A CURIOSITY—At the dining rooms of Messrs. Crook, Pox, I & Nash, Park Row, this city, we saw last week a curiosity in the form of a smelt inside the shell of an oyster. The j i oyster shell (lower valve) measured four and a half by three j inches and the smelt was five inches long, lying curved to i conform to the mouth of the shell and in a good state of pre-; servation. As the food of the oyster consists of nothing larger than the animaculfe of the salt water, it must therefore be inferred that the smelt was on an exploring expedi- tion while the oyster had his shell open for an airing, and I when that representative of the family clupeidcB intruded, the i oyster imprisoned him for ransom. i I THE PATENT SANDSTONE.—The recent fall of the church built of this material at Morrisania has set people to thinking what is likely to happen to the Preedmans Bureau buildings at Washington, built of the same worthless stuff at a cost of 200,000. The material is the very last we should j adopt for any structure required to be permanent, but perhaps permanency was not contemplated for the Preedmans Bureau. IT is gaid that contlactB have been made with a French j Company for opening a canal across the Isthmus in Nicaragua I and with an American Company for an Isthmus raijroad Work on4he latter is to begin in the spring, and the first 1 thirty miles of the canal are to be finished in eighteen months. I The contract price is ten millions of dollars. INTERESTING EXPERIMENTS BY PROP. TYNDALL.—Dr. Tyn-dall has made some very surprising experiments by passing vapors of different chemical substances into an exhausted glass tube, and then sending through them a beam of electric light. The vapor is at first invisible, but after the light has shone through it for a few seconds, it forms clouds of a blue, green, red, or mauve color, which break up into the most fantastic and beautiful forms, endowed with a rotary motion, which adds greatly to their effect on the eye. In some instances, the cloud takes the shape of funnels overlapping each other, and, curiously enough, the inner ones can be seen through the outer ones. The most surprising of all is the vapor of hydriodic acid. The cloud is seen cone-shaped, supporting vases of exquisite form, and over the edges of these vases fall faint clouds, resembling spectral sheets of liquid. Afterwards, a change takes place—roses, tulips, and sunftoers appear; then come a series of beautifully shaped bottles, dnf within the other, and on one occasion there was seen the shape of a fish with eyes, gills, and feelers. What, it may be asked, is the use of all this fantistic beauty ? The answer is, that Dr. Tyndall finds therein illustration of chemical decomposition, examples of molecular physics, and explanations of the formation of cloud and the blue color of the sky, whereof we shall hear more by-and-by, and by which science will be enriched. TEST FOE THE STRENGTH OP ALCOHOL.—Alcohol dissolves chloroform, so that when a mixture of alcohol and water is shaken up with chloroform, the alcohol and chloroform unite, leaving the water separate. On this fact Basile Rakowitsch, of the Imperial Russian Navy, has founded his invention. The instrument he uses is a graduated glass tube into which a measured quantity of chloroform is poured, and to this is added a given quantity of the liquid to be tested; these are well mixed together and then left to subside; the chloroform takes up the alcohol and leaves the water, which being lighter than the chloroform will float on the top; and the quantity of water that has been mixed with the spirit will be at once seen. N P. BTJRNHAM, of York, Pa., in a recent letter, says: I shall shortly send you an advertisement for my wheel; I have already received over one hundred letters from your description of it in your paper of the 9th Feb. This is a valuable endorsement of the SCIENTIFIC AMERICAN as an ad-. vertising medium. THIS WINTER although a very mild one has been a very hard one on proprietors of Skating Rinks in New York and Brooklyn, who have only saved themselves from ruinous loss-I es by adopting the velocipede. MR. FRANK BUCKLAND states that the skin of the salmon will make leather as tough as wash-leather and about the thickness of dog-skin leather, The scale marks give a vpry neat pattern to the leather.