192 



Experiments on the Preservation. 



[April, 



They were then taken up for examination, and presented the following 

 appearances. 



No. 



Varnish. 



Plates under water. 



Plates half above water. 



1 



Tar 



Perfectly preserved and free 

 from rust. 



A few dots of rust between 







wind and water. 



2 



Theetsee, .... 



Perfectly uninjured in ap- 



A line of rust at the level 







pearance. 



of the water. 



3 



Dhoona, .... 



White and pulverulent ; 



Large cracks from the con- 







soft and easily rubbed off while 



traction of the part exposed 







wet : rust here and there. 



to the sun, whitened where 

 thick, black where thin ; 

 plate preserved, above water. 



4 





Almost wholly disappeared, 



Paint uninjured above water 







and blotches of rust on the 



mark, and plate preserved, but 







surface. 



below water entirely removed. 



5 



Copal varnish, 



Whitened, pulverulent, and 



In air less, whitened spots 







soft ; but not much oxidated. 



of rust breaking out every 

 where. 



6 



Spiritvarnish, 



Whitened and very rusty. 



Very much corroded. 



7 





No trace of wax left, and 

 very rusty. 



This plate was all under water. 



8 





Flaky ; peeled off, and very 



In air remains on and acts 







much corroded. 



pretty well. 



9 





The clean iron excessively 



Much more rusty in the 







corroded and bad : the zinc 



lir than under water, where a 







also oxidated. 



kind of crust was formed. 



10 





The natural surface was a 



Rusty on the edges or where 







little whitened and pretty well 



it had been scraped ; else- 







preserved. 



where little injured. 



The superior preservative power of the coal-tar to all the substances 

 tried, with the exception perhaps of the theetsee, was evident ; the Bur- 

 mese 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 lacquer to the coal-tar : 

 the latter has on the other hand the advantage of drying and hardening 

 as soon as laid on. 



The change effected on the resinous varnishes is produced by an ac- 

 tual 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 elec- 

 tro-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 rapid oxidation of the latter 

 metal : (the impurity of the zinc was afterwards fully proved.) 



The wax and the white paint had entirely disappeared from the sur- 

 face of the metal under water before the plates were taken up ; it is im- 

 possible therefore to say in what way their removal was effected. 



The bituminous (coal-tar) coating was finally adopted, and it has been 

 successfully applied to the iron steamer, the Lord William Bentinck, 

 lately launched under Captain Johnston's superintendence. 



