Optical investigation of crystallised nitrogen, argon, methane, and some of the simpler organic compounds, of low melting points

Nothing is known about the crystallographic properties of the elements gaseous at ordinary temperatures and of many of the most simply constituted organic bodies. As it is not probable that the preparation of well-developed single crystals, nor the measurement of such crystals by the methods now used for crystal measurements, would be successful, some observations on the solid forms of these bodies by crystallo-optical methods have been undertaken and will be described in this paper. The experimental difficulties are too great to permit of an investigation as complete as that which may be undertaken with substances crystallising at ordinary temperatures, but in spite of the fragmental character of the results obtained, they throw light upon the general crystallographic properties of these substances, and may for certain general purposes be sufficient. Methods of Investigation and Apparatus. Of the two methods employed at ordinary temperatures for crystallo-optical investigations, namely that of cutting thin sections of the crystal in certain directions, and that of allowing the substance to crystallise in a thin layer between a slide and a covering glass under the polarisation microscope, only the latter method can be adopted for work at low temperatures. As the gas has first to be condensed to a liquid between the glass plates, and has then to be crystallised, I first tried to obtain a suitable crystallisation vessel by blowing a small bulb on to a glass tube and squeezing the bulb flat while the glass was still soft.