The Hon. Cassius M. Clay, late U. S. Minister to Russia, has recently returned from St. Petersburg, bringing with him some fine specimens of iron electrotypes, done after the process of Prof. Jacobi and Klein. We have before alluded to this important discovery. By its use, nearly all forms of electro-plating, such as engravings, stereotypes, medallions and ornaments, may be done in iron, with a fineness of texture which is really surprising. Its importance and value will be appreciated when we reflect that the iron electro-plates are about five times more durable than the ordinary copper electro.plates. Mr. Clay has presented us with an iron electro-plate copy 'Of a copperplate engraving of the Prince Imperial of Russia. This plate is six inches square,. and beautifully done. It is .one thirty-second of an inch in thickness, and has a color closely resembling that of zinc. These iron electrotypes are now Used by the Russian Government with complete success for the printing of bank notes. The process was patented in this country through the Scientific American Patent Agency, Sept. 29,1868, and further information can be had by addressing C. M. Clay&Co., 45 Liberty St., New York. The following description of the process we copy from the patent specification: ” Our invention consists in the application of a practical galvano-plastic process as to the deposits of iron on molds, or any other form, for reproducing engravings, stereotypes, and for other useful or ornamental purposes. ” The galvano.plastic bath we use is composed of sulphate of iron, combined with the sulphates of either ammonia, potash, or soda, which form, with sulphate of iron, analagous, double salts. ” The sulphate of iron may also be used, in combination with the chlorides of the said alkalies, but we still prefer the use of sulphates. ” The bath should be kept as neutral as possible, though a small quantity of a weak organic acid may be added, in order to prevent the precipitation of salts of peroxide of iron. ” A small quantity of gelatin will improve the texture of the iron deposit. ” As in all galvano-plastic processes, the elevation of the temperature of the bath contributes to the uniformity of the deposit of iron, and accelerates its formation. ” For keeping up the concentration of the bath, we use, as anodes. large iron plates, or bundles of wire of the same metal. ” Having observed that the spontaneous dissolution of the iron anode is, in some cases, insufcient to restore to the bath all the iron deposited on the cathode, we found it useful to combine the iron anode with a plate of gas-coal, copper, platinum, or any other metal being electro-negative toward iron, and which we place in the bath itself. ” As a matter of course, this negative plate may also be placed in a separate porous cell, filled with an exciting fluid, as diluted nitric or sulphuric acid, or the nitrates or sulphates of potash and soda. ” For producing the current, we usually take no more than one or two cells of Daniels' or Smee's battery, the size of which is proporlioned to the surface of the cathode. ” It is indispensable that the current should be regulated, and kept always uniform, with the assistance of a galvanometer, having but few coils, and therefore offering only a small resistance. ” The intensity of the current ought to be such as to admit only of a feeble evolution of gas-bubbles at the cathode, but it would become prejudicial to the beauty of the deposit if gas-bubbles were allowed to adhere to its surface. ” The same molds, as employed for de'positing copper, may also be used for depositing iron, only it is advisable, in employing molds made of lead or gutta-percha, to cover them previously with quite a thin film of galvanic copper, formed, in a few minutes, in the usual way, and then oring them, after having washed the molds with water, immediately in the iron-bath. ” The film of copper may be removed from the deposit either by mechanical means, or by immersion into strong nitric acid. ” The deposited iron is very hard, and rather brittle, so that some precaution must be taken in separating it from the mold. By annealing, it acquires the malleability and softness of tempered Condensed Food. Experiments have recently been made with satisfactory results to test the practicability of supplying the North German army and navy with compressed or condensed food. The principal object was to ascertain the best means of furnishing the soldier in the field with a three days' stock of provisions reduced to a minimum of weight and bulk. It has been found that a sort of meat-bread is admirably adapted for this purpose, as it may either be eaten dry in the form of cakes or can be converted with very little trouble into soup. Similar attempts have been made to compress hay and other provender for horses. [We find the above item in a recent number of the'"jJenin0 Post. The idea of using condensed food in the manner deScribed was first patented in 1850, by Ga,il :Borden, Jr., then a resident of Galveston, Tex'as, since better known in connexion with Borden's Condensed Milk, an article of large consumption in this and other cities. Mr. Borden has devoted a great deal of attention to the preparation of condensed food, and may be regarded as the pioneer in that branch. His patent of 1850 consisted in the concentrated extract. of alimentary ani mal substances, combined with the vegetable flour and meal, made into cakes and baked into bread, and was readily converted into a wholesome food.—EDS.