180 Professor Sir James Deicar [Jan. 20^ 



But at last the porosity must become so fine that absorption again 

 disappears. Thus Cumberland black lead (which is 96 per cent- 

 carbon), of sp. gr. 2*17, showed no absorption. 



From experiments, detailed on a former occasion,* undertaken ta 

 determine the effect of various substances in a state of fine division 

 placed within the walls of vacuum vessels in protecting the contents 

 of the vessels from external heat, by reason of bad thermal conduc- 

 tivity and the interference of the mean free path of the gas molecules, 

 I found that charcoal and lampblack were nearly equally good at the 

 temperature of liquid air, and that each was four times as good as 

 graphite. 



From som.e recent investigations I find that charcoals so different 

 in their origin as cocoa-nut charcoal and charcoal from cane-sugar 

 differ but little in their absorptive powers for a gas like hydrogen. f 



Mitscherlich specially studied the nature of porosity in connection 

 with the occlusion of gases. Taking a piece of charcoal weighing 

 • 9565 grm., and thoroughly saturating it with water, he found that 

 it weighed 2*2585 grm. in air, and O'llO grm. under water. Hence 

 of the gross volume of the charcoal, | was occupied by charcoal 

 substance, and f was free space into which gases might be absorbed. 

 Saussure found that charcoal at 12° C. and ordinary pressure absorbed 

 35 times its volume of carbonic acid, but as this occupied f of the 

 gross volume of the charcoal, it was actually forced into a space 

 equal to only -^^ of its original volume. He concluded, therefore, 

 that about a third of the gas was liquefied in the pores of the 

 charcoal, t 



He found that the cells of charred wood are on an average 

 ^^-^^ of an inch in diameter. Now, if a cubic inch of charcoal were 

 cut up into a number of small equal cubes each of whose edges was 

 -5^V?j iiich, the total area of their surfaces would be 100 square feet, 

 or taking into account the space occupied by the charcoal itself, it 

 would leave about 73 square feet. The thickness of the liquefied 

 carbonic acid over this area would, then, be about '000002 inch. 



This harmonises to a certain extent with Saussure's remark that 

 denser charcoals, that is, charcoals with pores of smaller diameters, 



* Liquid Air as an Analytic Agent, Boy. Inst., 1 Ap., 1898. 



t Approx. Series of 30/10/04, range 250 c.c. to 2750 c.c., gave 



log. ^ = --08+ -2670. Cocoa-nut. 



Series 4/11/04, range 250 c.c. to 2250 c.c, gave 



log. 'p = • 39 + • 195 c. Cane-sugar, 



lip = 100 mm., these give respectively 



c. = -— — - =7*8. Cocoa-nut ; 



and c. = TtjTTv = 8'3, Cane-sugar. 

 * 195 



X From Amagat's observations, the amount liquefied was almost exactly 

 one-fifth. 



