The experiments of the eminent philosopher just mentioned furnish a variety of suggestions on the radiation from heated surfaces. He found that, while the radiating power of clean lead was only 19, it rose to 45 when tarnished by oxidation, that the radiating power of plumbago was 75, and that of red lead 80. He also discovered that, while the radiating power of gold, silver, and polished tin was only 12, that of paper was 98, and lamp black no less than 100. He further says: "A silver pot will emit scarcely half as much heat as one of porcelain. The addition of a flannel, though indeed a slow conductor, far from checking the dissipation of heat, has directly a contrary tendency, for it presents to the atmosphere a surface of much greater propulsive energy, which would require a thickness of no less than three folds to counterbalance."
It is safe to infer from this analogy that the felt covering of boilers should not only be of considerable thickness, but should be protected by an external jacketing of some sort; for, though felt is a good non-conductor, it is a powerful absorber and radiator, more especially when it has been allowed to contract soot and dust.
Various experiments have lead to the general conclusion that the power of absorption is always in the same proportion as the power of radiation. It must be so. Were any substance a powerful radiator and at the same time a bad absorber, it would necessarily radiate faster than it would absorb, and its reduction of temperature would continue without limit. It has, furthermore, been proved that the absorptive property of substances increases as their reflecting qualities diminish. Hence, the radiating power of a surface is inversely as its reflecting power. It is for this reason that the polished metallic sheathing on the cylinders of locomotive engines, and on the boilers of steam fire engines, is not only ornamental but essentially useful. Decisive tests have also established the fact that radiation is effected more or less by color. "A black porcelain tea pot," observes Dr. Lardner, "is the worst conceivable material for that vessel, for both its material and color are good radiators of heat, and the liquid contained in it cools with the greatest possible rapidity; a polished silver or brass tea urn is much better adapted to retain the heat of the water than one of a dull brown, such as is most commonly used."
A few facts like those above stated afford more decisive information regarding the nature of heat than columns of theory or speculation. Yet it is rather strange that when so many learned and reliable men have, experimented so much and commented with such persuasiveness upon the subtile agency of heat and the vast amount of waste that must accrue by injudicious management, comparatively few have availed themselves of the united labors of these indefatigable pyrologists; manufacturing owners and corporations still persisting in having their steam boilers painted black or dull red and leaving them exposed to the atmosphere. Some persons, who pass themselves off very satisfactorily as clever engineers, affect a contempt for the higher branches of science, and assert, in a very positive and self-sufficient manner that experiments made in a study or laboratory are on too trifling and small a scale to be practically relied upon; that a tin kettle or a saucepan is a very different thing to the boiler of a steam engine.
This may be so in one sense, but the same chemical forces which operate upon the one will be just as active in a proportionate degree in their action upon the other. It was said by Aristotle that the laws of the universe are best observed in the most insignificant objects; for the same physical causes which hold together the stupendous frame of the universe may be recognized even in a drop of rain. The same observation may be applied to the laws of heat in all their ramifications; for, after all, our experiments are, in many instances but defective copies of what is continually going on in the great workshop of nature.
It would be needless to insist on the wasteful and destructive effects produced by the exposure of boiler surfaces to the open atmosphere. Such a practice can be neither supported by experience nor justified by analogy; and it is to be hoped that it may before long be consigned to the limbo of antiquated absurdities and be satisfactorily forgotten. Seeing that it cannot with any show of reason be affirmed that the boiler covering materials in present use possess the requirements necessary to recommend them; the question arises as to what is the best means of achieving the object required. This is an inquiry which it is the office of time alone to answer. As the problem is obviously one of primary importance, and well worthy of the attention of inventors, it is hazarding nothing to predict its satisfactory solution at no distant date.
The plain truth is, boilers have of late become gigantic foes to human life. Explosions have increased, are increasing, and should be diminished; and they are, in many instances, caused by boilers being strained and weakened by sudden contraction from having their surfaces exposed when the fire has been withdrawn from them. Boilers are also materially injured by the excessive furnace heat which it is necessary to maintain to compensate for the large amount of caloric which is dissipated from their surfaces, not only by radiation but from absorption by the surrounding atmosphere.
As the views here laid down are drawn exclusively from the region of fact and experiment, it is to be hoped that an enlightened sense of self-interest may prompt those whom the subject may concern, to give it that special attention which its importance demands.