On the Resistance of a Telegraph Receiving 'instrument. 449 



that Weiss's observations are erroneous. If a poor apparatus 

 be employed, the lines already mentioned in the spectrum of 

 nitrogen tetroxide appear to grow thicker when the tempe- 

 rature is raised ; the middle of the lines appears to occcupy a 

 new position. But if the same experiment be made with a 

 superior apparatus, it is found that a number of new fine lines 

 have appeared, and that there has been no real alteration of 

 the position of the original line. 



Weiss's so-called ''change in the position of the lines" is 

 thus shown to be a phenomenon exactly similar to that which 

 occurs when the D line is observed through a good spectro- 

 scope : instead of seeing one line, two become apparent. 



These observations on absorption-spectra enable us to under- 

 stand how valuable such spectra are in determining the che- 

 mical nature of any substance. Before, however, we can say 

 that each and every chemical compound possesses a charac- 

 teristic spectrum of its own, it would be necessary for us to 

 observe a very large number of absorption-spectra. Much 

 remains to be done in this direction. So far as experiment 

 goes, we know of no facts which positively contradict the 

 assertion that for each compound there is a characteristic ab- 

 sorption-spectrum. The same absorption-spectrum is said to 

 be common to more than one oxide of chlorine ; but we really 

 know very little concerning the exact composition of these 

 bodies. Although I have endeavoured to show that alteration 

 in temperature and in mass of material produces but slight 

 alteration in absorption-spectra, nevertheless I must not be 

 looked on as denying the possibility of several spectra being 

 possessed by one and the same element. The experiments of 

 PlUcker and Hittorf, of Lockyer, &c. show that certain ele- 

 ments do possess more than one spectrum. The spectra ob- 

 tained at low temperatures may probably be regarded as caused 

 by the molecules of the elements, while those obtained at high 

 temperatures may be looked on as caused by the atoms. 



A systematic observation of absorption-spectra, with especial 

 reference to the course of chemical change in solutions, would 

 certainly be of much assistance in solving some of the problems 

 of physical chemistry. 



LVIII. A Theoretical Deduction of the best Resistance of a Tele- 

 graph Receiving-instrument. By R. S. Bkough*. 



THE information given in the text-books regarding the 

 proper resistance of an electromagnetic receiving-instru- 

 ment to employ on any telegraphic circuit is meagre and in- 

 definite. The authors usually content themselves with saying 

 * From the Proceedings of the Asiatic Society of Bengal for August 

 1877. Communicated by the Author. 



Phil. Mag. S. 5. Vol. 4. No. 27. Dec. 1877. 2 G 



