INERT CONSTITUENTS OF THE ATMOSPHERE. 585 



a few such values have as yet been determined numerically; instances 

 may be chosen from the magnesium group, where the numbers run: 

 Magnesium = -f 1.2; Zinc = + 0.5; Cadmium = -f- 0.19; or from 

 the fluorine column, where the numbers are : Fluorine = — 2.0 ; 

 Chlorine^ — 1.6; Iodine = — 0.4. In each case the potential, posi- 

 tive or negative, is the highest for the element with smallest atomic 

 weight, and decreases with increase of atomic weight, for elements in 

 the same column. The order of some of the elements is: Cs Eb K 

 Na Li Ba Sr Ca Mg Al Mn Zn Cd Fe" Co Ni Pb H Cu Ag Hg'Pt'" ' 

 Au'"; and for electro-negative ions, S" 0" I Br CI F; the first ele- 

 ment, caesium, being the most electro-positive, and the last, fluorine, 

 the most electro-negative. 



The order given above corresponds fairly well with the order in the 

 periodic table, passing from left to right. But, as in the table, the 

 atomic weights follow each other continuously round the cylinder or 

 round the spiral, the abrupt change from elements of an extreme electro- 

 negative character, like fluorine to sodium, an element of highly electro- 

 positive character, or from chlorine to potassium, has always appeared 

 remarkable. The old dictum, Natura nihil fit per saltum, if not always 

 true (else we should have no elements at all, but a gradual and contin- 

 uous transition from one kind of matter to another — a condition of 

 affairs hardly possible to realize), has generally some spice of truth in 

 it; and it might have been predicted (and the forecast seems to have 

 been made obscurely by several speculators) that a series of elements 

 should exist which should exhibit no electric polarity whatever. Such 

 elements, too, should form no compounds, and, of course, should dis- 

 play no valency; they should be indifferent, inactive bodies, with no 

 chemical properties. 



The discovery of argon in 1894, followed by that of terrestrial 

 helium in 1895, and of neon, krypton and xenon in 1898, has shown 

 the justice of the foregoing remarks. In as much as the methods em- 

 ployed for the isolation of these elements illustrate their properties 

 and confirm the views as to their inertness and lack of electric polarity, 

 I propose to sketch shortly the history of their discovery. 



An accurate investigation of the density of atmospheric nitrogen 

 and of nitrogen prepared from its compounds led Lord Eayleigh to 

 inquire into the cause of the discrepancy, for the density of the nitro- 

 gen of the atmosphere was found to exceed that of 'chemical nitrogen' 

 by about one part in two hundred, whereas the accuracy of his experi- 

 ments was such that it would have excluded an error of one part in five 

 thousand. I need not here allude to the reasons which were at first put 

 forward to account for this anomaly ; suflBce it to say that they offered 

 no explanation; and that we ultimately traced the discrepancy to the 

 presence in 'atmospheric nitrogen' of a gas nearly half as dense again 



