THE POPULAR EDUCATOR. 



can be easily tried. Procure a flask, and having partly filled it 

 with water, place it over a lamp, and when the water is boiling, 

 cork the flask tightly, and remove it from the lamp. The air 

 has now been driven off, and if we pour cold water over the 

 outside it will condense the vapour within, and the pressure 

 being thus removed, ebullition will immediately commence 

 again. This experiment is a striking one, as the cold water has 

 the effect of making that in the flask boil. 



Having thus seen the effects of the pressure of the air, and 

 the mode in which the barometer serves as a means of measur- 

 ing it, we shall understand better the principle of the common 

 pump, and why water cannot be raised by it to a greater height 

 than about thirty feet. We shall also see the mode in which 

 the pressure-gauge of an air-pump, shown at Gin Fig. 2 (Vol. V., 

 pa'_ r e 298), acts. It is simply a straight glass tube, dipping at 

 tho lower end into a cup of mercury, and communicating at its 

 upper end with the exhaust pipe of the receiver. As the air is 

 removed the mercury risen, and the difference between its 

 height in this tube and that at which it stands in an ordinary 

 barometer shows the amount of air left in the receiver. With 

 a pump of the common description there is usually about one 

 inch difference, but if it be very carefully made, this may be 

 reduced to less than half an inch, showing that less than ^ of 

 the original volume of air remains in the receiver. 



There is, however, another piece of apparatus, known as the 

 Sprengel pump, by which a much greater degree of exhaustion 

 may be obtained ; and though it is seldom if 

 ever used to exhaust a large receiver, it is 

 frequently employed in chemical researches, 

 especially as it has the additional advantage 

 of rendering it easy to collect the air or gas 

 removed from any vessel. The principle of 

 this pump is very simple and yet very inge- 

 nious, a drop of mercury being made to take 

 the place of the piston in the ordinary pump. 



The annexed diagram (Fig. 13) will show its 

 construction and mode of action. A piece of 

 stout glass tubing, A B, about five feet long, and 

 having a bore about ^ of an inch in diameter, 

 is taken, and a funnel is fixed to the upper end. 

 Usually it is melted on so as to form all into 

 one piece. The lower end B is turned up a 

 little. A glass stop-cock, c, is also fixed a little 

 below the funnel, and a few inches lower 

 another glass tube, D E, opens into A B.. The 

 tube or vessel to be exhausted is then fixed 

 tightly to the end E. This may be done by a 

 good piece of india-rubber tubing, if the joint 

 be kept under water. The funnel is now filled 

 with quicksilver, and a vessel is placed at B 

 to catch all that runs down. The whole of 

 the apparatus is usually fixed to a piece of 

 board to guard against breakage. 



The tap is now turned on, and the mercury, 

 when it comes to D, is caused by the shape of 

 the tube to fall in a series of drops, each of 

 which acts as a piston, and carries down with 

 it a portion of the air from E, and in this way a nearly perfect 

 vacuum may be obtained. If a straight tube be fitted on E, 

 the lower end of which dips into a cup of mercury, the mercury 

 will rise in it till it stands at almost the same height as in a 

 barometer placed by it. In fact, the difference in height is 

 often almost imperceptible. 



In order to collect the gas or air, the end B' must dip into 

 a vessel of mercury, and the gas will bubble up through it 

 into a vessel placed to receive it, and may thus be saved for 

 analysis. 



We have now seen the most important effects of the weight 

 and consequent pressure of the air, and also the means of 

 measurjng this pressure and its variations. The question, what 

 is the true height of the atmosphere, is one which we cannot 

 fully solve. Each succeeding layer of air which we meet in 

 ascending is less dense than the one below it, and thus, theo- 

 retically, the limit to the height of the air is when the repulsive 

 force exerted by its particles on each other is exactly balanced 

 by the earth's attraction for those particles. We cannot calcu- 

 late exactly at what height this would occur, but the question 

 is of no great practical importance, and we may safely say that 



Fig. 14. 



Fig. 13. 



the limit of the atmosphere is about fifty miles above the sur- 

 face of the earth. A few scientific men have, as a result of 

 observations on the refractive power of the air as shown in its 

 causing twilight, stated the limit to be much above this ; but 

 if any air does exist at a greater height, it is in a state of such 

 rarity that it would scarcely be possible to prove its presence 

 even by the most delicate tests. 



We now pass on to notice the compressibility and elasticity 

 of the air. As we saw in our first lesson, if 

 any gas be confined in a vessel, it exerts a 

 pressure against the sides altogether apart 

 from its weight, and this pressure is exerted 

 against the upper part of the vessel as well 

 as upon the lower side. Now, this pressure 

 arises from the elastic force of the gas, and 

 depends alone upon its compression and 

 temperature. If the volume occupied by it 

 be in any way diminished, that is, if the 

 same quantity of gas be made to occupy a 

 smaller space, the pressure will be increased ; 

 or if the space occupied remain the same, 

 and the temperature be raised, the pressure 

 will also be increased. 



We have, then, to investigate the propor- 

 tion which this increase of pressure bears 

 to the diminution of volume. That air is 

 compressible to a very great extent, and 

 that the pressure it exerts increases with the 

 compression, is readily seen. Procure a 

 stout glass or metal tube, A (Fig. 14), with a stopcock, B, let 

 into it near the lower end, and a piston, c, fitting it air-tight. 

 When the tap, B, is open, we can place the piston at any part 

 of the tube, and the pressui-e on each side of it will be the same. 

 If now we close the stopcock, the air within will be cut off from 

 all communication with the external air, and therefore no pres- 

 sure will be communicated to the under side of the piston from 

 without; and yet it does not fall, though 

 pressed upon by the air with a force of 

 151bs. per square inch. The reason is 

 that the elastic force of the confined air 

 is sufficient to balance this pressure, and 



therefore the piston remains at rest. 

 Were we to place the whole under the 



receiver of an air-pump, and thus dimi- 

 nish the internal pressure, we should find 



that the elastic force of the air within 



would overcome the diminished pressure, 



and cause the piston to rise. If now we 



increase the pressure, either by adding 



weights to the top of the piston or by 



pressing with the hand, we shall find an 



increasing resistance to our efforts. This 



arises from the increased tension of the 



air, and if a weight has been placed on, 



we shall find that after sinking a little 



way the piston will come to rest, showing 



that the elastic force of the enclosed air 



is then equal to the pressure of 151bs. 



per inch, and to the added weight in 



addition. Let another similar weight be 



now added to the piston, and it will sink 



still lower, though it will not move as 



much as it did before. In this way we 



shall find that by adding more weights 



we can compress the air to almost any 



extent, the elastic force, however, in- 

 creasing greatly. If we now remove the 



pressure the air will expand again, so as 



to fill exactly the same space as it did 



before, and the piston will stand just 



where it did at first. 



A familiar example of this compressi- 

 bility of the air is seen if we invert a 



tumbler over a piece of cork floating on 



the surface of water. The water will rise a little way inside 



the tumbler, just as it does in a diving-bell, unless a fresh 



supply of air is introduced from above by means of the force- 

 pump. 



Fig. 15. 



