Unlike some other reproductions of classic texts (1) We have not used OCR(Optical Character Recognition), as this leads to bad quality books with introduced typos. (2) In books where there are images such as portraits, maps, sketches etc We have endeavoured to keep the quality of these images, so they represent accurately the original artefact. Although occasionally there may be certain imperfections with these old texts, we feel they deserve to be made available for future generations to enjoy.

Unlike some other reproductions of classic texts (1) We have not used OCR(Optical Character Recognition), as this leads to bad quality books with introduced typos. (2) In books where there are images such as portraits, maps, sketches etc We have endeavoured to keep the quality of these images, so they represent accurately the original artefact. Although occasionally there may be certain imperfections with these old texts, we feel they deserve to be made available for future generations to enjoy.

Thin-layer chromatography (TLC) has become a common and much favoured separation technique in laboratories in widely varied fields in recent years. Much of the credit for the introduction of this technique into analytical practice at the l 2 end of the 1950s is due to E. Stahl * * This method is simple and is characterized by high separation ability and sufficient sensitivity3; however, some analysts feel that it has passed the peak in its development and will gradually be replaced by the more modem high-performance liquid chromatography (HPLC). This is undoubtedly a very important analytical technique utilizing the specific separa tion properties of a large number of sorbents and the possibility of regulating 4 the flow-rate of the mobile phase by adjusting the pressure * Standardization of the experimental conditions is simpler in HPLC than in TLC, where the activity of the sorbent and flow-rate of the eitlent in the thin layer depend markedly on the relative humidity of the laboratory atmosphere and on the composition of the gaseous phase in the elution chamber. In addition, systems for quantitative detection of the separated ~ones are better developed for HPLC than for classical TLC, where, until recently, cumbersome and often even insufficiently reproducible chemical or gravimetric analysis of the extracts of scraped-off spots or densitometry of the separated zones, located first by pyrolysis or reaction s with suitable detection agents, were the predominant determination methods .