512 Dewars Method of 'producing High Vacua, 



with its attached charcoal absorber should be exhausted to as 

 high a degree as possible before the charcoal is cooled to the 

 temperature of liquid air. 



§ 23. Reviewing the results so far obtained, we see that they 

 may all be included in a relation of the form given in equa- 

 tion (v.). This expresses the fact that the rate of absorption 

 is proportional to the difference between the total amount of 

 air absorbed and the amount which has been absorbed at the 

 instant in question. In other words, the rate of absorption 

 stands in a constant ratio to the quantity of air that will still 

 be taken up by the charcoal. The constant is but little 

 affected by alterations in the pressure under which the 

 absorption takes place. 



The simplest explanation of the resnlt appears to be to 

 consider each element of the charcoal (probably a superficial 

 rather than a volume element) capable of taking up a definite 

 amount of gas. When the element has received this quantity 

 it becomes useless so far as further absorption is concernel. 

 On this hypothesis it follows that a given sample of charcoal 

 can absorb a definite amount of gas, the amount being inde- 

 pendent (or almost independent) of the pressure. Moreover, 

 the rate at which the absorption is proceeding at any instant 

 will be proportional to the number of elements which have 

 not yet taken up their quantum of gas; that is to say, to 

 the quantity of gas that the charcoal is still capable of 

 absorbing. 



This hypothesis is sufficient to account for the facts observed 

 in the simpler cases, but it may require some modification 

 when we are dealing with a limited quantity of air. For we 

 know that in this case the whole of the air is not absorbed 

 by the charcoal, but the pressure tends to a definite limiting 

 value *. 



It is to be expected that further light would be thrown on 

 the value of this limiting pressure, by a study of the absorp- 

 tion of pure gases by charcoal. In the case of air, the residual 

 pressure is due to some considerable extent to the less easily 

 absorbed gases, hydrogen, neon, and helium. Consequently, 

 if an extremely high vacuum is required, it is advisable to 

 remove as large a proportion of these gases as possible by a 

 preliminary exhaustion. 



Blythswood Laboratory, Renfrew. 



* In the experiment made on April 12th (Table V.) the final pressure 

 recorded was not altered by keeping the charcoal bulb in liquid air for a 

 further period of three and a half hours. 



