210 Proceedings of Royal Society of Edinburgh. [may 2, 
shall find a periodic recurrence of property or function in the series. 
The first element is a monad, the second a dyad, the third a triad, 
and the fourth a tetrad. Then we find the fifth a triad, the sixth a 
dyad, and the seventh once more a monad. Then follows a second 
series of seven elements, showing the same variation in atomicity ; 
this repeats itself right through the list of elements. This periodic 
recurrence of function is seen not only in the case of atomicity, hut 
it may he also observed in the atomic volumes, the fusibility and the 
electrical and other properties of the elements. There is then a 
general resemblance in physical properties between the first, eighth, 
fifteenth, &c., and between the second, ninth, and sixteenth 
elements. Mendelejeff has arranged the elements in the convenient 
tabular form given on the opposite page, which indicates these and 
some other important facts. 
Those elements which resemble one another, and which we can 
pick out by taking every eighth one from that one from which we elect 
to start, form what he calls a “ group,” and are arranged vertically. 
The sets of seven elements each, arranged horizontally, form twelve 
“ series.” 
There is yet another point of importance. The elements of a 
“ group,” which are in an even “ series,” are especially related to 
one another ; so in like manner elements in an odd series of the 
same group are similarly allied. Thus Li, Na, K, Cu, Rb, A g, Cs, 
Au, have all these properties in common ; but in this group Na, 
Cu, Ag, Au are most alike, and Li, K, Rb, and Cs, in like manner, 
are most closely related. 
In the paper to which I have already alluded I was able to 
demonstrate the fact that elements in the same group are capable 
of producing similar or related tastes. The power of producing a 
given taste is then a property which, like the ordinary physical 
qualities of the elements, follows the periodic law. As will now be 
shown, the same obtains for smell. 
In studying the facts of the case, let us start with Group VI. 
We find here, in odd series, three well-known substances whose 
compounds have strong and characteristic odours. Sulphuretted, 
selenietted, and teluretted hydrogen have all a disagreeable odour 
like that of rotten eggs. The compounds of the elements of this 
group with methyl and ethyl are disagreeable and alliaceous. In 
