PART I.
“To illustrate the practical importance of knowing the effects of reiterated heating and cooling on iron plates, one of the most obvious examples is the action of heat upon the plates of boilers which are alternately heated and cooled, as in use or otherwise. When in use, the plates above the fire are subjected to the fierce flame of the furnace on one side, and on the other side to a temperature approximating to that of the steam and water in the boiler. Where the conducting surfaces of the metal are thickened at the riveted seams, a source of danger is frequently revealed in the appearance of what are known as ‘seam-rips.’
“The long egg-ended boilers, much used in the North of England, are very subject to this breaking away of the seams. From some tests made by the writer on iron cut from the plates of two different boilers which had ripped at the seams, and one of which seam-rips had led to an explosion resulting in the destruction of much property, though happily of no lives, it was found that the heat acting on the bottom of the boiler had, through time, so affected the iron at the seam as to make it brittle, apparently crystalline in fracture, and of small tensile strength. Farther from the seam the iron appeared in both cases less injuriously affected. But although the alternate heating and cooling of the plates over a long period had produced this change in the molecular condition of the iron, a method of restoration presents itself in the process of annealing. In subjecting the pieces cut from the seam-rips to a dull red heat, and then allowing them to cool slowly in sawdust, the writer found that the fibrous character of the iron appeared again, and renewed testing showed that the ductility and tensile strength were restored.
“The same process of annealing is equally effectual in restoring the tenacity of iron in chains rendered brittle, and apparently crystalline, by long use, and is periodically applied where safety depends upon material in this form. Thus the heating and cooling of iron may be looked upon as the bane or the antidote according to the conditions under which the process is carried out. This affords an example of the importance of the physical effects produced by repeated changes of temperature. The change effected by one heating and cooling is so small that a cumulative method of experiment is the only one by which an observable result can be obtained, and this is the method adopted by the writer in the investigation now to be described.
“It is well known that if a wrought-iron bar be heated to redness, a certain expansion takes place, which is most distinctly observed in the direction of its length. It is also known, although not generally so, that if a bar be thus heated and then suddenly cooled in water, a contraction in length takes place, the amount of this contraction exceeding that of the previous expansion, insomuch that the bar when cooled is permanently shorter than it originally was. If this process of heating and cooling be repeated, a further amount of contraction is found to follow for many successive operations.
“Experiments Nos. 1 and 2 were made to verify this, and to show the increment of contraction after each operation.
“EXPERIMENTS ON WROUGHT-IRON BARS 11⁄8 IN. SQUARE
BY 30.05 IN. LONG, HEATED TO A DULL RED,
THEN COOLED SUDDENLY IN WATER.
| Experiment No. 1. | Experiment No. 2. | |||||
| Common Iron. | Best Iron. | |||||
| Contraction. | Percentage on original length. | Contraction. | Percentage on original length. | |||
| Inches. | Inches. | |||||
| After | 1st | cooling | .04 | .13 | .04 | .13 |
| „ | 2nd | „ | .10 | .33 | .10 | .33 |
| „ | 3rd | „ | .16 | .53 | .14 | .46 |
| „ | 4th | „ | .17 | .56 | .16 | .53 |
| „ | 5th | „ | .23 | .76 | .20 | .66 |
| „ | 6th | „ | .28 | .93 | .24 | .80 |
| „ | 7th | „ | .31 | 1.03 | .27 | .89 |
| „ | 8th | „ | .33 | 1.10 | .30 | 1.00 |
| „ | 9th | „ | .40 | 1.33 | .33 | 1.10 |
| „ | 10th | „ | .47 | 1.56 | .39 | 1.30 |
| „ | 11th | „ | .52 | 1.73 | .42 | 1.40 |
| „ | 12th | „ | .54 | 1.80 | .47 | 1.56 |
| „ | 13th | „ | .58 | 1.93 | .51 | 1.70 |
| „ | 14th | „ | .62 | 2.06 | .54 | 1.80 |
| „ | 15th | „ | .68 | 2.26 | .56 | 1.86 |
“The Table of Experiment No. 5 shows that at the twenty-fifth cooling a contraction of 3.05 per cent. had taken place, or an average of .122 per cent. after each cooling. This is almost identically the same average result as shown in Experiment No. 1 with straight bars.
“The above experiments only having reference to the permanent contraction of the iron in the direction of its length, the author made the following experiments to ascertain the effect in the other dimensions, and to see whether the specific gravity of the iron was affected in the reduction of dimensions.