ACTIOX OF WATKR ON IRON. 305 



it oscillated about 2° ; but when one half of each of the two 

 original pieces, i e. the black and the gray, were immersed, the 

 needles deviated at once from 78° to 80°, and on adding a sino-le 

 drop of hydrochloric acid, flew round. 



13 1 . This action in common water or sea water always showed 

 the softer iron positive to the other, and continued constant and 

 unchanged in an experinaent continued for some days. 



132. When two pieces of cast iron, such as the above, were 

 immersed in dilute hydrochloric acid, tied together and in con- 

 tact, the harder one remained quite bright and untouched be- 

 fore and after the acid was saturated, while the softer was rapidly 

 blackened and dissolved ; neither was any gas evolved from the 

 negative piece, unless the acid was concentrated. 



133. The subject is not in a state to do more now than state 

 the principle in view, and that so far as experiments have yet 

 gone, it is likely to add a new if not a better mode of protecting 

 iron to those already known ; if successful, its application to 

 engineering structures will afford many facilities of execution, 

 and be attended with much greater economy than any process 

 in which zinc is used possibly can. 



134. It already suggests to the engineer the importance of 

 preserving uniformity of texture and of chemical composition 

 in all parts of his structures of iron, in order that one part may 

 not accelerate the destruction of the other. It also shows the 

 necessity, when wrought iron (which is negative to all but chilled 

 cast iron) is applied in contact with cast iron, of allowing extra 

 substance in the latter to meet the increased corrosion produced 

 by the wrought iron at the points of contact, and it explains 

 why the vital injury is so often sustained in works in cast iron 

 of this metal being first eaten away round the bolt holes, as for 

 instance, in the air-pumps and condensers of marine engines, 

 where the parts of the work are secured together ; and it further 

 suggests, that where the sides of these bolt holes in the cast 

 iron can be chilled or cast on an iron core, as is often practica- 

 ble in such cases, while the bolt will slightly suffer, the cast 

 iron round it will be preserved. On a future occasion I hope 

 to lay the results of this branch of the investigation before the 

 Association. 



135. M. Payen's observations as to the power of alkaline 

 waters to protect iron from rust, though seldom applicable, are 

 worthy of further investigation, and an'attempt to discover their 

 rationale. It has very long been known that lime in powder, or 

 limewater, possesses a decided power of this sort, and both are 

 in use amongst workmen. 



Cases may be found in practice where solutions of an alkali 



