at the Royal Tnstitntion, 1 900-1 907. 397 



this purpose is depicted in fig. 14. In this arrangement A is a bulb 

 containing charcoal saturated with air or hydrogen or helium at any 

 desired pressure ; Y represents liquid air or hydrogen ; the space 

 between the bulb A and D is filled with the vapour of the liquid in 

 Y. On flashing even a feeble beam of light on the bulb, although 

 the rays must penetrate through the vessels and the liquid in Y, the 

 level of the liquid in the gauge is at once altered. The special 

 value of such an instrument is that it becomes more sensitive as the 

 temperature falls. 



Gases of the Atmosphere. 



The new field opened up within recent years by Lord Rayleigh's 

 classical discovery of argon in our atmosphere has been cultivated 

 with such brilliant success, particularly by Sir AYilHam Eamsay and 

 Professor Travers, that we now know that air contains besides this 

 gas four other elementary substances : helium, neon, krypton and 

 xenon, all of which belong to the class of inert elements — that is to 

 say, elements which are apparently destitute of chemical properties 

 and incapable, so far as we are at present aware, of forming com- 

 pounds with other elements — unless, as may well prove to be the 

 case, radium be a compound of helium. These gases are at present 

 the riddles of chemistry but it may well be that their true character 

 is not yet appreciated : indeed, presumptive proof that they are not in 

 reality inert is afforded by the fact that they give l^rilliant spectra, 

 if the argument be correct which is put forward on p. 32 that 

 electric discharges take place in gases within complex systems. That 

 even the heUum molecule has some degree of chemical activity is 

 shown by the fact that it is powerfully attracted by charcoal at very 

 low temperatures and that its heat of absorption is by no means incon- 

 siderable. Lastly, it may be pointed out that the argument which 

 has led to the assumption being made that the molecules of these 

 gases consist of single atoms is based on the slenderest of founda- 

 tions and is entirely inconclusive — atoms possessed of an extra- 

 ordinary high degree of affinity might well form molecules which 

 would be exceptionally sluggish in chemical behaviour : nitrogen, 

 in fact, is an illustration of the force of this argument, being almost 

 inert in the form of diatomic molecules, Ng' although an extraordin- 

 arily active substance in the elementary state. 



For these reasons the investigation of the properties of the gases 

 of the helium group is of exceptional interest and importance. 



Reference was made by Miss Gierke to the account given in 1901 

 by Liveing and Dewar of the separation of krypton and xenon from 

 hquefied air by a single process of fractional distillation and of their 

 observations on the spectra of these gases. Later observations on 

 the separation of the various gases by means of charcoal have been 

 referred to in an earlier section of this essay (p. 362). 



