•60 Proceedings of Royal Society of Edinburgh. [sess. 
polar atoms would cling all the closer, and would refuse to "be 
removed, therefore any additional one which could find a soft place 
to cling to would find the softest on some part of the equator, and 
as on the opposite side of the equator there would be an equally 
soft place, another would attach itself there. The valency in this 
way would advance from two to four, that is, two steps at a time. 
Now for the next two steps, from a tetrad to a hexad — let us 
suppose the conditions to be favourable for the change, which I 
again assume to be a decrease in temperature, or increase in 
pressure, or both, and consider where another fluorine atom could 
find a place to cling to, so as to be as far away as possible from its 
companions, and I think it will be found that this place would be 
where another great circle passing through the poles at right angles 
to the first would cut the equator ; and as the two points of 
intersection of the equator by this last circle would be equally 
favourable for the attachment of an atom, two would consequently 
attach themselves there, thus increasing the valency by another 
two steps. In this way we would have four atoms attached to the 
equator, all at equal distances from one another, and one each 
attached to the poles, and all separated by equal spaces. Now, I 
am not chemist enough to know if there are any cases in nature 
where the valency advances beyond six for artiads, but if there are, 
and carrying out the above process, I must admit that the next 
advance would be, not by two steps, but by four. I need not 
trouble to show how this would be, as readers can easily see for 
themselves that it must be so, according to the theory. Now the 
variable jperissads would vary on precisely the same principle as 
the artiads. Let us take phosphorus trichloride, P.C1 3 , as an 
■example ; I assume in this case that the three chlorine atoms 
would be attached to the phosphorous nucleus at equal distances on 
the equator, in such a way that the lines connecting the chlorine 
atoms through the phosphorous nucleus would form an equilateral 
triangle. It will be seen that the molecule in this way would 
be perfectly symmetrical. On the formation of P.C1 5 , which I 
believe takes place under a lower temperature or greater pressure 
of the chlorine, two more chlorine atoms would become attached to 
the nucleus, one to each pole ; thus again forming a symmetrical 
molecule. As in the case of the artiads, if there are any cases in 
nature where the valency of a perissad increases beyond five by the 
