12 REPORT—1843. 
$20. Since the publication of my last Report, a fact long doubted has been 
ascertained by myself and by others, namely, that water, when in the state of 
steam and under considerable pressure, is slowly decomposed by a surface of 
iron at temperatures far below visible ignition, even as low as about the 
melting point of lead, producing (as in the well-known case at the tempera- 
ture of ignition) the magnetic oxide of iron. This has been proved not only in 
Perkins’s closed tubes for heating buildings by hot water, or rather steam, but 
in a high pressure steam-boiler working at 65lbs. per square inch by myself. 
This decomposition appears always to go on in steam-boilers where the 
inner coating of deposit or sediment causes a plate to overheat, as in such 
cases oxide of iron is found lining the interior of the boiler; at that spot the 
deposited salts are probably decomposed here also in part. 
321. No mode of coating with zinc appears capable of preserving iron from 
the action of boiling salt water; on the contrary, the zinc oxidates with un- 
usual rapidity and the iron is not preserved. 
322. There is a great difficulty in making any experiments of practical 
-value or accuracy upon the questions proposed (2nd Rep. 166, &e.)’as to the 
temperature of the boiling sea water in marine iron steam-boilers, or what is 
the same thing, the degree of saline concentration at which the maximum 
corrosion takes place. From various circumstances attending the working 
of marine boilers, the waste of fuel appears to increase rapidly with the con- 
centration of the water beyond a certain point. 

A. B. Cc; D. 
Chloride of sodium ..... 2°50 | 16°00 | 25°50 | 20°80 
Chloride of magnesium .. . 0°35 0°46 1:07 4°85 
Sulphate of magnesia .... 058 | O80} 148] 9:50 
Carbon, lime and magnesia . 0°02 0:00 0:00 0:00 
Sulphate of lime. ...... 001 0°30 0°00 0°00 
Water. ..... cee eee se | 96°54 | 79°79 | 6914 | 64°85 
Sulphate of soda....... 0:00 2°65 2°81 0:00 




——$————— a 
100°00 | 100°00 |} 100-00 | 100°00 
The composition of sea water being on the average represented by the 
column A, specific gravity = 1:0278. When the water in the boiler has 
been concentrated to the specific gravity 1°140, its composition is shown in 
column B ; and when it has arrived at the density 1°220, it has the compo- 
sition in column C ; finally, when the greatest part of the common salt has 
deposited, the supernatant fluid has the composition in column D. Such are 
the accurate Berthier’s results. 
I believe the greatest amount of corrosion goes on in iron boilers (irrespect- 
ive of injury done by deposits) after sea salt has begun to deposit freely, 
when the boiling temperature is about 232° Fahr., combined air not being 
present in the water, and hence, as far as corrosion is concerned, the object 
of the engineer is to work at as low a point of concentration as possible, which 
comports well with all the other contingencies of the case. It would be de- 
sirable that the feed-water of marine boilers were heated to above 190° Fahr. 
before entering them, and means provided for the escape of the air disen- 
gaged, which now enters the boilers and aids much in corrosion. This could 
easily be done by Maudsley and Field’s beautiful arrangement of their feed 
and brine pumps. On this branch of the subject, however, I hope hereafter 
to present further and more complete results. 
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