PREFATORY NOTE. 



The present volume contains supplementary data on doubtful points, 

 which arose in connection with the preceding work on infra-red spectra. 

 The various phases treated, except Part VII, Chapter I, (4) and (5), and 

 Chapter II, were ready for publication during the summer of 1907, when 

 through the generosity of the Carnegie Institution of Washington it was 

 made possible to combine all the material into a new volume, which was 

 then delayed for the additional data on emission spectra of metal filaments 

 and insulators, thus rounding up the subject as completely as is possible 

 at this time. This completes, for the present, the "program of investiga- 

 tion." The subject, however, is not exhausted not even thoroughly 

 initiated for we know but little more of the cause of absorption and 

 of emission lines and bands, other than those due to several well-known 



o 



groups of atoms, than we did twenty years ago, when Julius and Angstrom, 

 independently, examined infra-red spectra. Each renewed effort is a step 

 in advance, as the present data on the effect of molecular weight illustrate. 

 The main problem is to obtain suitable material, which seems to be partly 

 a matter of chance. For example, one of the first substances ever 

 examined for selective reflection was quartz. It is the easiest obtainable, 

 and it illustrates the question of selective reflection better than any other 

 substance yet found, save carborundum, which was one of the latest min- 

 erals to be examined. On the other hand, after examining over 300 dif- 

 ferent substances, it has remained until the very last to find a series that 

 so well illustrates the effect of molecular weight as the data on the car- 

 bonates herewith presented. The reflection spectra of the carbonates and 

 nitrates in solution deserve further study. They are easily obtainable, and 

 their reflection bands, which are strong, lie in the region of the spectrum 

 where the radiation is quite intense, so that no serious difficulty need be 

 anticipated. Colloidal metals also deserve further attention. Artificial 

 substances, except carborundum, have never been examined, and it is 

 intended to make a study of the silicides, provided they can be melted 

 into homogeneous masses. 



In Part III (Carnegie Publication No. 65) the discussion of the accu- 

 racy attainable refers to the region up to about 12 /*. Beyond this point 

 the absorption of the rock-salt prism increases very rapidly, and it was 

 not possible to attain great accuracy. In fact, the writer has given but 

 little weight to his isolated observations lying beyond 14 n, although others 

 have been able to verify them, a notable example being calcite (Carnegie 



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