58 Proceedings of Royal Society of Edinburgh. [sess. 
In a compound molecule the component atoms group themselves 
symmetrically round the nucleus, because, as it seems to me, they 
find a better hold as far away as possible from where their 
companions are attached to it, the nucleus being less compressed 
at these parts. 
The traps and pitfalls for the unwary in dealing with this 
subject are almost innumerable. Before recognising the fact that 
the valency of an atom could be changed, by altering the velocity 
of its particles, from that of a monad up through all the stages of 
which it was capable — possibly to a hexad or higher — depending 
upon its atomic weight, and down again to that of a monad by 
mere increase of velocity, I was led into a perfect quagmire of con- 
tradictions; and although I must leave the subject still in a good 
deal of obscurity, for lack of mathematical and chemical knowledge 
to disentangle it, yet I hope I may have shed a little light on it, 
of which people in a better position than I am to avail themselves 
of it may take advantage, and make it clear as the sun at noon- 
day. 
"We may now inquire if the theory can give any account of the 
fact that when an atom changes its valency it does so by chang- 
ing two steps at a time. Thus, manganese is a hexad, but it can 
also act as a tetrad or a dyad, but never as a pentad, triad, or 
monad. Nitrogen is a pentad, and it can also act as a triad or 
monad, but never as a dyad, tetrad, or hexad. The first follows 
the order of the even number, and the last that of the odd number. 
“There are several of the chemical elements, and these among 
the most important and widely distributed, whose quantivalence 
appears to be invariable. This is especially true of hydrogen ; it is 
likewise true of the alkaline metals, lithium, sodium, potassium, 
csesium, and rubidium, and it is also true of silver, all elements 
whose atoms are univalent. It is further true of the trivalent 
element boron. Again, oxygen is always bivalent, and so are also 
the metallic radicals of the alkaline earths, calcium, barium, stron- 
tium, and magnesium ; and so are, moreover, the well known metallic 
elements, lead, zinc, and cadmium. Lastly, aluminium, titanium, 
silicon, and carbon are always quadrivalent, although, in the 
single instance of the molecule, CO, the carbon atom appears to be 
bivalent.” * 
* J. P. Cooke, “ The New Chemistry ” pp. 246, 247. 
