518 
Proceedings of the Royal Society 
at length, on principles which are stated, and a final mean of the 
whole is arrived at, giving for the solar parallax 8 - "877. This is 
then compared with the result of the so-called Great Pyramid Sun 
Distance, which, combined with the best modern determination of 
the size and shape of the earth, gives a parallax of 8‘"876. 
The direct results of modern science for the solar parallax vary 
anywhere between 8 - "80 and 8*"96; and to assist in apprehending 
the present state of a very contested question as to one of the most 
important of all physical facts, Mr Petrie added to his paper two 
very instructive diagrams, in one of which the chief data were 
arranged chronologically, and in the other quantitatively. 
2. On Chemical Structure. By Dr A. Crum Brown. 
In this paper the author drew attention to the nature of the 
process of Chemical Exchange (including substitution, double de- 
composition, and some cases of direct addition and subtraction), as 
a process by which the number of relations in which any given 
atom stands is unaffected. He pointed out the connection between 
this process and the notion of chemical structure, and discussed the 
nature of those chemical processes in which the number of relations 
of an atom is increased or diminished. He concluded by some 
remarks on the ambiguous structure of such bodies as aldehyde, the 
sulphites, and hydrocyanic acid, which from some points of view 
may be represented as — 
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