1 87 1.] Spectra of Metallic Compounds. 59 



thus preserved in a perfectly harmless condition, so as to be 

 capable of being taken up and examined, re-distributed, and 

 handled without the semblance of danger. But on the 

 presence of a hostile fleet in the neighbouring waters, in 

 chase, may be, of friendly vessels, it is necessary merely to 

 allow one's own ships to pass over the line, and when these 

 are safely beyond the boundary, the turn of a switch, or 

 depression of a key, suffices instantly to set up an impassable 

 barrier, and to convert the series of sunken buoys into active 

 submarine volcanoes of the most deadly and destructive 

 nature. 



V. SPECTRA OF METALLIC COMPOUNDS. 



jT^\U RING the last few years the constant employment 

 Ti^JJ of the spectroscope in qualitative analysis has 

 become so general, that it is of great importance to 

 detect and carefully remove any attendant sources of error. 

 Now, there are many other compounds besides those of the 

 alkalies and alkaline earths which afford spectra, and a 

 similarity in the position of their spectral lines and bands 

 causes them to be mistaken on cursory examination for 

 spectra of certain of the alkaline elements. An examination 

 of all the more important metallic compounds, and their 

 faithful mapping, so far as they differed from each other, 

 would eliminate this source of error. Such maps, moreover, 

 would enlarge the field of application of the spectroscope, 

 and enable us to detect the presence of many other bodies 

 than those to which its use is at present restricted. A 

 comparison of these spectra would set at rest many interest- 

 ing points of inquiry and speculation. As, for example, the 

 amount and kind of alteration which takes place in the 

 position, number, and relative intensity of spectrum lines at 

 various temperatures. For the variable influence — the 

 temperature — being but one function of the spectrum, it is 

 by no means to be concluded, without experimental inquiry, 

 that the less and more refrangible parts of spectra alter pari 

 passu. The fact that at high temperatures decomposition 

 takes place has already led (see memoir of M. Diacon, Ann. 

 de Chim. [4] , iv., 5) to a variety of interesting results. He 

 found that in certain cases, when mixtures of volatile com- 

 pounds were examined in the spectroscope, the spectrum 

 obtained was not that of the compounds previously existent 



