1843.] on the effects of Sea-water on Iron. 1071 



and for the sheathing of boats and other small crafts, which can be 

 hauled up from time to time. I beg to observe, that these remarks do 

 not apply to fresh water buoys or boats. In fresh water, the tinned iron 

 suffers corrosion, notwithstanding the presence of the zinc. 



Cements and Varnishes. 



13th. The plates forwarded to me by your Board from Captain 

 Kinsman were immersed in Sea-water. In a week all began to soften, 

 and in 14 days corrosion of the iron was taking place through the pores 

 in the substance of the cement. I consider this method quite inappli- 

 cable from these and many other reasons. 



14th. Caoutchouc. — This I applied in various forms/ I find to my 

 surprise that by three weeks' immersion, it softens so as to be detached 

 by the slightest friction. 



15th. Asphaltum. — This invaluable substance completely protects 

 the iron if rubbed on while hot : it resists even strong nitric acid for 

 months. But the application requiring the heating of the iron, opposes 

 a great practical difficulty to its use. I find by mixing a small 

 quantity (about one-eighth its weight) of fine sifted road dust with 

 the asphaltum while melted, that it will adhere to the cold iron. It 

 should be laid on by a wire brush, as no hair, or coir, or yarn brush will 

 stand the heat. Practical men in the dock-yards can soon turn this fact 

 to account. 



16th. Coal Tar answers well as a retarder rather than a preven- 

 ter of corrosion, provided all the volatile oil of the tar is entirely ex- 

 pelled by heat or exposure to the air, before the immersion of the iron. 

 If this be neglected, the varnish soon washes away. 



17th. Coal Tar boiled with one-eighth asphaltum gives an excel- 

 lent protecting varnish, and one which can be applied to cold iron, and 

 which dries much more perfectly than the coal-tar alone. 



18th. Along with those above-mentioned, I have tried many others, 

 such as the Gab juice, white and red lead paint, &c. but none approach- 

 ed in efficacy the articles above cited. Asphaltum is procurable cheaply 

 from the Persian Gulf, whence it is occasionally imported into Cal- 

 cutta under the name of Zift-i- Roomie. Purchasers not knowing its 

 exact qualities, will have boiled pitch palmed on them instead, by the 

 bazar venders. A simple test is exposure to the sun for from five to 



7 B 



