the Radioactive Emanations by Charcoal. 385 



(c) The surface exposed to the emanation. 



It is natural to suppose that the amount of absorption will 

 in some way depend on the amount of the charcoal absorbing 

 surface — that, in fact, the absorption will be increased if the 

 surface be increased. 



To prove the point experiments were performed, the results 

 of which are shown graphically in fig. 6. To save space, the 

 tables showing the actual numbers are omitted. Each division 

 of the ordinates corresponds to 6*17 x 10 — 13 ampere. 



For a particle of any material the ratio of its surface to its 

 mass becomes greater when the size of the particle is reduced ; 

 hence one lot of charcoal, equal in mass to a second lot, will 

 have a greater or less surface than the second according as 

 the size of its grains is smaller or larger than the size of the 

 grains of the second. 



In this particular experiment three equal masses ('2' 17 gms.) 

 of the same sample of coconut charcoal were used; but the size 

 of the grain was different in each case. The i-q curves (fig. 6) 

 were taken for all three. I. is the curve for the grains which 

 were just caught in a sieve of ten meshes to the inch, II. for 

 twenty meshes to the inch, and III. for thirty. It can be 

 seen that the larger the grain the less is the absorption, or, in 

 other words, the smaller the surface the less the absorption. 



The experiment shows that it is not the mass of the 

 absorbent on which the amount of absorption depends, but 

 rather the amount of surface which the mass exposes to the 

 emanation. 



By increasing the quantity of charcoal of a given size grain 

 the amount of absorbing surface is also increased, and conse- 

 quently there is increased absorption. This is shown by the 

 curves in fig. 7, which show the experimental i-q curves for 

 1*18, 2*17, and 3*35 gms. of the same sample of coconut 

 charcoal of the same size grain (just caught in a sieve 

 thirty meshes to the inch). 



It must be noticed that the greater the quantity of char- 

 coal — or, for that matter, the greater the power of absorbing 

 by any condition — the greater must be the speed of the air- 

 current to get any effect at all in the testing- vessel. Thus, 

 there is practically complete absorption for 1*18 gms. of 

 charcoal up to a speed of 0'5 c.c. per second, up to 0*8 c.c. per 

 second for 2*17 gms., and up to 1 c.c. per second for 3'35 gms. 



(d) The temperature of' the charcoal. 



The experiments of Dewar showed that when coconut 

 charcoal is used as an absorbent of gases, the absorption is 

 greatly increased by lowering the temperature of the 

 charcoal. 



