PHYSICS NINETEENTH CENT.SPECTROSCOPY. 503 



astronomy before Newton united them by the grand and simple con- 

 ception of universal gravitation. Observations, facts, and measure- 

 ments of the phenomena are still being accumulated, and they await 

 the advent of a spectroscopic Newton, who will one day promulgate 

 some comprehensive principle by which the relation of every line 

 of the spectrum of each substance to its chemical constitution and 

 physical state may be definitely expressed. Our knowledge of the 

 conditions of the phenomena is yet far from complete, and the facts 

 already known stand apart from the general theory of light. The 

 general doctrine of the undulations of the luminiferous ether has here 

 afforded little guidance. The conception of the particles of different 

 gases executing vibratory movements of definite periodicity, which 

 are communicated to the ether, and thus give rise to rays of certain 

 refrangibilities, is satisfactory so far as it goes, but it does not touch on 

 the relation of chemical constitution to the various kinds of spectra. 

 Why should the same substance give at one temperature one or two lines 

 only, and at a higher temperature give others in addition, or perhaps 

 a quite different set ? How are the differences between the spectra 

 of compounds and their elements to be explained ? Why does one 

 element give a spectrum having few lines, while an analogous element 

 may give one having many lines ? Can any relations be traced between 

 the positions of the lines and the physical or chemical properties of 

 the substances ? These are the questions which spectroscopists are 

 seeking to solve, and their solution involves a profounder insight into 

 the inner constitution of things than has yet been obtained. It is not 

 an unfair inference from what the spectroscope has already accom- 

 plished, that it will yet prove one of the most efficient of all instruments 

 for the attainment of this profounder insight. 



There have not been wanting speculations bearing upon the chemical 

 relations of the spectrum lines, but little appears to have been defi- 

 nitely made out. Helmholtz has propounded the theory that mole- 

 cules that is, the aggregation of atoms give band spectra, while free 

 atoms gave line spectra. Lockyer supposes that by increase of tem- 

 perature the molecules of nearly all our so-called elements may split 

 up into smaller groups of atoms, each of which yields its distinctive 

 line. This separation by increase of temperature is, in fact, known 

 to take place in certain cases, and is familiar to chemists under the 

 name of *' dissociation." This theory involves the supposition that 

 our so-called elements are not really simple substances, but that 

 they are resolved into true elementary bodies by a sufficient increase 

 of temperature. The dissociation is progressive, and its final stage is 

 reached at a temperature which is different for different substances. 

 These views may be illustrated by the diagram in Fig. 228, which re- 

 presents the lines due to a progressively increased temperature, in the 

 case of calcium spectra, according to Lockyer's views. The particular 

 compound of calcium existing in the flame of a Bunsen burner coloured 



