AN UNDISCOVERED GAs! 255 



vanadium, carbou and titanium are in each approximately the same as 

 that between helium and argon, viz, 30. If elements farther back in 

 the periodic table be examined, it is to be noticed that the differences 

 grow less the smaller the atomic weights. Thus, between boron and 

 scandium the difference is 33; between beryllium (glucinum) and cal- 

 cium, 31 ; and between lithium and potassium, 32. At the same time we 

 may remark that the elements grow more like each other the lower the 

 atomic weights. Kow, helium and argon are very like each other in 

 physical properties. It may be fairly concluded, 1 think, that in so far 

 they justify their position. Moreover, the pair of elements which show 

 the smallest difference between their atomic weights is beryllium and 

 calcium. There is a somewhat greater difference between lithium and 

 potassium. And it is in accordance with this fragment of regularity 

 that helium and argon show a greater difference. Then again, sodium, 

 the middle element of the lithium triad, is very similar in properties 

 both to lithium and potassium; and we might, therefore, expect that 

 the unknown element of the helium series should closely resemble both 

 helium and argon. 



Leaving now the consideration of the new element, let us turn our 

 attention to the more general question of the atomic weight of argon 

 and its anomalous position in the periodic scheme of the elements. 

 The apparent difficulty is this; The atomic weight of argon is 40; it 

 has no power to form compounds, and thus possesses no valency; 

 it must follow chlorine in the periodic table and pi ecede potassium, 

 but its atomic weight is greater than that of potassium, whereas it is 

 generally contended that the elements should follow each other in the 

 order of their atomic weights. If this contention is correct, argon 

 should have an atomic weight smaller than 40. 



Let us examine this connection. Taking the first row of elements, 

 we have : 



Li = 7, Be = 9.8, B = 11, O = 12, N = 14, < ) = 16, F = 19, ? = 20. 



The differences are: 



2.8,1.2,1.0,2.0,2.0,3.0,1.0. 



It is obvious that they are irregular. The next row shows similar 

 irregularities. Thus : 



( ? = 20), Na = 23, Mg = 24-3, Al = 27, Si = 28, P = 31, S = 32, CI = 35.5, 



A = 40. 



And the differences : 



3.0, 1.3, 2.7, 1.0, 3.0, 1.0, 3.5, 4.5. 



The same irregularity might be illustrated by a consideration of each 

 succeeding row. Between aigon and the next in order, potassium, 

 there is a difference of — 0.9; that is to say, argon has a higher atomic 

 weight than potassium by 0.9 unit; whereas it might be expected to 



