52 MM. Warburg and v. Babo on the Connexion 



elasticity, holds only approximately, and even that only at 

 moderate degrees of density; at higher densities, according 

 to the investigations of Natterer, Andrews, Cailletet. and 

 others, the connexion between elasticity and density is not 

 even approximately given by Boyle's law, but is apparently 

 more complicated. It can, however, according to van der 

 Waals *, be explained from the kinetic theory of gases, if the 

 volume of the molecules and the attraction between them be 

 taken into account. 



Corresponding investigations in relation to the viscosity of 

 gases have hitherto been carried out only so far as Kundt and 

 one of us t have studied the deviations of Maxwell's law at 

 very slight densities. For higher degrees of density the con- 

 nexion between viscosity and density has not yet been inves- 

 tigated. For the solution of this problem (treated in the pre- 

 sent paper for one substance, viz. carbonic acid) the corre- 

 sponding values at constant temperatures of the coefficient of 

 friction, the density, and, for many reasons, the pressure must 

 be determined. 



We employ as the measure of the pressure the inverse 

 value of the volume of a mass of nitrogen at constant tempe- 

 rature of the apartment, the volume of that mass at the pres- 

 sure of one atmosphere being put =1. To measure the pres- 

 sure according to this definition a nitrogen-manometer was 

 employed, which was always attached to the principal appa- 

 ratus, and permitted pressures between 30 and 120 atmo- 

 spheres to be evaluated. 



The density of the substance heated above the critical tem- 

 perature we determined by a volumetric measurement of the 

 carbonic acid, which at each transition from a greater to a 

 less density was liberated from our apparatus, the volume of 

 which was known to us ; the density of the mass in the appa- 

 ratus after the conclusion of a series of experiments we calcu- 

 lated from the pressure, which then amounted to about 30 

 atmospheres, by Clausius's formula}, which at so small a 

 pressure is sufficiently accordant with the observations. At 

 the temperature 32 0, 6 our experiments comprise the interval 

 of densities between O'l and 0"8. 



For the determination of the friction-coefficient we em- 

 ployed the method of flow through capillary tubes. The 

 capillary, placed vertical, ended below in a measuring-tube 

 which dipped in mercury, above in a space A, which could 

 from time to time be shut off from the rest of the space by a 

 cock, and in which a diminution of pressure could then be 



* Dissertation: Leyden, 1878. t Berlin Monatsberichte, 1875, p. 100. 



I Wiedemann's Amiafm, ix. p. -'148. 



