At the regular weekly meeting of the Polytechnic Association of the American Institute, held on Thursday evening, the 25th ult., the subject of boiler clothing was discussed at some length, but without any decisive conclusion being arrived at respecting the most serviceable and economical material for that purpose. It appeared from the testimony adduced, that though there is a variety of substances in use, even those which are practically acknowledged as being the most efficient are far from coming up to the required standard of utility, and are characterized by defects which are at once forced upon us by a little close examination. Felt is an admirable non-conductor of heat, but owing to its combustible nature it is quite unreliable when subject to the heat of a high pressure of steam. A large fragment of this material which had been taken off the boiler of a North River steamboat was exhibited at the meeting, scorched and charred as if it had been exposed to the direct action of fire. For these reasons felt covering is, generally speaking, confined to boilers in which a comparatively low pressure of steam is maintained. But even under the most favorable circumstances of actual wear its durability is limited to a short period.

Powdered charcoal possesses the elements of efficiency as a non-conductor in an eminent degree; but its susceptibility of taking fire militates strongly against its adoption as a boiler covering.

Besides the materials above mentioned, there are some which come under the denomination of cements; but the use of such is somewhat at variance with what a dull world would call "facts." Employing them as a clothing for a vessel in which it is necessary to retain heat is certainly the wrong way of doing a light thing, if the evidence of distinguished experimenters be worth anything.

The researches of most well-informed physical philosophers go to prove that the conducting properties of bodies are augmented by cohesion, and that heat is conveyed profusely and energetically through all solid and ponderable substances. Thus gold, silver, and others of the most solid metals are the best conductors. Next to the pure metals in conducting powers are rocks, flints, porcelain, earthenware, and the denser liquids as the solutions of the acids and alkalies. As a further evidence to prove that the passage of heat through all substances is increased by cohesion, even some of those which are known to be among the best conductors are deprived of this property by a division or disintegration of their particles. Pure silica in the state of hard, rock crystal is a better conductor than bismuth or lead; but if the rock crystal be pulverized, the diffusion of heat through its powder is very slow and feeble. Heat is conducted swiftly and copiously through transparent rock salt, but pulverization converts the solid mass into a good non-conductor. Caloric has for the same reason a stronger affinity for pure metals than for their oxides.

Again, wood is known to be a better non-conductor when reduced to shavings or sawdust than when in the solid state. It is probably on this account that trees are protected by bark, which is not nearly so dense and hard a body as the wood. Wool, silk, and cotton are much diminished in conducting qualities when spun and woven, for the reason that their fibers are brought closer together.

Count Rumford discovered that hot water, at a given temperature, when placed in a vessel jacketed with a clothing of twisted silk, and plunged into a freezing mixture, cooled down to 185° Fah. in 917 seconds. But when the same vessel was clothed with an equal thickness of raw silk, water at the same heat and under the same process required 1,264 seconds before it reached the same decrease of temperature. It was also found by Sir Humphry Davy that even metals became non-conductors when their cohesion was destroyed by reducing them to the gaseous state.

It is now generally admitted that, heat being motion, anything, which, by the cohesion of particles, preserves the continuity of the molecular chain along which the motion is conveyed, must augment calorific transmission. On the other hand, when there is a division or disintegration of atoms, such as exists in sawdust, powdered charcoal, furs, and felt, the particles composing such bodies are separated from each other by spaces of air, which the instructed among us well know are good non-conductors of heat. The motion has, therefore, to pass from each particle of matter to the air, and again from the air to the particle adjacent to it. Hence, it will be readily seen, that in substances composed of separate or divided particles, the thermal bridge, so to speak, is broken, and the passage of heat is obstructed by innumerable barriers of confined air. The correctness of these assumptions has been so abundantly proved by experimental demonstrations, that every mind that is tolerably informed on the subject must be relieved of every shade of doubt respecting the greatly superior non-conducting powers which bodies consisting of separate atoms possess over those of a solid concrete nature.

The next matter of interest connected with the subject under notice is its relation to the philosophy of radiation. It has long been known that the emission of heat from a polished metallic surface is very slight, but from a surface of porcelain, paper, or charcoal, heat is discharged profusely. Even many of the best non-conductors are powerful radiators, and throw off heat with a repellent energy difficult to conceive.

"If two equal balls of thin, bright silver," says Sir John Leslie, "one of them entirely uncovered and the other sheathed in a case of cambric, be filled with water slightly warmed and then suspended in a close room, the former will lose only eleven parts in the same time that the latter will dissipate twenty parts." The superior heat-retaining capacity which a clean tin kettle possesses over one that has been allowed to collect smoke and soot, lies within the compass of the most ordinary observation.

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.