1834.] On the Preservation of Sheet Iron from Rust. 272 



when the water of the canal was only slightly salt. 



They were then taken up for examination, and presented the 

 following appearances. 



No. 



1 



2 

 3 



Varnish. 



10 



Tur, 



Theetseey 



Dhoontty 



Painlj 



Copal varnishy 



Spirit varnish 

 Wax, 



Lime, 



Zinc, 



None, 



Plates under water. 



Perfectly preserved and 

 free from rust. 



Perfectly Qninjured in ap- 

 pearance. 



White and pulverulent;! 

 soft and easily rubbed off 

 while wet ; rust here and 

 there. 



Almost^wholly disappear- 

 ed, and blotches of rust on 

 the surface. 



Whitened, pulverulent, 

 and soft; but not much ox- 

 idated. 



Whitened and very rusty. 



No trace of wax left, and 

 very rusty. 



Flaky ; peeled off, and 

 very much corroded. 



The clean iron excessively 

 corroded and bad : the zinc 

 also oxidated. 



The natural surface was a 

 little whitened and pretty 

 well preserved. 



Plates half above water. 



A few dots of rust betweea 

 wind and water. 



A line of rust at the level 

 of the water. 



Large cracks from the con- 

 traction of the part exposed 

 to the sun, whitened where 

 thick, black vt^here thin ; 

 plate preserved, above water. 



Paint uninjured above wa- 

 ter mark, and plate preserv- 

 ed, but below water entire- 

 ly removed. 



In air less, whitened spots 

 of rust breaking out every 

 where. 



Very much corroded. 



This plate was all under 

 water. 



In air re'lnains on and acts 

 pretty well. 



Much more rusty in the 

 air than under water, where 

 a kind of crust was formed. 



Rusty on the edges or where 

 it had been scraped; else- 

 where little injured 



The superior preservative pov/er of the coal-tar to all the substan- 

 ces tried, with the exception perhaps of the theetsee, was evident ; 

 the Burmese varnish laboured under the disadvantage of being a 

 single coat, otherwise it would doubtless, from its hardness, its firm 

 adherence, and its inalterability by water, prove fully equal as a lac- 

 quer to the coal-tar ; the latter hys on the other hand the advan- 

 tage of drying and hardening as soon as laid on. 



The change effected on the resinous varnishes is produced by an 

 actual chemical combination with the water ; the soft pulverulent 

 matter is analogous to the white powder obtained by the addition 

 of water to an alcoholic or of acid solution of rosin. 



The failure of the zinc guard, which was expected to act as an 

 electro-positive protector to the iron, may I think, be attributed to 

 its being adulterated with lead, which being negative with respect to 

 iron, would cause, as was actually the case, a more ra|>id oxidation 

 of the latter metal ; (the impurity of the zinc was afterwards fully 



proved.) 



The wax and the white paint had entirely disappeared fiom thd 



