1 877] . The Chemistry of the Future , 301 
approximation to the atomic weight of the corresponding 
member of column 4. 
Returning to the second and third series, we find that the 
corresponding degrees of each have the same atomicity. 
Only the last four members of each column can combine 
with hydrogen, forming respectively RH 4 , RH 3 , RH 2 , RH. 
Thus we see that the power of uniting with hydrogen, absent 
in the first members of each series, becomes high in the 
middle and declines again to the end. The permanence or 
instability of their compounds, their acidity and power to 
exchange hydrogen for metals, change according to the 
same gradation. Thus HC 1 is a well-marked acid, H 2 S a 
much feebler acid, H 3 P can no longer be called an acid, and 
in H 4 Si there is no residue of the acid character. In the 
corresponding members of the first column, again, HF 1 is 
manifestly acid, H 2 0 neutral, H 3 N basic, and H 4 C neither 
acid nor basic. 
All the members of the second column combine with 
oxygen, forming salifiable bodies, as— 
Na 2 0 , Mg 3 0 3 (Mg 0 ), A 1 2 0 3 , Si 2 0 4 (Si 0 2 ), P 2 0 3 , 
S 2 0 6 (S 0 3 ), CL 0 7 ; 
two atoms of the element thus combining with a number of 
atoms of oxygen, increasing from 1 to 6. This arrangement 
corresponds with the decrease of basicity and the increase 
of acidity. At the beginning of the series is a decided base, 
soda ; at the end a powerful acid, the chloric ; and in the 
middle is alumina, which may be either a feeble base or a 
feeble acid. 
A regular connection may be traced, not merely in the 
combining numbers, but in other chemical and physical 
characteristics. At the beginning of the first, second, third, 
and fourth series we find in each case bodies unmistakably 
metallic ; at the opposite end come non-metallic substances, 
whilst those which stand on the boundary between these two 
classes take an intermediate position. 
Again, if we consider the same set of elements with re- 
ference to specific gravity and atomic volume, we find 
periodic changes. Thus in specific gravity we have — 
Na = o v 97, Mg=i*75, Al = 2*67, Si = 2 ‘49, P~ 1*84. 
S = 2* o 6, Cl (liquid) = i’33. 
Plere the numbers, as will be seen, increase up to aluminium, 
and then decrease again, save that sulphur is anomalous, 
and has a higher specific gravity than phosphorus, as the 
