SOO Mr Stromeyer on Metallic Iron and its Oxides. 



modifications, I have suggested the three following queries, 

 which I hope some of your learned correspondents will be able 

 to solve : 



Query, How is vision performed in those insects of the but- 

 terfly kind, since they are not provided with stemmata, nor 

 have lenses in the hexagonal apertures, in the external covering 

 of their eyes ? 



Query, How is vision performed in those insects which we 

 find unprovided with stemmata, but in the external covering of 

 whose eyes we find the hexagonal apertures filled with double 

 convex lenses ? 



Query, How is vision performed in those insects which we 

 find provided with stemmata, but want lenses in their com- 

 pound eyes ? 



If the foregoing remarks are deemed worthy of a place in 

 your valuable publication, your inserting them will oblige, Sir, 



Your very humble Servant, 

 Mitchell- Street, Glasgow, 



%\st August 1826. William Ewing. 



Art. XVIII. — On Metallic Iron and its Oxides. By F. 

 Stromeyer, M. D. F.R. S.E. &c. &c. Professor of Che- 

 mistry in the University of Gottingen. 



The third volume of Poggendorff's Annalen der Physik und 

 Chernie, contains some observations by M. Gustav Magnus on 

 the spontaneous combustion of certain metals, in which that 

 property has not been previously noticed. He finds that nickel, 

 cobalt, and iron, reduced from their oxides by means of hy- 

 drogen at a very low heat, undergo spontaneous combustion 

 when they are exposed to the air at common temperatures ; 

 whereas they are not subject to the same change if the reduc- 

 tion is effected by hydrogen in a strong fire. M. Magnus as- 

 cribes the difference of combustibility to the density of the iron 

 being greater in the second than in the first case, owing to the 

 more intense heat which is employed in the operation. 



In the sixth volume of the same Journal, Professor Stromeyer 

 has offered a different explanation of the phenomena, at least 



