864 TRANSACTIONS OF THE AMERICAN INSTITUTE. 



Oxygen in water, as well as that of acids held in water, cora- 

 biniiio- with wrought-iron forms a protoxide which is porous and 

 absorbing, admitting water through it to constantly act on the 

 iron until it is wholly destroyed, whereas the same action upon 

 cast-iron forms a peroxide or a different combination with oxygen, 

 the scale or film of which is impervious to an addition of oxygen; 

 hence it becomes a protection to the metal against further action. 

 Cast-iron cooking utensils of many years' use attest this fact. 



I- have seen an ordinary cast-iron tea-kettle for family use, which 

 presented no visible depreciation for efficiency after an uninter- 

 rupted use of thirty-seven years. 



The officers of the Manchester (England) Boiler Association 

 reported in 1862, that with an average of 1,400 boilers under 

 their care, 83 were positively dangerous, 37 dangerous, 987 with 

 objectionable defects, and 270 objectionable from corrosion alone. 



Safety from Destructive Explosion. 



In the three systems of steam boilers in common use we have 

 first the horizontal, with and without return flues, second the 

 horizontal tubular, and third the vertical, which usually is. also 

 tubular. 



In each case increased power necessitates increased diameters 

 or length of boiler, or both, and this in turn requires a proportion- 

 ate increase in the requisite quantity of water the boiler is to 

 hold, which involves as, compared with a boiler of less dimensions, 

 not only greater liability to rupture, but a destructive power of 

 still greater ratio. 



Mr. Colburn states, " The danger attending the presence of a 

 large quantity of heated water in the boiler is now well understood, 

 the boiler is weakened in proportion to its increase of diameter, the 

 bursting pressure to a given thickness of plates, being inversely 

 as the square of the diameter, so that in a boiler of given length 

 the elements of weakness and destruction are collectively relative 

 to that of the diameter." 



It is now well known that the water in a steam boiler is charged 

 with steam, the pressure of which is equal to that of the steam 

 above it, and of a volume equal to the bulk of water. 



It is from this cause that, in the rupture of an ordinary steam 

 boiler, a volume of steam equal to the whole contents of the boiler 

 is instantly liberated, and with it the water, slightly, if at all, 

 obstructed in its wave, is projected with such velocity and force 



