at the Royal Listitutioa, 1 900-1 907. 361 



as a mere physical change but as a chemical change, inasmuch as it 

 involves a change in the number of kinetic units, in other words, in 

 molecular composition, such as is exemplified bv the expression 



In a solid, the association of the molecules which are the units in 

 the gas is of a rigid character ; so soon as they oscillate outside 

 certain mean positions, the mass ceases to be solid and passes into 

 either the viscous fluid or the liquid state. 



When charcoal is brought into contact with a gas, presumably its 

 surfaces become coated initially with a layer of molecules which may 

 be regarded as practically solidified. If the temperature be low 

 enough, this surface layer of molecules attracts other molecules, thus 

 forming the equivalent of a liquid layer. The ofl&ce of the charcoal 

 may therefore be supposed to be that of acting, in virtue of its rigidity 

 and its affinity, as an anchorage, as it were, for the molecules which are 

 presented to it in the gaseous state ; once anchored, these molecules 

 can exercise their blandishments over their own kind in a manner 

 which is impossible so long as they are flying about violently 

 in every possible direction. 



Density of Gases Absorbed in Charcoal. — As Sir James Dewar 

 points out, the surface presented by charcoal is enormous. Accord- 

 ing to Mitcherlich, the cells of charred wood are on an average ^twq 

 of an inch in diameter, A cubic inch of charcoal cut up into small 

 equal cubes having edges ^ 4V(j of an inch long Avould offer a surface 

 area of 100 square feet ; taking into account, however, the space 

 occupied by the charcoal itself, the area would be about 73 square 

 feet. If the amount of carbon dioxide absorbed by such an amount 

 of charcoal under ordinary conditions were spread as a liquid over 

 such an area, the thickness of the layer would be about • Oo< m)(>2 of 

 an inch. 



Taking into account the real and apparent density of charcoal 

 such as has been used, the average pore space in 100 grammes may 

 be taken as being about 15 cubic centimetres. Calculating with the 

 aid of this value the average density of the material condensed within 

 the charcoal and contrasting it with that of the liquid obtained on 

 condensing the gas alone, it seems that the density of the absorbed 

 material is equal if not a little superior to that of the same sub- 

 stance in the liquid state at the same temperature. The results are 

 given in the table opposite. 



It will be noted that the density of the occluded hydrogen at 80° 

 absolute is practically that of liquefied hydrogen ; as this temperature 

 is about four times that at which hydrogen boils, the density of the 

 occluded hehum at 15° absolute being 0*17, it may be inferred that 

 this would be about the density of liquid helium at its boiling point, 

 about 4° absolute. Actually, Prof. Onnes has found the density of 

 liquid helium at about -4" '5 to be 0*15. 



