No. 2.] INFRA-RED ABSORPTION SPECTRA. 125 INFRA-RED ABSORPTION AND REFLECTION SPECTRA.1 BY WM. W. COBLENTZ. INTRODUCTION. ANY chemical compounds contain both oxygen and hydro­ M gen, which, on applying heat, pass off in the form of water. That the water is not united so tenaciously as the other constituents is evident from the fact that in many instances it can be more easily removed; many salts give up their water if exposed to dry air at ordinary temperature. From the circumstance that many of these compounds are crystalline the water is said to be present as water of crystallization. The manner in which the water exists in the crystal is not understood. By some it is considered a part of the chemical molecule; by others it is thought that the molecules of water exist in their entirety among the molecules which constitute the crystal. It is characteristic of water of crystallization that it is expelled at a temperature far below red heat, and frequently below ioo° C. Another characteristic of minerals containing water of crystallization is their property of reabsorbing water after it has been removed. Copper sulphate is an excellent example; on applying heat, the blue crystal becomes a crumbling white mass, which, if permitted to stand in dry air absorbs water and resumes its blue color and crystalline structure. On the other hand, the water which is given off only at a red or even a white heat can scarcely be present in the compound in the same manner as the water of crystallization, and is distinguished as water of constitution. In this case the water is not supposed to exist as such in the mineral, but to result from the union of oxygen and hydrogen or from the hydroxyl groups contained in the compound. 1 Extracts from a memoir submitted to the National Bureau of Standards for publication. 126 W. W. COBLENTZ. [VOL. XXIII. Of course there are minerals which contain water in both these forms. In copper sulphate, CuS04 + SH20, four molecules of water are given off at ioo° and the fifth at 200° so that the latter is pos­ sibly combined in some manner different from the former. In epsomite, MgS04 + 7H20, six molecules of water are given off at 132° while the seventh is held more tenaciously and is not given off until heated to 2io°. However, in cases like these, where successive portions of water are given off at different temperatures, it is difficult to draw a dis­ tinction between water of crystallization and water of constitution. In the minerals just quoted, however, it has been found that the heat of hydration of the last molecule of water is different from that of the molecules of water which pass off at a lower temperature, which confirms the belief of a difference in the bonding in the two cases. As a whole, the question of the association of the atoms of oxygen and hydrogen in certain compounds is far from settled, while the difficulties involved in its investigation are very great. The existing data bearing upon the subject is practically nil. PRESENT METHOD OF INVESTIGATION. In a preliminary paper bearing upon the question of water of crystallization l the writer described the infra-red absorption spectra of two numerals, selenite, CuS04 + 2H20, and brucite, Mg(OH)2, in which the atoms of oxygen and of hydrogen are thought to be com­ bined in a different manner. It was the application of the results of previous work in which it was abundantly proven that certain groups of atoms have characteristic absorption bands. Hence, if the oxygen and hydrogen, which enter into the composition of certain minerals, are bonded as they are in a molecule of water then one would expect to find the absorption spectra of such minerals to be a composite of the bands of ordinary water and of the bands caused by the other constituents. The only previous investigation bearing on this subject, from the standpoint of infra-red absorption spectra is that of Konigsberger,2 1 PHYSICAL REVIEW, 20, p. 252, 1905. 2 Konigsberger, Ann. der Phys., 61, p. 703, 1897. No. 2.J INFRA-RED ABSORPTION SPECTRA. 12J who studied the pleochroism of several minerals including selenite. Unfortunately his plate of selenite was too thick, so that no energy was transmitted beyond 2.5 ti. He calls attention to the fact that a small absorption band at 1.5/* coincides with that of water, from which it would appear that in selenite the absorption of the water of crystallization does not appear to be different from that of ordinary water. Thinking that it would be fairer to select the large water bands at 3.0, 4.75 and 6/i as a criterion, the writer (loc. cit.) examined thin sections of several minerals and found as anticipated that minerals containing water of crystallization have large absorption bands coinciding with those of ordinary water. The selection of selinite, CaS04 + 2H20, and brucite, Mg(OH)2, as types of water of crystallization and of constitution was a fortunate one, for two rea­ sons. (1) The selenite curve showed all the absorption bands of water in their proper positions and intensities, except the 4.65 [JL band which is shifted and too deep for the thickness of water con­ tained in the plate under examination. The fact that the band is shifted caused me to suspect that it may be complex, and, from the fact that it lies in the region where the NCS-radical and certain sul­ phur compounds have a strong absorption band, that it may be due to the S04-radical, a surmise that since then has been unexpectedly verified. (2) Although the brucite curve did not contain the water bands, thus showing the difference between water of constitution and of crystallization, it contradicted my previous work * in which it was shown that the OH-radical in the alcohols has an absorption band at 2.95 fi. From the brucite curve the conclusion was drawn that the OH-radical is inactive. Since then I have studied the chemical side of the question more thoroughly from which it appears that there is no marked difference2 in the activity of the OH-radical in the hydroxide studied, and hence that the brucite curve should have a band at 3.0// instead of the one shifted to 2.5 fx. In other words, the present research, which is in part the outcome of the investigations of Infra-red Spectra, Washington, 1905. PHYS. REV., Vol. 22, 1905. 2 From the chemical standpoint, however, the OH in alcohol and in H2S04 is more active, since it is replacable by a metal (more acid) than the OH in brucite MgO(OH)2, which is not replacable (more basic). 128 W. W. COBLENTZ. [VOL. XXIII. aforesaid discrepancies, has to a very unexpected degree furnished us with an abundance of new proof that certain groups of atoms have definite absorption bands, which really means that these groups of atoms, or "ions" enter into the various compounds in a similar manner. To sum up, in the present investigation the criterion for distin­ guishing water of crystallization from water of constitution, is the presence of absorption bands at 1.5, 2, 3, 4.75 and 6 /*, which is the location of the absorption bands of ordinary water. If there are no other absorption bands nearby then the intensity of these bands should be somewhat like that of ordinary water, viz. : the bands at 1.5, 2, and 4.75 p. are weak, while the bands at 3 and 6 fi are very strong. A hydroxyl group will cause an absorption band at 3 fi. Sili­ cates may have a small band shifting from 2.9 to 3.1 /ut, but since it is weak, there is no danger of confusing it with the strong water band in the same region. PLEOCHROISM. Since the present research involves the examination of minerals which are not isotropic, the question presents itself whether it would be better to examine them in natural or polarized light. In the former case we obtain all the absorption bands, irrespective of the direction of transmission, but their observed intensities will not necessarily be real. In the latter case where the intensity of the transmitted energy depends upon the direction of vibration of the polarized rays with respect to the axes of the crystal (pleochroism) the observed intensities will be real, the number and position of the absorption bands will depend upon the direction of observation, but the accuracy is less the farther one penetrates the infra-red, due to the loss in the polarizing the source of energy. By these statements it is not intended to give the impression that pleochroism is not worth considering. The work of Merrittl and of Konigsberger2 is especially valuable in demonstrating t'hat ab­ sorption is dependent upon the direction of vibration of the incident energy. The visible spectrum is so narrow that it is difficult to 1 Merritt, PHYS. REV., 2, p. 424, 1894. 2 Konigsberger, Ann. der Phys., 61, p. 687, 1897. No. 2..] INFRA-RED ABSORPTION SPECTRA. I 29 separate the absorption bands, while, on the other hand, the infra­ red is a vast and almost unexplored region, and it is here that one would expect to resolve wide absorption bands such as exist in the visible spectrum. The curves of Merritt and of Konigsberger show this, especially the calcite curves, which are entirely independent for the ordinary and extraordinary rays. Nevertheless, in looking over the previous work it seemed to the writer that in the present state of our knowledge of absorption a greater advance would be made by simply mapping the spectra of a great many minerals for energy transmitted through them in its natural mode of vibration.