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Article XIV. 



Selections from a Memoir on the Expansion of Dry Air. By the 

 late Professor F. Rudberg. 



[From Poggendorff's y^?ina/ew, B. 41. S. 271.] 



Among the constants in physics there is certainly not one 

 Avhich is usually considered to be determined with greater preci- 

 sion than the expansion of dry air, or of dry gases generally, under 

 a constant pressure, between the standard points of the thermo- 

 meter scale. The numerous experiments made by Dalton and 

 Gay Lussac, almost at the same time, about the beginning of 

 the present century, appeared to show, beyond all doubt, that 

 the amount of this expansion from 0° to 100^ C, under a con- 

 stant pressure, was 0*375 of the volume of the air at 0°. Their 

 great skill in experimenting, and the magnitude and number of 

 the services they had rendered science, left no room for any 

 doubt as to the accuracy of this result ; consequently, for more 

 than thirty years in all computations in which the expansion of 

 gas occurs, it has been assumed to be 0*375. 



The constant in question is undeniably of the greatest im- 

 portance in Physics, since it forms the basis of all methods of 

 measuring temperature ; it is used in the explanation of most of 

 the phaenomena caused by heat ; and lastly, is requisite in the 

 reduction of many observations in Physics and other sciences ; 

 as, for example, in determining the velocity of sound, in the 

 measurement of heights by means of the barometer, and in com- 

 puting astronomical refractions. This being the case, it will no 

 doubt appear surprising, that the value of this constant, which 

 has been employed up to the present time, is erroneous to no 

 email amount, since, as will be shown in this memoir, it appears 

 to be not more than from 0*364 to 0*365, instead of 0*375. 



The change of volume produced by heat can be determined, 

 cither by heating cold air and measuring the increase of its vo- 

 lume, or by cooling warm air and determining the diminution 

 of its volume. I have adopted the latter method, as being by 

 far the most accurate. 



In most of the experiments, a glass globe, having a neck 

 made of thermometer tube ABC (fig. 1 .), and capable of con- 



