ON MARINE BOILERS. 183

and tubes were found to be eaten into., indented,or ‘ pitted.’ The indentations varied in dia-meter from the smallest speck to inch, andin depth from the merest impression to theentire thickness of the plates or tubes. Andalthough they were formed all over the boilers,they were most frequently found and were mostnumerous just over the fireplaces, and in thoseparts immediately in connection with thegreatest heat. In some of these parts thesurface was entirely covered with the indenta-tions; while in other parts as much as a squarefoot of plate, although subjected to the greatestheat, was free from them. The plates andtubes in all cases have been of the best ironand by good makers, and the ‘ pittings ’ occurin what looks like iron of good quality, with agood fibre, no slag or cinder being perceptible.So destructive was this pitting in boilers, usingthe same water over and over again, that inone instance the tubes of new boilers wereactually eaten through at the end of two orthree voyages, extending over only a fewmonths altogether, and it became necessary toput in new tubes, and to use a portion of saltwater for feed, to keep up an incrustation, sothat the boilers should not be acted upon. Ifthe iron of the boilers had been all of one make,it would naturally have been concluded that the‘ pitting ’ was due to the quality of the iron ; butas the iron of different boilers had been obtainedfrom different makers from time to time, thequality of the iron could not be blamed.

“ The presence in the boiler of a soft metal,such as copper from the condenser tubes, it wasconsidered would induce a galvanic action suchas might affect the iron in some way. But theanalysis which was made of the deposit scraped

from the boiler shows that there was scarcelya trace of any foreign metal there. Indeed itmight have been concluded that a soft metalcould not be present, for the tubes of thecondenser and the copper pipes were all in aperfect condition. Even at the joints, madetight by india-rubber, hardened by vnlcanis-ing, there was scarcely a speck of corrosion.

“A search was then made to ascertainwhether the gluey deposit was present thatarises from the decomposition of the tallow andoil used for lubrication, as Mr. Jack hadfrequently heard that such a deposit took placein boilers where Hall’s surface condensers wereused. For the purpose of ascertaining this, themud cocks of a vessel were not opened forsome time before arriving in port; and the fireswere then put out on arrival and the mud dis-charged, when the only substance found wasthe watery brownish deposit before referred to.The deposit remaining in the bottom of tbeboiler was carefully examined, but here againthere was only the same deposit. As it wasbelieved that the lubricating material carriedinto the boilers with the feed might, by con-tinued subjection to heat, form an acid capableof producing the effects observed, the kind oflubricating material employed was noticed, inorder to ascertain whether animal or vegetableoils acted most injuriously; but it was foundthat the action went on as much with the one oilas with the other. In case however a fat acid,formed as already mentioned, might be thecause, pieces of chalk were put into the boilers,and from time to time fresh pieces were added ;carbonate of soda was also mixed with the feedwater in regular doses ; but all to no purpose;the action went on getting worse and worse.