Mb. J. Napieb on Incrustations in Steam Boilers. 193 



of lime is more soluble in a solution of common salt than it is in dis- 

 tilled water ; and so also is common salt in solution of other salts. The 

 quantity of sulphate of Hme which boiling sea water will dissolve is not, 

 so far as I am aware, ascertained ; but from a quantity of water drawn 

 from a boiler at Ailsa Craig, that had been fed with sea water, I found it 

 to contain 203 grains of sulphate of lime per gallon, nearly half an ounce. 

 Sulphate of lime is said not to be soluble in water at 300° Fah. ; but 

 whether this would be the case in water containing salt in solution, I 

 do not know. Bicarbonate of lime in solution in water suffers decom- 

 position ; as the water is heated to boiling, it loses an equivalent of car- 

 bonic acid, and passes into the state of carbonate, which, not being 

 soluble, falls as a precipitate. In boilers where water containing carbo- 

 nates exist, and that are allowed to stand over night, this precipitate 

 settles on the boiler, hai'dens, and forms a crust. Such crusts are 

 generally composed of thin layers, or laminae, each representing a day's 

 work. Bicarbonate of iron in water undergoes a similar decomposition 

 when the water is brought to boil. Carbonic acid is evolved, and car- 

 bonate of iron is precipitated, which is shortly after decomposed. The 

 iron is converted into a peroxide, and the remaining carbonic acid 

 liberated. This salt is generally in very small proportion to the lime, 

 and only imparts to the crust formed a brownish tint. I have seen, 

 however, a highly chalybeate water used in a boiler, which in a few 

 weeks, when blown off the boiler, was a red colour, and gave the engine- 

 man the impression that it was blood. A portion taken from the boiler, 

 and allowed to settle, deposited 190 grains per gallon, containing 130 

 grains peroxide of iron ; still no caking had taken place, nor ever took 

 place with these waters ; but care was taken to blow off the red sediment 

 from time to time, and regularly. 



Sulphate of lime in water has a different action — is not precipitated 

 but as the water is evaporated — after it has got its maximum quantity of 

 salt in solution — the sulphate of lime crystallizes upon the surface of the 

 boiler, and forms that hard, crystaUine cake, or scale, which adheres so 

 tenaciously to the boiler plates. This salt constitutes three-fourths of 

 the incrustation upon stationary or land boilers, and is the cake on all 

 marine boilers. This sort of crust cannot be avoided by care or mecha- 

 nical means, except by keeping the salt in the water under its crystalliz- 

 ing quantity, which would necessitate such an amount of blowing off 

 and supply as would render it expensive ; but with the carbonates, or 

 such salts that are precipitated, a little attention and blowing off at 

 particular times wUl prevent entirely the formation of a cake or crust 

 upon the boiler. The other salts held in solution in water are never 

 found as crusts upon boilers ; for example, I have never found a crust 

 Vol. IV.— No. 1. 2 c 



