- . PROCEEDINGS OF THE POLYTECHNIC ASSOCIATION. 863 



severs the frail post into fragments, which now settle to the bot- 

 tom. The deposit of another day falls upon and among this ruin, 

 another breaking up from like cause occurs, the process continu- 

 ing until the blow-oiF cock is opened, say once a week, after the 

 fires are drawn, with full pressure; when the whole deposit will 

 be ejected, leaving the globes always entirely clear and clean. 



I have seen specimens of this deposit from water holding lime, 

 silica, iron and salt, one piece nearly as large as a hen's egg. It 

 was composed of fragments of scale about one-quarter inch square 

 and one-sixteenth inch thick, all concreted like volcanic breccia. 

 In this case the water was very bad, and the attendant had neg- 

 lected to blow off on the regular day. 



With a cylinder boiler such a breaking up can rarely if ever 

 occur. The scale in that case finds a medium of adhesion or 

 cementation in the protoxidized surface of the plate, to which it 

 thereby became a fixed enamel. The increased diameter of the 

 boiler, the enlarged surface of the scale, with its greater adhesion 

 to the plates, all aid to keep it in situ. When the plates expand 

 from heat, there being a slight elasticity in the scale, permitting 

 it to yield with the bending iron; it holds fast to its seat, increas- 

 ing in thickness until dissolved out, or removed by hand, an 

 inconvenient if not a pernicions process, as doubtless the plates 

 of many boilers have been crystalized by the hammering neces- 

 sary to remove scale. 



Durability. 



Mr. Colburn further says: "I can but call attention to that 

 great source of boiler casualties, corrosion, which has been com- 

 pared to that fearfully fatal disease, consumption in human life, 

 except that in the boiler it is external as well as internal, and the 

 better or purer the iron the more rapid the corrosion; whereas, 

 iron containing carbon, and particularly silica, is less destructible 

 under like circumstances." ' 



And he further says: " Since superheated steam began to be 

 generally employed, much difficulty has been experienced from 

 rapid corrosion of the superheaters, until Messrs. Richardson of 

 Hartepool adopted cast iron, and this material shows no corrosion 

 at all after four years' use. Cast iron bridges are indestructible 

 by rusting, while large quantities of rust scales are being removed 

 annually from all wrought-iron bridges, the Conway and the 

 Britania Tubulars in particular." 



