514 
EDINBURGH MEETING. 
so on. We find, further, that an atom which in one compound is singly-related, 
becomes in another trebly- or fivefold-related, and that a doubly-related atom 
may, in the same way, become fourfold- or sixfold-related ; the change in the 
number of relations (what is called the change of u atomicity ”) taking place, 
in the vast majority of cases, by two at a time, so that an atom which has an 
odd number of relations in one compound has an odd number in all, and may, 
therefore, be called oddly-related ( u perissatomic ”), and an atom which has an 
even number in one, has an even number in all, and may be called evenly-related 
(“ artiatomic ”). 
These relations have been expressed in a great variety of ways; most com¬ 
pletely by the system of “ graphic formulae,” first introduced by Professor 
Kekule in 1859. These graphic formulae have received many modifications ; in 
one of these, which I proposed in 1861, each atom is represented by a circle 
containing the symbol of the atom, and the relations by lines proceeding out¬ 
wards from the circumference and connecting the circle with others. Such 
graphic or pictorial representations have not unfrequently been objected to as 
expressing more than we know, and as leading readers to imagine that they 
show the relative position of the atoms, of which, of course, we are entirely 
ignorant. I think I shall be able to show you that this accusation is unfounded, 
and, at the same time, illustrate the meaning of chemical constitution, by ap¬ 
plying the same graphic method to a case where the idea of position in space 
does not enter at all. Let us suppose that John is occupant of a house which 
belongs to Mr. Smith, and is situated in Edinburgh, and that Thomas occupies 
a house belonging to Mr. Brown in Glasgow ; then representing John by J, 
his house by E, Smith by S, Thomas by T, his house by G, and Brown by B, 
S B 
we may express their relations thus : | and | without meaning that 
E—J G—T 
the landlord is situated above and the tenant at the side of the house. Now if 
it is convenient for John to go to Glasgow and Thomas to Edinburgh, they may 
SB SB 
change houses, and we have | and | becoming | and | 
E—J G—T E—T G—J. 
Here we have two groups, one in Edinburgh and one in Glasgow, before the 
change and after it. The only difference is, that we have John and Thomas 
each in the position previously occupied by the other. These two men may be 
totally unlike in every other respect, but they are capable of replacing each 
other in this particular relation. So in chemistry ; when chloride of sodium 
and nitrate of silver are brought together, the silver and the sodium change 
places, each taking up the relations previously held by the other. This is an 
example of what is called double decomposition. 
Now let us suppose a slightly more complicated case. Let John be proprietor 
B 
as well as occupant of his house. Here we have E = J and | In this case 
G—T. 
John is doubly-related and united by both bonds to his house. If he now 
B 
I 
change with Thomas, the state of matters will be represented thus : G—J 
I 
E—T. 
Before the change two groups, but only one after. The corresponding chemical 
reaction is termed addition, and groups to which addition can be made in this 
way are said to be condensed. The condensation obviously consists in the atoms 
being united together by more bonds than are necessary to ensure the unity of 
