226 REPORT—1840. 
case, but on scraping the rust off the metal is found unequally 
removed to a greater or less depth beneath it, so as to leave a 
pitted uneven surface ; 5th, Tubercular, where the whole of the 
rust which has taken place at every point of the specimen has 
been transferred to one or more particular points of its surface, 
and has there formed large projecting tubercles*, leaving the 
rest bare. 
In one or other, or some combination of these forms, every 
sort of iron, cast and wrought, which I have noticed, corrodes 
when exposed to the action of air and water, by which is meant 
water holding air in combination, such as all water at common 
temperatures found in nature does. 
153. The 12th column in these tables contains the amount of 
water, if any, absorbed by the specimen of iron. It was con- 
ceived possible, from some known factst, that cast iron long 
under the pressure of water might from its porous crystalline 
grain absorb the fluid more or less like a sponge ; if so, it was 
necessary to know the amount of this as influencing the weigh- 
ings. Means were therefore taken to determine the point, and 
the tables show that in almost every case no absorption has 
occurred; where it has, the result is to be attributed without doubt 
to a minute “ blow-hole,”’ or cavity in the casting. 
It is certain, however, that under a sufficient pressure, cast 
iron may be caused to absorb water or other fluids, and experi- 
ments are in progress to determine the conditions of this ques- 
tion, which is not without interest and utility. The difficulty of 
obtaining cast iron impermeable to fluids is well known to the 
makers of hydraulic presses. 
154. The tables Nos. VI., VII. and VIII. reduce into a small 
compass the whole of the results of the preceding ones; their 
own headings sufficiently explain their particular objects; it 
is therefore only necessary here to make some general obser- 
vations on a few of the more striking results arrived at so 
far. 
155. On the average it will be seen that the metallic de- 
struction, or corrosion of cast iron, is a maximum in the clear 
sea water at the high temperature of 115° Fahr. (y), and that it 
is nearly as great in the foul sea water (8), while it is a mini- 
mum in the clear river water (¢). 
156. The temperature is not higher in the first case than iron 
in works of engineering, or in iron ships is likely te be exposed 
to in different parts of the world, as the following data in- 
dicate :— 
* Report, § 49 to 59. + Report, § 37 to 58. 
