A Cubious Substance.—About three hours after the late explosion of the Lexington,some gentlemen at Hawesville, ten miles below the scene of the accident, saw at least a cart-load of substance, that had the appearance of dirty foam, floating in the river. They put off to it in a skiff and found that it bore a strong resem-blanee to very porous pumice-stone. When a piece of it was taken from the water it smoked and emittted a very offensive gas for a few moments, and if afterwards put back in the water,sank.A considerable quantity of this substsnce was saved, but most of it was permitted to float on down the river, A piece of it has been presen-sen ted to us, and it has the appearance of sand solidified and partially vitrified by intense heat. It is full of large and small* cells, and of course j very li^ht, and, although the gentlemen at Hawesville testify that the substance, after having been taken from the water, would sink, the specimen in our possession is still light enough to float.The probability is, that there was a very large quantity of sand In the boilers of the Lexington, and that the heat, to which it was here exposed, gave it the form in which it afterwards was found floating in the river.—Louisville Journal.