separate, by means of the continuous jig, minerals so near in specific gravity as barite and galena. This whole subject of ore dressing is one of the most important questions connected with the future of mineral industry in America. In the Mississippi valley everything connected with metallurgy, from the fuels to the finished metal, will one day be closely dependent on it. OCEAN ECHOES. Prof. Henry communicated to the National Academy at Philadelphia his latest researches into the subject of sound, and among them an explanation of the echo observed on the water. This echo he had formerly been inclined to attribute to reflection from the crests of the waves. Tyndall holds that it is due to reflection from strata of air at different densities. Prof. Henry's present explanation is that this echo is produced by the reflection of the sound wave from the uniform surface of the water. The effect of the echo is produced by the fact that the original sound wave is interrupted. It has what the learned Professor calls _shadows_, produced by the intervention of some obstacle in its path. Sound is not propagated in parallel, but in diverging lines, and yet there are some cases where what may be called a "sound shadow" is produced. For instance, let a fog-signal be placed at or near water level on one side of an island that has a conical elevation. Then the signal will be heard distinctly by a vessel on the opposite side of the island at a distance of three miles. But when the vessel sails toward the island (the signal being on the opposite side), the sound will be entirely lost when the distance is reduced to a mile, and in any smaller distance it is not recovered. In this case the station of the vessel at the shorter distance is in the "sound shadow." The termination of that shadow is the point at which the diverging beams of sound, passing over the crest of the island, bend down and reach the surface of the water. The formation of the sound echo may be explained by this extreme divergence of the sound waves, for it is rational to suppose that at a great distance from the source of sound some of the dispersed waves will reach the water surface at such an angle as to be reflected back to the hearer. This was well illustrated by an experiment made to test Tyndall's theory. A steam siren was pointed straight upward to the zenith, but no echo from the zenith was heard, though the presence of a cloud from which a few raindrops fell certified the presence of air strata of different densities. But, strange to say, an echo _was_ heard from every part of the horizon, half of which was land and half water. The only explanation of this fact is that the sound waves projected upward were so dispersed as to reach the earth's surface at a certain distance, and at that point some of them had curled over and assumed a direction that caused their reflection back to the siren. THE DELICACY OF CHEMISTS' BALANCES.