46 



ELASTICITY OF AIR. 



weight of the atmosphere, and will be subject to no pressure from below, be- 

 cause of the absence of air beneath it. It will then stop the admission of air 

 from above the aperture, and will maintain the vacuum below. The elastic 

 force of the air contained in the vessel R, now acting upward against the ex- 

 hausting valve V, will raise it, and the air will escape through the space be- 

 tween it and the surface surrounding the aperture, and will thus fill the barrel 

 above ; but the air having expanded into an increased space will have an 

 elastic force less than that of the external air, and consequently the piston-valve 

 V' will be pressed down by a greater force than it is pressed up, and will 

 therefore remain closed. Let the piston be now depressed ; as it descends, 

 the air enclosed in the cylinder acquires increased elastic force, and pressing 

 upon the exhausting-valve V, causes it to close, so as to intercept the air in 

 the cylinder from the vessel R. When the piston has descended in the barrel 

 through such a space as to condense the air beneath it, so as to give it a greater 

 elastic force than the external atmosphere, it will press the piston-valve V 

 upward with a greater force than the external air presses it downward. Con- 

 sequently the valve V' will be opened, and the air confined beneath the piston 

 will begin to escape through it. When the piston has arrived at the bottom of 

 the barrel, the whole of the air will thus be expelled. This process is re- 

 peated whenever the piston is raised and depressed, and thus the valves, which 

 in the form adapted for explanation, required constant manipulation, acquire a 

 self-acting property. This form of the instrument, which is that commonly 

 used, is attended with an obvious limit to its operation, which does not exist in 

 the theoretical form represented in fig. 1. It is evident that the operation of 

 the valves depends upon the presence of air of a certain determinate elastic 

 force, in the vessel R, which elastic force it is the purpose of the instrument 

 to reduce indefinitely. When the elastic force of the air contained in R, is so 

 far diminished that it is only equal to the force required to raise the valve V, 

 the action of the machine must stop, for any further diminution would render 

 the air confined in R unable to open the valve, and therefore no more air could 

 pass into the barrel A, B. This is a practical limit of the power of the ex- 

 hausting syringe. The degree of perfection of which the instrument is sus- 

 ceptible, therefore, depends upon making the valve V, offer as little resistance 

 to being raised as is consistent with its being perfectly air-tight when closed. 



But we have another limit to the operation of this instrument, arising from 

 the piston-valve V. This valve is closed, not only by its own tension, but 

 also by the weight of the incumbent atmosphere above it. When the piston is 

 depressed, the air included in the barrel must first attain a degree of elastic 

 force by condensation equal to the pressure of the atmosphere, before it can 

 open the valve V'. But this is not sufficient : it must acquire a further in- 

 creased elastic force equal to the tension of the valve V, over the aperture, in 

 order to raise that valve and escape, and therefore the perfection of this valve 

 also depends on having as little tension as is consistent with being perfectly 

 air-tight from above. 



The efficiency of the instrument will also depend upon the accuracy with 

 which the piston fits the bottom and sides of the barrel. When the piston is 

 depressed to the bottom, it is considered in theory to be in absolute contact, so 

 as to exclude every particle of air from the space between it and the bottom. 

 But in practice, this perfection can never be obtained. It may, however, be 

 very accurately fitted, and the air retained between it and the bottom may be 

 reduced almost without limit. The small hole which passes from the valve 

 V' to the bottom of the piston, will still remain, however, and will continue to 

 be a receptacle for air, even when the piston is in close contact with the bot- 

 tom. This space, therefore, produces a defect in the machine which is not 



