14 INFRA-RED TRANSMISSION SPECTRA. 



PREPARATION OF THE MINERAL SECTIONS. 



Whenever possible the crystals were split parallel to a cleavage plane, 

 since many crystals would break in grinding. The remaining ones were 

 ground thin in the ordinary manner. To accomplish this they were 

 mounted upon glass plates by means of pitch having a low melting- 

 point, so as to avoid disturbing the water of crystallization which is 

 expelled from some minerals at a low temperature. The sections were 

 cleaned in chloroform and mounted upon heavy cardboard having 

 rectangular holes about 4 by 10 mm. cut into them. That the crystals 

 were not affected by grinding and the cement was proven in a number 

 of cases where cleavage sections of the same mineral were examined. 

 Sometimes the sections were mere fragments 5 mm. long, so that two 

 pieces had to be placed end to end with a slight overlapping, but not 

 glued at the center. Since we are not concerned with total transmis- 

 sion, but simply with absorption bands, this does not interfere with the 

 investigation. Of course if a glue could have been found which is free 

 from large absorption bands, the sections could have been mounted upon 

 rock-salt plates and ground much thinner. As it was, the sections 

 were entirely free, except at the ends, which rested upon the cardboard, 

 which was then mounted upon the holder, which moved in vertical 

 ways before the spectrometer slit. 



In the description of the different minerals, the direction of cutting 

 the sections is parallel or perpendicular to some crystallographic axis. 

 In many crystals there is usually a perfect cleavage parallel to one of 

 the crystallographic axes, which fact was taken advantage of in pre- 

 paring the sections. The cleaving was done by means of a small knife- 

 blade struck with a light piece of metal, for example, a file. It may be 

 of interest to note that with crystals, for example anhydrite, which have 

 perfect cleavage in several directions, it was necessary to strike the 

 knife-blade a sudden blow, while in others, like scolecite, a series of 

 light blows started the cleavage and produced large plates. A great 

 many minerals like copper sulphate and alum could not be used, because 

 a slight warming was sufficient to dehydrate them, when the surface 

 became rough and scattered the incident radiation. 



In the present research the examination of the sections after being 

 prepared was overshadowed by the difficulty in obtaining specimens of 

 sufficient size and in grinding them thin enough. In my previous work 

 it was found that as a general rule the compounds examined increase 

 in their general absorption beyond 5 /*, so that the (liquid) films had 

 to be reduced to o.oi mm. In the present work it was impossible to 

 reduce the thickness to much less than o.i mm., with the consequent 

 result that many sections became opaque beyond 5 /JL. 



