84 Proceedings of the Royal Irish Academy. 
Second Period, or Silicon Period. 
ELEMENT. 
ATOMIC WEIGHT. 
VALENCY. 
1. 
Sodium. 
lYlOnaCL. 
2. 
Magnesium. 
24 
Dyad. 
3. 
Alumiuium. 
27 
Triad. 
4. 
Silicon. 
28 
Tetrad. 
5. 
Phospliorus. 
31 
Triad, 
6. 
Sulphur. 
32 
Dyad. 
7. 
Chlorine. 
35i 
Monad. 
In Plate v. I show, on an enlarged scale, Dr. Eeynolds' diagram 
for the above-named Carbon and Silicon Periods, following hydrogen. 
This diagram is formed by joining together by right lines the succes- 
sive fourteen points, commencing with litbium and ending with chlo- 
rine. If the points were at random, the number of right lines would 
be thirteen ; and this would be the order of the curve passing through, 
the fourteen points, and an infinite number of such curves could be 
drawn, and the problem would be geometrically indefinite. The points 
being supposed placed at random, it is well known that a single general 
quartic curve can be drawn through them, and one only, thus giving 
an unique geometrical solution. 
A general quartic curve is therefore the simplest solution that the 
collocation of fourteen points admits of in its most difficult form ; but 
this solution may become simpler, if the points are arranged by a law 
or method, and not at random. 
In the problem now before us, we find on inspection that five of the 
fourteen points lie on the same straight line,^ viz. : — 
C I N I 0 I Mg I Si. 
This is shown in Plate vi. 
This line joins the two tedrads, carbon and silicon, and passes 
through the point 20 of the line of atomic weights. 
The chances are millions to one against this happening at random. 
