46 Prof. C. Barus on the Absorption of the 



long tubes become dull, and are unsuitable for sharp com- 

 parisons. One may state that more dust is required for long 

 than for short tubes, but that the difference is a relatively 

 vanishing quantity and out of all proportion with the data 

 for small tubes. The difference between the present and the 

 preceding experiments with tubes is this, that whereas in the 

 latter case (small bore) the saturated air is conveyed in the 

 undiluted condition through the tubes, in the case of wide 

 tubes (5 cm.) the saturated dust is necessarily diluted on 

 being introduced into the tube, having its own independent 

 current of air. The present series shows the remarkable 

 preservative tendency of this operation of dilution, and points 

 out a reason for the constancy of behaviour of the colour- 

 tube itself after the nuclei have once been captured. 



5. In the endeavour to frame at least a working hypothesis 

 for these phenomena two possible occurrences are prominent: 

 the first is the decay of the particle so far as its activity in 

 producing condensation is concerned. This may be due to 

 growth and loss of charge, to the action of ordinary dust par- 

 ticles floating in the air, or any similar cause whatever. It 

 constitutes a loss within the ionized medium itself. The second 

 relates to losses at the boundary, to the motion of the ionized 

 particle, whether stimulated by an electric field, or a diffusion- 

 gradient, or not, occurring in the latter case as a mere ionic 

 velocity. Since electric field is absent, it would at the outset 

 be natural to treat the motion as a case of diffusion, and due 

 to a concentration gradient. It seems hardly probable, how- 

 ever, that in a swift turbulent current of air diffusion can be 

 recognized. I have therefore thought it best to regard the 

 nucleus as moving with a definite (absorption) velocity k 

 independent of direction and (for a given class of experiments) 

 independent of concentration. So circumstanced, the swarm 

 of nuclei are transferred by the air-current. As the nucleus 

 impinges upon but does not rebound from a barrier, k may 

 still be regarded as an external diffusion coefficient, corre- 

 sponding to the constant in Newton's law of cooling. 



Let dx be the thickness of an infinitesimal right section of 

 the absorption-tube of radium r, traversed at velocity v by an 

 air-current charged with nuclei. Let n be the number per 

 cubic centim., and k the number absorbed per square centim. 

 per second if n=l. Hence h is the absorption velocity* 

 discussed. 



* If, as in cases considered elsewhere, the absorption-tube were a con- 

 denser with the field acting radially outward, k would be replaced by 

 UeE/(R 2 — R,), where e is the charge of an electron, U the velocity of 

 the ions relatively to each other, E/(R 2 - Rj the potential gradient, 

 R T being the axial and R 2 the circumferential radius of the condenser. 

 1 mention this here for future reference. 



