515 
of Edinburgh^ Session 1865 - 66 . 
they are in many points of view forced and unnatural ; in other 
words, that they are not what an unbiased consideration of the 
composition and properties of the substances would naturally lead 
to. This is still more the case with those substances in which the 
sulphur appears to be hexatomic. These are SO3, M2SO4, M2S20e, 
and the numerous derivatives of H^SO^, which contain (SO^/ ; as 
these bodies, like the sulphites, contain, in the molecule, only two 
monad atoms, it is, of course, 'possible to represent their composi- 
tion by formulse containing diatomic sulphur.* Thus — 
Sulphurous Anhydride. Sulphite. Sulphuric Anhydride. 
Sulphate. 
Examining these formulas we find that these stable substances 
have some of the oxygen equivalents saturated by oxygen ; this 
is, of course, possible ; indeed we know bodies such as H2O2, Ba02, 
Brodie’s peroxides, &c., in which this arrangement is pro- 
bable ; but all these bodies are remarkable for instability, and for 
the readiness with which they lose oxygen. They present ab- 
solutely no points of resemblance to the substances we are now 
considering. The striking analogy of SO2 and SO3 to 00 and CO2 
also leads to the conclusion that the former^ is S^^02 and the latter 
S^'Og, and that in both of them all the oxygen is combined with 
sulphur. If we accept this view, and follow out the analogy 
between the carbon and the sulphur compounds, we get a simple 
explanation of many relations otherwise difficult to be understood. 
For instance, hypo-sulphuric acid obviously stands in the same 
relation to sulphurous anhydride and sulphuric acid as oxalic acid 
^ I use liere, for the purpose of representing the chemical structure of 
molecules, the graphic method which I first introduced in my thesis pre- 
sented to the Medical Faculty of the University in 1861, and which I have 
more than once employed in papers read before this Society. I am happy to 
observe that its advantages are appreciated by others. Dr Hofmann, for in- 
stance, using a system almost identical in his lecture delivered before the 
Koyal Institution on the 7th of April of this year. 
