300 The Chemistry of the Future, [July, 
time which we may wish to particularise ; but if we remem- 
ber its derivation we see that it means a “ journey round,” 
and t.hat it is applicable only to such portions, of time or of 
anything else, as exhibit some series of changes tending first 
in a certain direction and then returning to, or at least to- 
wards, the point of departure. Thus a year is legitimately 
a “period,” because in it certain phenomena— astronomical, 
meteorological, and organic— go through a circle of changes 
which necessarily ends where it began ; but a stretch of io, 
or 20 , or ioo years cannot be called a period, unless we can 
discover in it some phenomenon which has its increase and 
decrease, or its recurrence in such a term of years. In like 
manner, if we take a group of animals, of minerals, or of 
elementary bodies, and find that in them some one attribute 
increases and then decreases again to its former condition, 
or suffers any other cyclical variation, we may call such a 
group a “ period,” and the change in question “ periodic.” 
We trust that none of our readers will feel aggrieved at 
being thus reminded of what doubtless most of them are 
aware, since a correct understanding on this point is abso- 
lutely necessary for the intelligibility of Mendeleeff’s law. 
At the same time we should be very happy to find some 
word incapable of being misunderstood. 
We now turn to Table I., which exhibits the known ele- 
ments arranged in the arithmetical order of their atomic 
weights. In the second and third columns we find all whose 
weights range from 7 to 36. Here we perceive that the 
characters of the elements change gradually and regularly 
with alternating magnitudes of the atomic weights. These 
changes are periodic, taking place in both columns in the 
same manner, so that the corresponding members are 
analogous. If we compare respectively— 
Li Be B C N O FI 
Na Mg A 1 Si P S Cl 
we must admit that there is, between the members of each 
of these pairs, a resemblance, which in the cases of carbon 
and silicon, fluorine and chlorine, is especially striking. 
We notice, further, that the atomic weight of each member 
of the first group differs from that of the corresponding 
member of the second almost exactly by one and the same 
number, viz., 16. A correspondence of this latter nature, 
indeed, can no longer be traced between the corresponding 
members of the remaining columns. If, however, we add 
the atomic weight of an element in column 3 to that of its 
representative in column 5, and divide by 2, we obtain an 
