92 ANNUAL OF SCIENTIFIC DISCOVERY. 



American ingenuity long since superseded the artificial teeth which were at 

 first manufactured by the French. In twenty years the number of teeth 

 made here has increased from 250,000 to 5,000,000. For all these grinders 

 we cannot find occupation, and a large portion are exported. The capital 

 employed in this single branch of industry is upwards of $500,000. A single 

 firm in Philadelphia use 700 moulds, producing 9,000 different shapes and 

 styles of teeth, costing upwards of $18,000. Of platina alone 300 ounces 

 a month are used simply for pins to fasten the teeth in their places. This 

 firm manufactures 180,000 finished teeth per month. The value of gold-foil 

 it sells amounts to $109,200 per annum. It is estimated that the 5,000 den- 

 tists in the country use no less than $2,500,000, worth of gold per annum. 



MEDALS IN ALLOYS OF PLATINUM AND IRIDIUM. 



M. Pelouze recently presented to the Academy of Sciences at Paris, in the 

 name of M. Jacobi, medals of different sizes struck in alloys of platinum 

 and iridium, fused at the laboratory of the Ecole Normale, by the process of 

 MM. Deville and Debray. The alloys contained respectively twenty, ten, 

 and five per cent of iridium. According to the declaration of M. Jacobi, 

 they were rolled cold and without annealing, with great ease, and presented 

 the characters of the most ductile metals. Under the press they take a 

 polish equal to that of coins; and the alloys rich in iridium showed a 

 hardness rather greater than that of gold of 0'916. This hardness is pro- 

 portioned to the quantity of iridium, as is also the resistance of the alloy 

 to aqua-regia, which attains its maximum when the quantity of iridium 

 reaches twenty per cent. 



WEAR OF GOLD AND SILVER COINAGE. 



The Gazette of St. Petersburg gives a curious account of an experiment 

 recently made at the mint of that city for the purpose of ascertaining the 

 comparative loss by ordinary wear of gold and silver coin. It appears, con- 

 trary to the generally received opinion, that gold wears away faster than 

 silver. The means employed were as follows : Twenty pounds of gold half- 

 imperials, and as much of silver copecks, coins of about the same size, 

 were put into two new barrels, mounted like churns, which w r ere kept turning 

 for four hours continuously. It was then found, on weighing the coins, that 

 the gold coins had lost sixty-four grammes, while the silver coins had lost 

 only thirty-four grammes ; but as the number of gold pieces was twenty- 

 eight per cent, less than those of silver, the proportion is greater to that 

 amount in favor of the latter. It must, however, be mentioned that the 

 silver contained more alloy than the gold, the standard of the former being 

 868-lOOOths of pure metal, and that of the latter 916-lOOOihs. The result of 

 the experiment is, that the pecuniary loss on the wear of gold coin is about 

 thirty times more than on silver. 



STONE-DIGGING AND WALL-LAYING MACHINE. 



~\Ve were recently invited to witness, says the New York Tribune, the trial 

 of a machine for digging stones and building wall. The novelty of a ma- 

 chine for such labor excited a good deal of incredulity as to the possibility 

 of substituting mechanical for manual labor in this hardest of farm work; 



