the Intensity of the Lines of the Carbon Spectrum. 203 



in a later paper (Phil. Trans. 1868), so that apparently he 

 considered the result to be unquestioned. 



As no details of the experiments were given, and in fact 

 no further reference to this particular question could be 

 found anywhere, the following experiments were undertaken 

 at Professor Schuster's suggestion, whose constant advice I 

 had the advantage of in the conduct of them. 



The red line of carbon is a double one ( \ „„ Jt j, the more 



refrangible one being the stronger. When the slightest 

 trace of hydrogen was present along with the carbonic acid 

 gas, in which the spark was taken in the first instance, the 

 red line of hydrogen (X 6562) appeared ; as more and more 

 hydrogen was admitted it was found that the carbon red line 

 very rapidly diminished in brightness, so that with a compa- 

 ratively small proportion of hydrogen in the mixture the 

 carbon line appeared very faint. 



The other principal line of carbon seen when the spark is 

 taken in carbonic acid is a broad one in the violet (X 4266). 

 The behaviour of this line in the presence of hydrogen was 

 quite different; as the relative proportion of hydrogen was 

 increased this line merely suffered a gradual diminution in 

 brightness (along with several oxygen lines in its neighbour- 

 hood), but it never disappeared, being quite distinct when 

 the amount of hydrogen present w r as far in excess of the 

 carbonic acid ; indeed it was quite recognizable with an ex- 

 ceedingly small proportion of carbonic acid in the mixture, 

 and seemed, if anything, to be more persistent than the 

 oxygen lines near it in the violet. 



Two other carbon lines were examined (X 5640 and 

 X 5145); they apparently suffered merely a gradual weaken- 

 ing as the proportion of hydrogen was increased, but as they 

 are much fainter than the two principal lines they were not 

 attended to in the subsequent experiments, which were 

 directed solely to investigating the difference in behaviour of 

 the two strong lines. Thus it appeared from these initial 

 experiments that the red line of carbon seen under these 

 conditions is affected by the presence of hydrogen in a totally 

 different w r ay from the other carbon lines. 



After these few preliminary experiments, in which Sir 

 William Huggins's observations were generally confirmed, 

 several series of experiments were made with mixtures of 

 carbonic acid and hydrogen in known proportions by volume. 

 The gases, supplied by two Kipp's apparatus, were passed 

 into an aspirator which was graduated so that definite 

 volumes of each gas could be admitted ; from the aspirator 



