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XV. On some of the compounds of chromium. By Thomas 
Thomson, M. D. F. R. S. L. and E. Professor of Chemistry , 
Glasgow. 
Read March 29, 1827. 
The principal object of this Paper is to give an account of a 
singular combination of chromic acid, and chlorine, which I dis- 
covered about a year and a half ago. But as the investigation 
of this compound led me to a more careful examination of the 
oxides of chromium, and a more accurate knowledge of their 
composition than had been hitherto attained, I shall also state 
the facts which I have ascertained with respect to these bodies. 
In a Paper inserted in the Annals of Philosophy (1st Series, 
vol. 16, p. 321), I have shown that the atomic weight of 
chromic acid is 6*5. Two other compounds of chromium 
and oxygen being known, namely, the green oxide , and the 
brown oxide , I was induced, from analogy, to consider the 
atomic weight of the former to be 4-5, and that of the latter 
5*5 ; and to make the atom of chromium 3-5. These views 
were rather favoured by some experiments on the chloride 
and muriate of chromium, which I have stated in my “ First 
Principles of Chemistry/' vol. ii. p. 52. But these experi- 
ments deviating more than two per cent from the theoretical 
number, could not be considered as decisive. How far my 
former notions on this subject were correct, will appear from 
the following statements. 
