De. Wallace on the Incrustation of Marine Boilers. 319 



the hydrate. The sulphate of lime appears to occur as the hydrate 

 described by the late Professor Johnson as having been found by him 

 in a distinctly crystallized condition in a high-pressure steam-boiler, its 

 composition being represented by the formula 2 (CaO, SO3) -j- HO. 



The crust formed in steam-boilers was formerly supposed to consist 

 essentially of carbonate of lime, and the substances proposed as remedies 

 had for then.' object the prevention of the precipitation of this com- 

 pound. The distinguished German analyst, Fresenius, was, I beUeve, 

 the first to correct this erroneous notion. He found, experimentally, 

 that waters containing no sulphate of lime, although highly charged 

 with carbonate, gave no crust, but only a loose, incoherent precipitate, 

 which made the water muddy ; and he found that the various boiler- 

 crusts which he had an opportunity of examining consisted of carbonate 

 of Hme and other matters, hound together by sulphate of lime. The 

 result of Mr. James Napier's analyses, and those given in this paper, 

 amply confirm this statement, so far as sea-water is concerned. The 

 crusts formed by river-water vary much in composition ; but, so far as 

 my experience goes, they all contain sulphate of lime, and their degree 

 of hardness appears to depend on the proportion of that salt which 

 they contain. 



The crust-forming ingredient of water being sulphate of lime, the 

 preventive substance must be a body that will either decompose it or 

 prevent its adhesion to the boiler. We have thus mechanical and 

 chemical preventives. Among the former the following substances 

 have been recommended, and used with greater or less success : — 

 Potatoes, tallow and other fats and oils, sawdust, clay, oak bark and 

 spent tar, oak wood, treacle and molasses. 



There is, practically, only one chemical preventive — carbonate of soda. 

 Carbonate of potash has the same action, but is far too expensive. 

 Under the influence of carbonate of soda the sulphate of lime is con- 

 verted into the still more insoluble carbonate ; but this compound is 

 not capable of attaching itself to the boiler-plates, but remains sus- 

 pended in the water. Some preparations for preventing incrustation 

 are said to dissolve oflF a crust already formed ; but these are mere 

 deceptions. All the mixtures I have examined consist essentially 

 of an alkaline carbonate ; and I have found that a boiler-crust, such 

 as No. 2, may be boiled with a strong solution of carbonate of soda, 

 without suffering the slightest diminution of weight. There can be 

 no doubt, as regards land boilers using river-water, that incrustation 

 can be easily and completely prevented, and that without risk of corro- 

 sion, priming, or other inconvenience, by the use of carbonate of soda ; 

 and this may be done at a very trifling cost. When, however, we have 



