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LIV. Water as a Catalyst. By R. E. Hughes, B.A., B.Sc, 

 F.C.S., late Scholar of Jesus College, Oxford, Natural 

 Science Master, Eastbourne College *. 



SOME further chemical changes have been investigated, 

 and the influence of the absence or presence of water on 

 the progress of the change has been determined. Last year 

 the author showed (Phil. Mag. xxxiii. p. 471) that dried 

 hydrogen-sulphide gas has no action on the dried salts of 

 lead, cadmium, arsenic, &c; and, in conjunction with Mr. F. 

 Wilson, it was shown that dried hydrogen-chloride gas is 

 without action on calcium or barium carbonates (Phil. Mag. 

 xxxiv. p. 117). 



Silver chloride prepared in the dark, dried perfectly in an 

 air-bath, and then placed on a watch-glass in a desiccator 

 partially exhausted, was found to be not perceptibly darkened 

 in sunlight even after an exposure of some hours ; whereas a 

 rapid darkening takes place if moisture is introduced. 



It is a well-known fact that paper, especially ordinary glazed 

 writing-paper, when moistened with a solution of potassium 

 iodide and exposed to the light, becomes of a brownish-violet 

 tint — due doubtless to the decomposition of the KI and libe- 

 ration of the iodine. 



The author finds that the progress of this change is subject 

 to several conditions. A solution of potassium iodide placed 

 on glass or porcelain becomes brown only after an exposure 

 of some days. This change, it is suggested, is due to either 

 the organic matter or the ozone in the atmosphere. 



A piece of ordinary filter-paper soaked with a saturated 

 solution of KI was dried in the dark. When placed on a 

 watch-glass under a desiccator, no change took place on ex- 

 posure even after some days ; although crystals of the salt 

 were formed on the paper. Moreover, generally speaking, 

 the wetter the paper the deeper was the tint produced. In 

 fact the tint was proportional to the quantity of water present. 

 When no water was present, then no change took place. 

 Further, a strip of filter-paper floated on the top of a solution 

 of KI for quite two hours in bright sunlight before any per- 

 ceptible darkening occurred, although the underside had 

 assumed a deep brown tint. 



But this change also depends on the kind of paper used ; 



* Communicated by the Author. [As some of Mr. Hughes' results 

 have been anticipated by Mr. Baker (Proc. Chem. Soc. May 4, 189o, 

 p. 129), he wishes it to be stated that the MS. was received by us on 

 April 21st.— Eds.] 



