188 EFFECT OF SURFACE CONDENSATION ON MARINE BOILERS.

by the corroded parts. It is found in practice,as already alluded to, that the surfaceof the plate presents minute hollows, ofdifferent shapes, and in some cases separatefrom each other, therefore, correctly denom-inated “ pitting.” Now while observing thatthis pitting is far from what might havebeen supposed to be the result of galvanicaction, viz., a gradual wearing or thinning ofthe plate throughout, it must be rememberedthat the material in question is not as homo-geneous in its property as the copper, and thisdoubtless is one of the causes for the unevencorrosion. Again, for example : the propertieswith which the water is impregnated may notcombine under a given temperature, formingthereby a separation of metallic matter in thefluid, and thus another cause added to thatalready alluded to.

Apart from the metallic agent, another hasbeen proved to be equally destructive—thelubricant used in the cylinders. Not only is itcorrosive, but it also forms a sediment both inthe condenser and boiler, and therefore also achemical action is doubtless the result ofcontact; which has been proved, by the greaseforming an acid of great consuming power onmetallic surfaces.

It will thus be seen that, if the marineengineer gained one desired effect by theintroduction of surface condensers, he intro-duced others of an evil nature, almost sufficientfor him to regret the production of his powersof thought. It may be stated that the corrosionof the boiler plates has ensued without the feedwater coming in contact with copper or brasssurfaces ; this is accounted for by the fact thatthe water contained some mineral property,

and thus a galvanic action ensued. In otherinstances, the oil or tallow used in the cylindercaused a chemical action, and thereby the sameresult as to corrosion.

Thus far having treated of the baneful effectsof surface condensation on marine boilers, atten-tion must now be given to the best means yetpractised to counteract or prevent the same.

It will be noticed that entire sea water as feedwater produces certain incrustation ; it has alsobeen stated that distilled water obviates thesame. Now the latter destroys or corrodes thesurface of the plates in actual contact with it.It is, therefore, obvious that, to prevent cor-rosion by galvanic or chemical action, an inter-ception must be formed between the fluid andthe plate, and be a relative non-conductor atthe same time. Here also is the fact thata sediment is requisite, but the main questionis; by what means, and the required thick-ness to admit the penetration of the caloricfrom the furnace? Practice has, however,some time ago settled this matter, of whichwe have had some experience, the solutionof the problem being thus effected. Thesteam when in the cylinder has not beenpermitted to be impregnated with any acid—therefore the use of tallow or grease for internallubrication has been abolished. Also, it hasbeen proved, the incrustation formed by entiresea water is not injurious to the boiler, bychemical or galvanic action, and the lodgmentof sediment of the required thinness is therebyattained — by the introduction of a smallquantity of sea water in the distilled-watertank, before feeding the boiler—and thusa slight incrustation is formed, not in-jurious as a non-conductor of heat, but