In dealing with the problem of tar one must first define what is meant by tar. To use the term "tar" as if it were a definite chemical compound, readily obtainable from the pharmacist, is erroneous. A brief résumé of the present, somewhat limited, knowledge of tars in general will illustrate this. Much of the information has been gathered from the writings of Porter1 and Perutz and Siebert.2 Since the latter authors have covered all the older literature on the subject we shall not cite it here. A few of the more recent publications have been cited by Nelson and Osterberg.3
Any product obtained by destructive distillation of organic substances is a tar. It is thus readily understood that the chemical composition of tars varies with the substances from which they were derived and with the temperatures to which they are subjected. There are four main groups
OBERMAYER ME, BECKER SW. A STUDY OF CRUDE COAL TAR AND ALLIED SUBSTANCES: PRELIMINARY REPORT. Arch Derm Syphilol. 1935;31(6):796–810. doi:https://doi.org/10.1001/archderm.1935.01460240021002
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