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.