CHEMISTRY. 197 



affoctod when 10 litres of the same air, without the interposition 

 of the manganese tube, were passed over it. 



"But the action of heat furnishes the most unequivocal proof of 

 the identity of the body in the atmosphere with ozone. In a former 

 communication ('Philosophical Transactions ' for 1856, p. 12), I 

 showed that ozone, whether obtained by electrolysis or by the action 

 of the electrical brush upon oxygen, is quickly destroyed at the tem- 

 perature of 237° C. An apparatus was fitted up, by means of wliich 

 a stream of atmospheric air could be heated to 2G0° C. in a glob- 

 ular glass vessel of the capacity of 5 litres. On leaving this ves- 

 sel, the air was passed through a U-tube, 1 metre in length, whose 

 sides were moistened internally with water, while the tube itself 

 was cooled by being immersed in a vessel of cold water. On 

 passing atmospheric air in a favoral)le state through this appa- 

 ratus, at the rate of 3 litres per minute, the test-paper was dis- 

 tinctly tinged in 2 or 3 minutes, provided no heat was applied to 

 the glass globe. But when the temperature of the air, as it passed 

 through the globe, was maintained at 260° C, not the slightest 

 action occurred upon the test-paper, however long the current 

 continued to pass. Similar experiments, with an artificial atmos- 

 phere of ozone, — that is, with the air of a large chamber contain- 

 ing a small quantit}'' of electrolytic ozone, — gave precisely the same 

 results. On the other hand, when small quantities of chlorine or 

 nitric-acid vapor, largely diluted with air, were drawn through the 

 same apparatus, the test-paper was equally affected, whether the 

 glass globe was heated or not. 



" From these experiments I consider myself justified in conclud- 

 ing that the body in the atmosphere, which decomposes iodide of 

 potassium, is identical with ozone." — Proceedings of the Royal 

 Society. 



Ozone is a powerful disinfectant, and, when generated on the 

 large and cheap scale possible by Wilde's electro-magnetic ma- 

 chine, may be of very great service in hospitals and other build- 

 ings where lai'ge collections of people vitiate the air. 



CORROSION OF CAST IRON. 



It has been often stated that cast iron, when exposed to the 

 action of sea-water or to atmospheric influences, under certain 

 conditions, becomes "rotten;" an expression which is intended 

 to indicate a loss of strength or cohesion without a corresponding 

 alteration of volume or size. This phenomenon is entirely differ- 

 ent from common oxidation, or rusting, which latter process shows 

 itself by attacking the surface, and gradually reducing the size of 

 the article, which, so far as it remains intact by this external re- 

 duction, does not seem to lose its qualities, so that the reduced 

 strength of a rusted bar is simply proportionate to the reduction 

 of its original section. The state of corrosion which would justily 

 the term "rotten" is a reduction of cohesion without any appar- 

 ent removal of material, and is not easily recognized externally. 

 The nature of this change has for a long time remained uuex- 



17* 



