326 



Fe 3 O 4 , with a tendency to separate again into FeO, Fe 2 O 3 ; that it is 

 really in combination as Fe 3 O 4 , but ready to yield to slight causes 

 and become FeO, Fe 2 O 3 . This would explain Mr. Mercer's experi- 

 ment (quoted by Brande, I. 716, edition 1848) of the chemical union 

 of a mechanical mixture of protoxide and peroxide of iron. 



Perhaps the sesquioxides occupy a middle place in the scale of 

 effects. Take the case of iron ; we have 



Fe + 0. pyrophorus, violent oxidation, 

 Fe 2 O 3 +NH3+0 alternate reduction and oxidation, 

 Fe O 3 +NH 3 (in water), reduction. 



To show the last, add ammonia to a solution of FeO 3 , KO, and 

 Fe 2 O 3 will be precipitated. 



A mixture of ten parts by weight of powdered chlorate of potassa 

 with one part of Fe.j O 3 disengages oxygen with extreme facility and 

 great economy of heat as compared with the oxides of copper and 

 manganese ; and it is the more convenient because n grains of the 

 mixture will represent almost exactly n cubic inches of disengaged 

 oxygen. 



A state of mechanical division is not absolutely necessary for the 

 catalysation of some inflammable vapours by Fe 2 O 3 ; an old nail, 

 entirely transmuted into rust, will perform the operation ; and when 

 we consider that in many cases of fermentation, decay and putrefac- 

 tion, this oxide may be present, divided or aggregated, while heat is 

 evolved, and inflammable gases and vapours are set free, we may 

 hereafter be able to trace some instances of " spontaneous com- 

 bustion " to the catalytic action of the sesquioxides of iron. 



III. " Ocular Spectres and Structures as Mutual Exponents." 

 By JAMES JAGO, A.B. Cantab., M.B. Oxon., Physician to 

 the Royal Cornwall Infirmary. Communicated by Pro- 

 fessor STOKES, Sec. R.S. Received March 5, 1855. 



The present communication is a revised and modified version of a 

 paper bearing the same title, which was read on the 18th of January 

 and 1st of February, and which was, by permission, withdrawn. The 

 chief modification applies to the author's views respecting the struc- 



