262 Dr. Mac Culloch on Reflecting Telescopes. 



deterioration, as may easily be imagined without further ex- 

 planation. To this cause, indeed, we must, I believe, trace 

 some of the deteriorations of specula which do not depend on 

 tarnish or corrosion, though I must also add, in concluding, 

 that the crystallized specula are more easily tarnished than 

 others, probably from the irregular action of moisture on them, 

 or, possibly, because the definite compound and its associate 

 thus intermixed, one or other, or both, offer less resistance to 

 the air than that mixture of tin and copper which constitutes 

 a good speculum. 



On the concealed Agency of Carbonic Acid in determining the 

 Decomposition of Water by the Contact of Iron. By Mar- 

 shall Hall, M.D. F.R.S.E. &c. &c. 



A' SERIES of experiments were made several years ago, nearly 

 simultaneously by M. Guibourt and myself, upon the question 

 of the decomposition of water by the contact of iron*. 

 Previously to this period, it was generally admitted, by the 

 chemists of this country at least, that iron possesses the 

 property of decomposing waterf . M. Thenard, indeed, held 

 the opposite view, that iron does not decompose water at or- 

 dinary temperatures, and founded upon it the definition of 

 the third section of his interesting classification of the metals. 

 This discrepancy of opinion was not removed by the two 

 series of experiments above alluded to, M. Guibourt having 

 concluded, that although iron does not decompose water when 

 it is in small proportion to the fluid, yet, that a decomposition 

 is effected when the relative quantity of the metal is great. 

 My own experiments, on the contrary, appeared to establish the 

 fact, that water does not undergo decomposition by the contact 

 of iron, when both are perfectly pure. 



The two Memoirs published by M. Guibourt and myself, 



♦Journal de Pharmacie, torn. iv. p. 241; Journal of the Royal Institution, vol. 

 vii. p. 55. 



f Davy's Elements, p. 385 ; Thomson's System, ed. 5th, vol. i. p. 368 j Henry'$ 

 Elements, ed, 7th, vol.i. p. 105 &c. 



