4 INFRA-RED ABSORPTION SPECTRA. 



However, in infra-red work we are confronted with a difficulty not 

 encountered in the optical region, viz., the permanent registration of 

 the spectra under investigation. In the optical region we can photo- 

 graph the spectra without much delay. In the infra-red, photography 

 has not yet been possible beyond 1.2 jx, and, instead of being able to 

 procure the entire spectrum simultaneously by projecting it upon a pho- 

 tographic plate, it is necessary to begin at one end of it, and with suit- 

 able apparatus explore point after point until we reach the other end. 



As a consequence, the process of mapping infra-red spectra, whether 

 due to emission or absorption, is, at best, a slow and tedious one. How- 

 ever, as a compensation for all this, with the limited dispersion at our 

 disposal, the spectra, which are a series of curves, are quite simple, so 

 that the process of " measuring up the plates " is of no importance. 

 After this we are ready to study the curves individually and collectively. 



The present investigation was begun while the author was a graduate 

 student, at Cornell University, during the first three months of 1903. 

 During that time the absorption spectra of 38 compounds were explored 

 to 14 /A. It then became evident that, in order to gain a better knowl- 

 edge of infra-red spectra than obtained at that time, a very extended and 

 systematic investigation would be necessary. This was made possible 

 under an appointment as Research Assistant by the Carnegie Institution 

 of Washington during the year following June, 1903. 



The successful completion of this investigation has placed me under 

 deep obligations to numerous persons, to all of whom I am very grateful 

 for services rendered. In particular would I mention Prof. E. L. 

 Nichols, who placed every facility of the laboratory at my disposal and 

 did for me many other deeds of kindness, and his colleague, Prof, E. 

 Merritt, whose advice and suggestions were also asked. Professor 

 Nichols has also read several chapters of this manuscript. In the De- 

 partment of Chemistry I am indebted to Prof. L. M. Dennis for the 

 facilities of the gas laboratories, to Prof. W. R. Orndorfif for the use 

 of chemicals, and, last but not least, to Dr. J. R. Teeple, whose advice 

 and suggestions on the chemical side of this subject were constantly 

 sought. To Profs. A. C. Gill and H. Ries, of the Department of Geol- 

 ogy, I am also indebted for material placed at my disposal. It is a 

 further pleasure to acknowledge the generosity of Prof. C. F. Mabery, 

 of Case School of Applied Science, who presented me with 25 selected 

 samples of pure distillates of petroleum, which could not have been pro- 

 cured elsewhere. When one considers that it has taken seven years 

 to prepare them, and that his collection is the most complete in exist- 

 ence, the value of the accession of these samples to the list of compounds 

 investigated becomes apparent. 



