in the tube then descends to a height equal to that of the barometer above the level of the mercury in the lower cup, and a vacuum is left in the top of the tube. This is always alluded to as a Torricellian vacuum, and is found in the ordinary barometer. In 1855, Geissler invented a mercurial air pump in which the vacuum is produced by connecting a receiver with a Torricellian vacuum. The original form of Geissler's pump is shown in the accompanying diagram, which will serve to illustrate the principle of the operation of pumps of this class, though they have received numerous modifications and improvements. In most mercury pumps the parts are made of glass, the connections being made with rubber tubing. In the diagram, A is a large bulb. B is a tube about 3 feet long, C a rubber tube uniting the lower end of B with the vessel D, which is open on top. A can be connected with either of the tubes G or F, but not with both at once, or it can be shut off from both. The receiver to be exhausted is connected with G, and F leads to the open air. Enough mercury is used to fill A, B, C, and D, as shown, and the vessel D is capable of being raised or lowered. The operation of the pump is as follows: Suppose the vessel D is raised a little higher than A, as in the figure. The mercury will flow into the bulb A, which it fills if the cock E is turned so as to connect A with the outside air, F. The cock is then turned so as to connect A through the tube G with the vessel to be exhausted, the air in which at this stage is at atmospheric pressure. D is then lowered, and the level of the mercury in A is lowered in consequence, the mercury running down B and C to D. As the mercury in A descends, air is drawn from the receiver through G into A, so when the mercury has descended below A the whole space is filled with the air drawn through G, which, having expanded from the receiver attached to G, is at less than atmospheric pressure. The cock E is then turned so as to cut off communication between A and G. D is then slowly raised, and the mercury flows gradually back into A, compressing the air above it until it is at atmospheric pressure. At this point the cock E should be turned to connect A with the outside air F, and as D continues rising the mercury continues to drive out all the air at F, until the bulb A is filled with mercury to the cock E, which is then closed so as to cut off all communication with A. When D is again lowered, the mercury does not begin to fall in A until D is about 30 inches below A. It then begins to descend, leaving a Torricellian vacuum above it, and D is lowered until A is empty. The cock is then turned so as to connect A with the receiver through G, and the remaining air in that vessel expands and fills A. The cock E is next turned off. D is raised, and the mercury rising in A compresses the air above it until it is let out at F by turning the cock. By repeating this operation a sufficient number of times a vacuum is gradually produced in the receiver connected to G. When the operation is nearly finished great care must be taken not to raise the vessel D too rapidly, or the impact of the mercury against the top" of the bulb A will break the apparatus. It will also be seen that when the vacuum is nearly reached the mercury in A will be at the top of the bulb when D is about 30 inches below. If the valve should be turned to F at this point, the inrush of air would drive the mercury down. Therefore, no communication between A and F must be made until D has been raised on a level with E, and no communication between G and A must be made until D is lowered 30 inches again, otherwise mercury will run through G into the receiver which is being exhausted.
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Geissler Pump.
The Geissler pump just described may be taken as the type of mercury pumps, which are classified as upward-driving, and, while a number of improvements in details have been introduced, making them of a more practical type for factory use, these pumps all operate on the principle of connecting the receiver to be exhausted with Torricellian vacuum.
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Sprengel Pump.
Sprengel brought out his well-known form of mercury pump in 1865, and the diagram shows it in its simplest form. The Sprengel pump is a general type of what are classified as downward-driving pumps. A is a funnel having a stop-cock C, and B is a tube of small bore, called the shaft or fall-tube. The receiver to be exhausted is connected to the tube G, which branches off from near the top of the shaft. The tube B terminates very close to the bottom of the vessel D, which is provided with a spout F, as shown, leading to the cup E. The distance from the branch G to the top of the mercury in the vessel F must be at least three feet. A is filled with mercury, which flows down the shaft B, the rate of flow being regulated by the cock C, so that a very small stream is allowed to fall. This mercury in falling breaks up into short lengths, between which are small columns of air which flow in at the junction of G with the shaft B. The weight of the mercury forces these short columns of air down the shaft B to the mercury in D, from the surface of which they escape. The mercury as it runs into the cup E must be poured back into the funnel A. This operation continues until no more air is carried down with the mercury. When the vacuum is nearly completed, the mercury in the fall-tube will fall with a sharp, rattling noise, showing that there is not enough air carried down with it to act as a cushion. With all kinds of mercury pumps, however, it is necessary to continue the operation for a considerable time after the receiver is apparently exhausted. Even when no more air appears to be carried on by the pump, the vacuum will improve as the operation continues. The reason for this is that the air sticks to the surface of the glass, forming a sort of coating, which is swept off the surface by the pump, but very slowly. The simple form of Sprengel pump is better than the simple Geissler pump, but is not well suited to fac-
