42 NEW METHOD OF DETERMINING COMPRESSIBILITY 



be distorted by the strain of compression ; but with the help of our 

 glass jacket the result is very easily attained. 



If glass were a definite substance, the figures given in the table of 

 data concerning the compressibility of water alone or of mercury alone, 

 would at once afford the desired intelligence. By plotting the results 

 in the second and third column on page 38 for example, it is readily 

 seen that in a glass jacket containing 18.75 milliliters of water and 1.9 

 milliliters of mercury at 20 , 100 kilograms per square centimeter 

 would correspond to 1.050 grams of added mercury, 200 units of 

 pressure would correspond to 2.086 grams of added mercury and so 

 forth. The same proportion of change of volume to total volumes of 

 water and mercury would exist in a jacket of any other size. Unfor- 

 tunately, however, the compressibility of glass is not uniform enough 

 in different samples to make such an inference more definite than 

 within three tenths of one per cent. 



On the other hand, the difference betxveen the compression of tvater 

 and mercury, as found by a jacket of this kind, is perfectly definite 

 and free from all uncertainty connected with the glass. This difference 

 is plotted as the curve for water, on the larger diagram (Fig. 5), and 

 will serve at any time as a means of comparing any other gauge with 

 that made by Schacffer and Budenberg, thus enabling any one who has 

 a less accurate gauge to correct its readings, or any one who has a more 

 accurate guage to correct ours. 



The best method of making this comparison would be to make suc- 

 cessive series of experiments first with mercury and afterwards with 

 water in a given glass jacket, in the way described above, and then to 

 plot the results and compare the differences with ours. The curves are 

 so nearly straight lines that they may be drawn with great accuracy by 

 bending a thin ruler made of wood with an even grain, until all the 

 points are covered. 



From our preliminary experiments it seems probable that the sen- 

 sitiveness of this manometer is very great. Under favorable condi- 

 tions the method is able to detect -?\ atmosphere in 1 ,000 atmospheres 

 or one part in 20,000. 



It is our purpose to carry out the evaluation of this manometric 

 method with much greater precision than has been heretofore possible, 

 in an apparatus free from ground glass joints. The present results in 

 this direction must be considered as merely preliminary, but even these 

 may serve an end hitherto unattainable. 



It is a matter of great regret that the scientific world has not agreed 

 upon a less arbitrary unit of pressure than the "atmosphere." The diffi- 



