NATURAL PHILOSOPHY. 109 



IMPROVEMENT IN ELECTROTYPING. 



The National Intelligencer says an improvement in the process of electro- 

 typing has been made, by which electrotypes can be produced with great 

 rapidity and accuracy. The improvement consists in covering the face of the 

 wax, or other material of which the matrix is made, with fine metallic leaf 

 before the impression is taken. In this way a perfect conducting metallic 

 surface is obtained ; that is, over the entire face of the letters, as well as over 

 the spaces between the lines. 



The sides of the letters do not, as a general thing, have a metallic conduct- 

 ing surface, inasmuch as the types, when the impression is taken, cut the leaf, 

 and force a part of it down into the matrix, thus leaving the wax exposed 

 on the sides of the letters. This cutting of the leaf, however, is rather an ad- 

 vantage, since such exposed parts of the wax are the very parts where a slow 

 deposit is preferred, and which is effected by touching such parts over with 

 plumbago. The advantages are these:" The moment that the mould or 

 matrix is placed in the bath and the battery applied, the deposit of metal 

 commences at once on the entire surface, the deposit being more rapid, 

 however, on the face of the letters, and on the spaces between the lines than 

 on the sides of the letters; and this is just what is wanted, since it prevents, 

 especially when the letters are small and deep, what is termed "bridging 

 over" (hollow letters). By the use of silver leaf an electrotype may be pro- 

 duced with a bright silvered face, a feature of considerable importance in 

 all cases where the plates are to be laid aside for future use, inasmuch as the 

 face of the letters will not be so easily injured by long and continued expo- 

 sure to air and moisture, as when of the usual copper face. 



ELECTRIC DISCHARGES IN AIR HIGHLY RAREFIED. 



On making a current of static electricity to pass through a tube of rarefied 

 air, a luminous arc is obtained, which experiences modifications, when sub- 

 jected to the action of the poles of a powerful magnet. This fact, which 

 calls to mind the corresponding effect experienced by a luminous arc pro- 

 duced by a powerful galvanic battery, was discovered by De la Rive in 184U. 

 He first used as the source of the electricity the hydro-electric machine of 

 Armstrong, afterwards a common electric machine, and quite recently Kulnn- 

 korff 's apparatus. M. Pliicker, of Bonn, has tried the same, and his results 

 are published in a recent number of Poggendorff 's Annalen. 



According to De la Rive, it is necessary for success that the tube or globe 

 should contain some vapor, equivalent in tension to six millimeters of mer- 

 cury, and the vapors answering best are those of alcohol, sulphuret of car- 

 bon, and camphine. De la Rive has applied the experiments to the illustra- 

 tion of the Aurora Borealis, so frequent in the Polar regions. 



ON A MODIFICATION OF RUHMKORFF'S INDUCTION COIL. 



At the last meeting of the British Association, Mr. W. Ladd, presented the 

 results of a very extensive course of experimenting with Ruhmkorff 's in- 

 duction coils, with a description of the machine, as it is now constructed. 

 His object,. he said, was not to make very large machines, but to obtain the 

 greatest results from a three-mile coil, that being sufficiently large for all ordi- 

 nary purposes. I find the best length for the iron core to be thirteen inches 



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