Chemistry and Physics. 



167 



increases slightly. With helium the increase is more rapid than 

 with hydrogen ; with argon there is first a considerable decrease 

 followed at very high pressures by a gentle increase, although the 

 product does not reach the theoretical value at 100 atmospheres 

 pressure; with krypton the change with rise of pressure is a still 

 more marked decrease, and with xenon the decrease is very sud- 

 den. At the higher temperature the results are more difficult to 

 interpret ; while nitrogen maintains its nearly constant value for 

 PV, helium decreases rapidly, then increases, and the same pecu- 

 liarity is to be remarked with the other gases, although they do 

 not give the product of PV coinciding with that calculable by 

 assuming that the increase of PY is proportional to the rise of 

 absolute temperature. 



These last experiments must be taken as merely preliminary ; 

 but they show that further research in this direction would be 

 productive of interesting results. 



The spectra of these gases have been accurately measured by 

 Mr. E. C. C. Baly, with a Rowland grating ; the results of his 

 measurements will shortly be published. It may be remarked, 

 however, that the colour of a neon-tube is extremely brilliant and 

 of an orange-pink hue ; it resembles nothing so much as a flame, 

 and it is characterized by a multitude of intense orange and yellow 

 lines ; that of krypton is pale violet ; and that of xenon is sky- 

 blue. The paper contains plates showing the most brilliant lines 

 of the visible spectrum. 



That the gases form a series in the periodic table, between that 

 of fluorine and that of sodium, is proved by three lines of argu- 

 ment : 



(1) The ratio between their specific heats at constant pressure 

 and constant volume is 1*66. 



(2) If the densities be regarded as identical with the atomic 

 weights, as in the case with diatomic gases such as hydrogen, 

 oxygen, and nitrogen, there is no place for those elements in the 

 periodic table. The group of elements which includes them 

 is : — * 



Hydrogen. 



Helium. 



Lithium. 



Beryllium. 



1 



4 



7 



9 



Fluorine. 



Neon. 



Sodium. 



Magnesium. 



18 



20 



23 



24 



Chlorine. 



Argon. 



Potassium. 



Calcium. 



35%5 



40 



39 



40 



Bromine. 



Krypton. 



Rubidium. 



Strontium. 



80 



82 



85 



87 



Iodine. 



Xenon. 



Caesium. 



Barium. 



127 



128 



133 



137 



(3) These elements exhibit gradations in properties such as 

 refractive index, atomic volume, melting-point, and boiling-point, 

 which find a fitting place on diagrams showing such periodic 



* For arguments in favour of placing hydrogen at the head of the fluorine 

 group of elements, see Orme Masson, Ghem. News, vol. 73, 1896, p. 283. 



