400 



THE POPULAR EDUCATOR. 



of a vessel of mercury, there is merely a U-shaped tube, with 

 the bend filled with mercury and the open end connected with 

 the boiler. It acts, however, in the same way. 



If, instead of the pressure on any gas being 'greater than that 

 of the air, it is less, the gas still expands in the same proportion. 

 Thus, if one-half the pressure be removed it will fill twice the 

 space. To prove this, a graduated tube is nearly filled with 

 mercury, and inverted into a tubular vessel filled with the same 

 liquid. It is first sunk so deep that the level of the mercury is 

 the same inside as outside, and the volume of the contained air 

 is carefully noted. The tube is now raised till the air has ex- 

 panded to exactly double this volume, and the mercury in the 

 tube will then be found to stand at just half the height of the 

 barometer above that outside. 



A simple experiment shows that if external pressure be re- 

 moved air will expand forcibly. Procure a very shrivelled apple, 

 and having placed it under the receiver of the pump, remove 

 the air. The apple will expand, and look quite plump and fresh. 

 If, however, you admit the air in order to remove and enjoy the 

 apple, the pressure at once shrivels it up as before. 



Several experiments may also be easily performed showing 

 the large amount of elastic force which may be stored up in 

 compressed air. The air-gun is, perhaps, the simplest illustra- 

 tion of this. In it the elastic force of the air takes the place of 

 gunpowder, and propels the bullet with great velocity. A 

 strong copper ball is made to screw on just below the lock of 

 the gun. By means of a condensing syringe, air is powerfully 

 compressed into this, and when the trigger falls it presses a pin, 

 and thus opens a valve in the ball and allows a portion of the 

 air to escape. This strikes the bullet, and imparts such velocity 

 to it as to make it a very deadly missile. If the ball be well 

 charged, the gun may be discharged from twelve to twenty times 

 successively without condensing the air in it afresh ; the power, 

 however, diminishes slightly each time that it is fired, as the 

 air becomes less dense. 



This expansive force of compressed air is sometimes employed 

 in the construction of a fountain. A strong metal vessel is 

 constructed, and a tube, dipping nearly to the bottom, is fitted 

 tightly to its mouth. A stopcock is inserted in this tube, and 

 a screw is also cut in the upper end of it. The vessel is then 

 filled to about three-fourths of its height with water, and, by 

 means of a condensing syringe screwed on to the pipe, the air 

 within is powerfully compressed, the fresh air bubbling up 

 through the water. The tap is then closed and the syringe re- 

 moved, and when it is desired to start the fountain we have only 

 to screw a jet on the tube, and on turning the tap the tension 

 of the air will be such as to force the water through the jet with 

 sufficient velocity to rai?e it to a considerable height. 



The air, it must be remembered, does not create any force ; 

 it merely stores up the force exerted by the hand in working 

 the syringe. It is, in fact, a reservoir of power, and in many 

 instances it becomes of great service by its action in this 

 way. 



In this case the air was condensed, and its elastic force 

 thereby greatly increased ; at ordinary pressures, however, it 

 has quite enough elastic force to act in the same way if we 

 allow the jet to play into a vacuum. There are two modes 

 of showing this experiment. 



If a small vessel, similar to that described above, be placed 

 under a tall receiver, and the air rapidly removed, the effect will 

 be seen. The simplest plan of making the vessel is to take a 

 small flask with a tightly-fitting cork, through which is passed 

 a glass tube, drawn to a jet at the upper end, and reaching 

 nearly to the bottom of the flask. To show the experiment 

 well, the receiver must be very rapidly exhausted ; otherwise the 

 air slowly expands, and merely causes the water to run slowly 

 out of the jet. The plan usually adopted is to exhaust a second 

 large receiver on another pump-plate, and so arrange the two 

 that a connection may be made between them by opening a'tap. 

 The air is thus almost instantaneously rarefied to a considerable 

 extent, and the experiment answers. The second plate in this 

 description of pump is known as the transfer-plate, and is fre- 

 quently found very convenient. 



The other mode of exhibiting the experiment is rather simpler. 

 A jet is screwed into the aperture of the pump-plate, and the 

 pump is so constructed that the plate, together with a portion 

 of the exhaust-pipe closed by a stopcock, may be removed with- 

 out the admission of any air. The end of this tube is then 



plunged beneath the surface of water, and, on opening the tap, 

 the water will be forced up through the jet by the pressure of 

 the air, and thus produce a very pretty fountain in vacuo. 



Having seen the mode of ascertaining the alteration which is 

 caused in the volume of any quantity of a gas by variation in 

 the pressure, we have now to examine the effects produced by 

 variations in the temperature. These variations are consider- 

 able ; it is therefore necessary, in order to measure the exact 

 quantity of a gas, to bring it to a standard temperature, and, 

 as already stated, 60 has been fixed on as the most convenient. 

 There is, however, often difficulty and loss of time in bringing 

 a gas exactly to any temperature ; we want, therefore, when we 

 know the volume at any other temperature, to be able to calcu- 

 late what it would be at 60. 



If we dip the neck of a retort beneath the surface of water, 

 and apply heat to the bulb, we shall find a number of bubbles 

 of air passing off through the water ; and when the source of 

 heat is removed and the air cools again, the water will rise in 

 the neck of the retort to take the place of the displaced air. 

 So, likewise, if we nearly fill a bladder with air, and, having 

 tied the neck tightly, place it before the fire, the air in it will 

 expand so as completely to distend, and perhaps burst the 

 bladder. We see, then, that the air alters in its volume by a 

 change of temperature. 



This property of air is sometimes employed in the construc- 

 tion of a thermometer. Two forms of air-thermometer are re- 

 presented in Fig. 16. In one, a straight 

 glass tube, B, with a bulb blown at one 

 end, is placed with its open end down- 

 wards in a vessel f coloured water, A. 

 Heat is first applied to the bulb, c, greater 

 than that which it is required to indicate ; 

 a portion of the air is thus driven out of 

 the tube, B, and the water rises to replace || B 

 it. The height at which this water stands 

 depends upon the pressure of the air in 

 the bulb, c, and as this varies with the 

 temperature, the column serves as a ther- 

 mometer. In the other form represented, 

 the tube is turned up so that a small 

 quantity of air may be included in the 

 bulb, c, above the water, and this, as it 

 expands or contracts by the heat, causes 

 the water to rise or fall in the limb, A B. Fig. 16. 



Both are graduated by comparison with a 



standard thermometer. These instruments are highly sensitive 

 to slight changes of temperature ; they are, however, affected 

 by the height of the barometric column, and therefore a certain 

 amount of uncertainty is introduced into their indications. 



The law showing the relation between the temperature and 

 the volume of any gas was discovered by Dalton, and has been 

 checked by many philosophers since that date. It may be 

 stated as follows : 



If any gas be allowed to expand freely under a constant 

 pressure, its increase of volume when raised from 32 to 212 a 

 will be equal to 0'366 of its original volume, and this law of 

 increase holds true in the same proportion for intermediate 

 temperatures. 



Now, there are 180 between these two temperatures ; the 

 expansion for each degree is, therefore, y^ of 0'366, or about 

 jj-j, and this fraction is called the co-efficient of expansion. A 

 gas, then, expands 33-2 of its volume at 32 for each degree that 

 it is raised above that point. This rule enables us to make the 

 calculations we required, for 492 cubic inches at 32 will occupy 

 493 at 33, 510 at 50, 520 at 60, and so on. Suppose, then, 

 we have the following question : A quantity of gas is measured 

 at a temperature of 76, and is found to occupy 427 cubic- 

 inches ; what is its volume at 60 ? We first find the proportion 

 between the space a gas occupies at 60 and at 76, and, as we 

 have seen, 492 cubic inches at 32 will occupy 520 at 60, and 

 536 at 76. The volumes are therefore in the proportion of 520 

 to 536, and the following rule of three sum will therefore give 

 us the required volume : 



As 536 : 520 : : 427 : 414J. 



Very often in chemical experiments corrections have to be 

 made for pressure as well as for temperature. The process is, 

 however, the same, and each correction may be made separately. 



