Fitz Gerald and Joly — Measurement of Small Pressures. 129 



be plenty of force to turn a light mirror ; and so a spot of light, 

 instead of a microscope, might be used to observe changes in the 

 level of the liquid. A method depending on changes in the posi- 

 tion of Newton's rings might also be used. In all methods when 

 liquids are used the vapour pressure of the liquid is one of the 

 great difficulties in measuring the pressure of the rest of the gas, 

 and can only be reduced by using liquids that have a very small 

 vapour pressure. In some cases we can practically get rid of it by 

 only opening temporary communication between our measuring 

 apparatus and the gas, whose pressure we wish to determine, and 

 not allowing time for any serious amount of diffusion to take place. 

 At very low pressures, as the flow of gas through even relatively 

 large tubes is of the nature of diffusion, a great additional diffi- 

 culty is introduced. 



Another method of measuring very small pressures is to observe 

 the position of the centre of a flat elastic membrane when subject 

 to a difference of pressure at its opposite surfaces. A flat film of 

 indiarubber, such as is used in toy balloons, stretched across a 

 circle 5 cm. in diameter, can easily show, when observed by a 

 microscope, a difference of pressure of the millionth of an atmo- 

 sphere. The very smallest difference of pressure will move a flat 

 film, and thin flat films of glass can be made which can be moved 

 by very small pressures. There is an objection to using inexten- 

 sible films like glass because of the uncertainty as to what point 

 moves most, but there is a great advantage in their non-liability to 

 diffusion or evaporation, both of which faults exist in indiarubber 

 films. We have not made sufficient experiments on glass films to 

 speak with any degree of confidence as to their being capable of 

 measuring such small pressures as the millionth of an atmosphere. 

 If a lens were placed near them, and displacements of Newton's 

 rings observed, it seems most likely that a very much less pressure 

 could be detected. Of course soap-bubble films might in some 

 cases be employed, though they are liable to all the faults of india- 

 rubber films and to others of their own, such as the difficulty of 

 attaching a suitable pointer to be viewed through the microscope, 

 their want of permanence, especially if vertical, and the danger of 

 dust, liquid, &c, deflecting them if they are horizontal. An aneroid 

 barometer is practically a film such as we are suggesting, and one 

 on a large scale observed with a powerful microscope would pro- 



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