118 AXXUAL OF SCIENTIFIC DISCOVERY. 



which affect the magnetism of one pole of a magnet equally affect 

 that of the other pole. 



SACEIFICIAL METALS. 



It is a fact perfectly well known to chemists, but one which mechan- 

 icians have not heeded nearly so much as it deserves, that when two 

 metals are retained in contact, and conjointly exposed to chemical 

 influences during long periods of time, one metal sacrifices itself to 

 ensure the other's preservation. The history of ships' copper sheathing 

 shall be taken as our first illustration of the sacrificial metallic function. 

 Sir Humphrey Davy, as is well known, devised a method of checking 

 or altogether obviating the destruction of ships' copper sheathing. He 

 accomplished this by attaching to the ship's l3ottoni, at suitable inter- 

 vals, slabs of the metal zinc, and which he called protectors. Pro- 

 tectors they were, in the following manner: The zinc rapidly 

 corroded, and was lost ; but, so long as any zinc remained for the sea- 

 water to act upon, the copper remained untouched. So far as 

 copper preservation is concerned, the method adopted must be pro- 

 nounced efficient. Practically, it failed, indeed, to secure the advan- 

 tages described, but not for any reason that concerns us here. It 

 was found that when copper no longer slowly dissolved, it ceased to 

 be a poisonous metal. Barnacles and seaweed attached themselves, 

 just as they might have done to an uncovered wooden bottom ; and, 

 moreover, owing to a galvanic action set up, the ship's copper was 

 rapidly fouled by a deposition of magnesia and lime, precipitated from 

 the niagnesian and calcareous soluble salts always present in sea-wa- 

 ter. Failing, then, to achieve what was intended by it, the copper- 

 protecting process of Davy ceased to be employed ; but the failure of 

 it is that which alone concerns us here, as illustrating what we would 

 wish to convey by the words " sacrificial metal." 



Take, as the next example, the illustration afforded by the setting 

 of an iron paling-rail in a bed of lead. The most casual observation 

 as one passes along the streets of a city may be made more preg- 

 nant with facts bearing upon this matter than the longest homily. 

 For a time, varying as to length with locality, external influences, 

 and perhaps, in some degree, on the varying quality of the iron and 

 lead brought into contact, both metals remain sound. But after a 

 time decay inevitably sets in, and, when once commenced, marches to 

 the issue of final destruction with wonderful rapidity. The remark- 

 able fact is that both metals do not decay simultaneously ; it is the 

 iron which corrodes, whilst lead the softer metal remains intact. 

 Let the mechanician do what he will, this result cannot be prevented ; 

 a law of nature having decreed the sacrifice, it must and will be 

 achieved. 



Our next illustration shall be ordinary tin plate iron coated 

 with tin, as is well known. Well, what sort of destruction is set up 

 when tin plate has been exposed for a long duration of time to the 

 atmosphere ? Is the tin destroyed ? does it tarnish, even ? Never. 

 Iron is the sacrificial metal here ; and as surely as the iron is any- 

 where exposed, it perishes, crumbles, and dissolves away, with de- 

 structive rapidity. The rate of destruction of iron totally unprotected 



