred by a current of air; temperature was determined with the thermometer numbered 2053.

at once.

While the differential manometer was covered with ice, it was not easy to adjust the volume of mercury in it so that the two surfaces should be tangent to the two platinum points. Accordingly, two pressures were alternately measured which should bring the surface b to the right-hand point, and then the surface c to the corresponding point. The mean would be the pressure which would make both surfaces tangent to the two points at the same time, provided the volume of mercury were properly adjusted.

The following determinations of the pressure required to produce equilibrium of the gauge were made; all the readings are corrected for expansion of the mercury and of the FIG. 12.-Mounting of dif- scale of the barometer, but not for error in the length of the

ferential manometer to main

tain its position undisturbed. scale.

DETERMINATION OF PRESSURE OF HYDROGEN FOR COMPARISON WITH PRESSURE OF OXYGEN.

35

During the determination of the pressure of the gas in the globe a, Fig. 11, the cylinder containing it stood at the centre of the tank ff, so that a sufficient thickness of ice could be placed on every side of the globe. But for the subsequent use of the apparatus this globe had to be near the side of the tank. The cylinder bb was therefore held free from the tank while the latter was moved into its new position, when the globe full of hydrogen was again supported by resting the containing cylinder on the bottom of the tank. The differential manometer was not disturbed.

22.-OXYGEN BY SECOND METHOD.

DESICCATORS FOR HOLDING GLOBES.

The globes used for weighing oxygen in this series of determinations were kept, during the whole course of an experiment, in desiccators, one of which is shown in Fig. 13. A copper vessel has a flange and cover at a, and a glass plate secured to a flange at b. At the centre of this plate is a fitting into which may be screwed a socket carrying the glass tube c. The whole is water-tight.

The counterpoised simply rests on a pan e, except when the cover is removed and the counterpoise suspended to the balance. The globe f, however, must be firmly held in order that the key of its stopcock may be manipulated. It is accordingly grasped by the clamp shown in Fig. 14. The jaws a a are moved in opposite directions by the right-hand and lefthand screws on the shaft b. A key which turns this shaft can be removed and

FIG. 13.-Desiccator for containing and manipulating globes during filling, weighing, and exhaustion.

the opening stopped with a cork. When the globe is held by this clamp, a handle g, Fig. 13, can be pushed forward so as to turn the key of the stopcock. When this handle is withdrawn and the globe is suspended from the balance, the jaws are made to release the globe which then hangs FIG. 14.-Clamp for holding perfectly free. The glass tube c, Fig. 13, is removed during

globe while in the desiccator.

the weighing.

When it is desired to exhaust a globe in its desiccator, it is secured by the clamp, a ground joint is fitted in place, as in Fig. 6, and the desiccator is set near

the Toepler air pump. Connection is made by fusion, as in the previous series of experiments. The stopcock is opened by the handle g, Fig. 13, and when the exhaustion has been completed, the stopcock is closed in the same way, the handle is withdrawn, the ground joint is removed, and the desiccator is put in place under the balance. The counterpoise is hung on one pan of the balance and the globe. on the other and then released from the clamp, when it is ready to be weighed.

23.--OXYGEN BY SECOND METHOD. DRYING THE AIR IN THE DESICCATORS.

It would be difficult to keep dry the air in desiccators as large as these, especially when open to the air during the weighings, unless the air is dried in some other apparatus and introduced into them. A current of dry air was accordingly brought into the desiccator through a tube for the purpose, h, Fig. 13, which delivered it at the bottom of the desiccator in a fanlike horizontal current. The air was forced by a hydraulic blower through two carboys containing pure sulphuric acid. While the globe was at the balance, this current passed through the axis of the mechanism for weighing the globes by reversal, which was used in the second series of determinations, and is described at page 42. The current was stopped a few minutes before each weighing, but suffered little other interruption. When the globes were filling or exhausting, one opening of the desiccator was closed and the other was loosely stopped with cotton-wool; an occasional introduction of dry air was then thought sufficient.

24. OXYGEN BY SECOND METHOD.

FILLING GLOBES WITH OXYGEN.

When it was desired to fill a globe with oxygen, the desiccator containing it had the cover a, Fig. 13, put in place. The ground joint of the globe was cemented into its corresponding piece, and to the latter was fused a tube long enough to project above the tube c, Fig. 15, which was then screwed into position. The desiccator was then placed in the tank a a, near the globe of hydrogen and the differential manometer. The tube was then fused to the tubes leading to the air pump, the manometer, and the apparatus for producing oxygen. The filling with oxygen was conducted in all respects as in the first series of determinations, the only difference being that the key of the stopcock of the globe could not be reached by the hand but by the handle g, Fig. 13.

While the globe was filling, beams dd, Fig. 15, were secured in position to hold down the desiccator containing the globe and also the cylinder containing the globe serving as a standard of pressure and temperature. The tank was then filled with water, which was well stirred by a current of air. When the pressure of the

same as that of the

The pressure of the

oxygen in the globe in the desiccator was very nearly the hydrogen in the standard globe, the tube e was fused off. oxygen was then reduced by drawing off mercury from the stopcock g, till equilib rium was produced at the differential manometer. The water was well stirred, and the equilibrium was maintained for a sufficient time, sometimes for four hours, commonly for one hour. When the equilibrium was regarded as satisfactory, the stopcock of the globe was closed, the water was drawn off from the tank, the con

FIG. 15.-Filling globe with oxygen; use of desiccator; hydrogen used for comparison.

nections of the globe to the other part of the apparatus were severed, the desiccator was lifted out of the tank, dried, and taken to the balance. The cover a, Fig. 13, was removed, the tube c was unscrewed, the ground joint of the globe was cleaned, and the globe and its counterpoise were suspended from the pans of the balance

as before.

25.-OXYGEN BY SECOND METHOD. THE BALANCE WHICH WAS EMPLOYED.

It

The balance used in the weighings of this series of determinations was made by Rueprecht, of Vienna, and was lent to me by the Smithsonian Institution. twelve hundred grammes in each pan. The excursions of the beam are read by the telescopic observation of the image of a scale of equal parts which is

will

carry