CHAPTER III. 



DESCRIPTION OF APPARATUS AND METHODS. 



The work involves two distinct kinds of activity, viz, mapping the 

 spectra, by means of a series of curves, and studying them. The spec- 

 trum is produced by means of a rock-salt prism mounted on a mirror 

 spectrometer. For a source of radiation the " heater " of a Nernst 

 lamp is mounted before the collimator slit. The distribution of the 

 energy in this spectrum forms a smooth, continuous curve. If we inter- 

 pose a hollow cell of rock salt, containing some compound, between the 

 " heater " and the collimator slit, the distribution of the incident energy 

 in the resulting spectrum will no longer form a smooth, continuous 

 curve. The intensity will now be found to rise and fall, forming a 

 series of maxima and minima. These maxima and minima are char- 

 acteristic of each compound interposed ; and it is with these curves that 

 we are concerned in this work. To obtain these curves very narrow 

 portions of the spectrum are successively projected upon a device (a 

 Nichols radiometer), which is sensible to heat radiation. This is a 

 slow process, but it is the most successful yet devised. 



The number of organic compounds is so large, while many are so 

 mtimately related, that, after the preliminary work, it became evident 

 that, in order to gain more definite information in regard to absorption 

 spectra than had been obtained by previous investigations, a large 

 number of compounds would have to be examined. This has been 

 done. The absorption spectra of at least 130 compounds of hydrogen 

 and carbon have been explored, the majority of them to 14 /x and 15 ix. 

 About 30 compounds were exam.ined twice, while 19 were explored to 

 2.5 IX, using a quartz prism. They include solids, liquids, and gases, 

 and belong to the principal groups of organic compounds. 



The field is large, while many of the compounds ordered could not 

 be obtained in commerce; hence it was necessary to proceed in this 

 manner. It really amounts to a preliminary survey of the whole field. 

 Subsequent work must be a detailed study of individual compounds 

 belonging to a particular group, now that we know which groups are 

 the most promising of results. 



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