produced in the Prismatic Spectrum. 83' 



gen is usually considered an organic radicle, one a compound 

 of such a radicle as aether ; the remaining one, chloride of 

 sulphur, possesses ])roperties referable to none of these heads. 



2. The mere presence of colour is no indication of the ex- 

 istence of lines. Vapours which the eye cannot distinguish 

 from each other will be found in one case, as with bromine, 

 to give numerous lines, in another, as the chloride of tungsten, 

 to give none at all. 



3. The probable position of the lines cannot be inferred 

 from the colour of the gas : with the green perchloride of 

 manganese the lines are most abundant in the green, whilst 

 with the red vapours of nitrous acid they increase in number 

 and density as we advance towards the blue end of the spec- 

 trum. 



4. Simple bodies, as well as compounds, may produce lines; 

 and two simple bodies which singly do not produce them, may 

 in their compounds occasion them abundantly. Neither oxy- 

 gen, nitrogen, nor chlorine, when uncombined, occasions 

 lines 5 but some of the oxides, both of nitrogen and of chlo- 

 rine, exhibit the phaenomena in a most striking manner. 

 There are, however, oxides, both of nitrogen and of chlorine, 

 which do not occasion the appearance of lines. 



5. We find also that lines may exist in the vapour of simple 

 substances, as in iodine, which disappear in their compounds ; 

 this is exempl^ed in the hydriodic acid, a combination of 

 equal volumes of gaseous iodine and hydrogen, united without 

 condensation. 



6. Sometimes the same lines are produced by different 

 degrees of oxidation of the same substances, a remarkable in- 

 stance of which is furnished in the oxides of chlorine. Here 

 chemical considerations of interest may assist in explaining 

 the cause. 



7. The lines are increased in number and density by in- 

 creasing the depth of the coloured stratum through which the 

 light is transmitted, or by any cause which increases the in- 

 tensity of the colour; proving that more lines exist than our 

 instruments or eyes are capable of discerning when the vapour 

 is dilute. This is especially exemplified with iodine and bro- 

 mine. 



8. These lines occur both with polarized and non-polarized 

 light. There is little doubt, although I have not yet made 

 the experiment, that the lines thus produced, as with the or- 

 dinary solar lines, indicate an absence of chemical influence, 

 as well as of the luminiferous portion of the ray. 



In all the experiments on transmitted light, the luminous 



G2 



