142 ANNUAL OF SCIENTIFIC DISCOVERY. 



bined. These anomalies appeared to admit of an easy explanation 

 on the supposition that, in any case, the compound is decomposed in 

 flame, either simply by the high temperature, just as water is, as shown 

 by Grove, or, in all other cases of the production of bright lines by 

 the introduction of a metallic salt into a flame of burning bodies, as 

 shown by Deville. In the voltaic pile the decomposition must of ne- 

 cessity take place by electric action. The compound gases, protoxide 

 and biuoxide of nitrogen, gave, when electrified, the same series of 

 bright bands, as Pliicker had shown, which their constituents when 

 combined furnish. Aqueous vapor always gives the bright lines duo 

 to hydrogen and hydro-chloric acid, the mixed system of lines, which 

 could be produced by hydrogen and chlorine. The reducing influence 

 of the hydrogen and other combustible constituents of the burning 

 body would decompose the salt, liberating the metal, which would im- 

 mediately become oxidized or carried off in the ascending current. 

 There was obviously a marked difference between the effect of intense 

 ignition upon most of the metallic and the non-metallic bodies. The 

 observations of Pliicker upon the spectra of iodine, bromine, and chlo- 

 rine show that they give, when ignited, a very different series of bands 

 from those which they furnished by absorption. But it was interesting 

 to remark that in the case of hydrogen, which, chemically, was so sim- 

 ilar to a metal, we have a comparatively simple spectrum, in which the 

 three principal bright lines correspond to Fraunhofer's dark lines, C, 

 F, and G. 



PRACTICAL VALUE OF THE STEREOSCOPE. 



A novel application of the stereoscope was announced a year or two 

 ago by Prof. Dove, of Berlin. It consists in the detection of reprinted 

 matter in the case of books, pamphlets, etc., and was based upon the 

 impossibility, or at least extreme difficulty, of compositors, when setting 

 up a page of type with the intention of producing a fac-simile of a page 

 of printed copy, making the blank spaces between the separate words 

 in a line exactly the same width in the copy as in the original. Our 

 readers may not all be aware that the blank spaces between the words 

 which they are now looking at are made by placing very thin strips 

 of lead or type-metal, technically called " spaces," side by side between 

 each group of types forming a word, and so arranging them as to ob- 

 tain each line of the proper length. These lead " spaces " are so thin 

 that in ordinary work it is never attempted to get exactly the same 

 number between each word, but they are put in in greater or less 

 number, according to the way in which the words fall at the end of a 

 line ; that is to say, if the line, as it is set up in type, falls a trifle short 

 of the proper length, it is " spaced out ; " and if it exceeds that length 

 by a letter or two, some of the " spaces " are removed, or thinner ones 

 used. In this manner it will be perceived that however accurately the 

 compositor follows the words of his printed copy, and sets up his page 

 in imitation of the original, he is sure to be sometimes incorrect with 

 the spacings between the words. A knowledge of these facts led Prof. 

 Dove to imagine that if a stereoscopic slide were so mounted as to have 

 the original printed page on one side and the recomposed fac-simile on 

 the other half, an inspection in the instrument would at once detect 

 the reprint. And so it was seen to be on trial. The page of print 



