The present procedure for certification is outlined in United States Department of Agriculture Miscellaneous Circular 52; the chemistry of the permitted coal-tar food colors and the revised methods of their analysis are fully described in the following pages. CHEMISTRY OF THE PERMITTED COAL-TAR FOOD DYES In the preparation and purification of dyes to be certified care must be taken not only in the final steps of their manufacture, but also in the synthesis or purification of their intermediates. Many intermediates are commercial products containing relatively large proportions of isomeric and subsidiary compounds, which will produce isomeric and subsidiary colors during the preparation of the dye. Some of these impurities are harmful, and all are undesirable adulterants. Purification of adulterated intermediates is not worth trying. The only safe method is to use intermediates of the best possible quality. The Bureau of Chemistry has refused to approve for certification several batches of dyes because they contained foreign coloring matter.

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been fairly generally accepted as standard-that is, where all substances involved are a t the standard pressure (more strictly, fugacity) of 1 atmosphere. To take a particular case, consider the reaction 2CO + 5Hz = CiHe + 2H20.represents the maximum work obtainable (free-energy increase) in the reversible formation of 1 mol of CzHG at a pressure of 1 atmosphere when each of the substances involved is maintained at the beginning and throughout the process a t a constant pressure of 1 atmosphere. This free-energy increase is then a measure of the tendency for this reaction to take place. It is true that in practice a process such as this in which the pressures of all the substances involved are 1 atmosphere a t the beginning and throughout the process would never be encountered. But this, nevertheless, is the process the writer has chosen to consider as a standard for comparison, and this is the general practice in thermodynamic considerations. Thus, when the value of the constant of the reaction, K , is larger, the thermodynamic teiidency for the reaction to take place is greater. And, although in forming a mol of one hydrocarbon the molal proportions of reactants and resultants required are different from those required for another hydrocarbon, all the free-energy quantities that have been compared refer to the formation of 1 mol of hydrocarbon from the same reactants and resultants a t the same partial pressure. The actual processes corresponding to given free-energy changes are set forth especially clearly by Noyes and SherrX5The statement in the first conclusion of the writer's paper2 concerning the relative tendencies for different hydrocarbons to form from water gas, of course, refers to the temperature 6 "Chemical Principles," The Macmillan Co , New York, 1922. range considered (0" to 400" (3.). Table I in that paper clearly shows that the temperature coefficients of the different reactions are different. A relation between the equilibrium constants of the different reactions, which holds for one temperature range, may not hold for another.In conclusion, a few minor errors in Francis' paper3 should be pointed out. It is inconsistent to use the present writer's value for the free energy of ethane, which was derived from his value for methane, unless the tatter value is accepted. This, however, makes an error of but about 500 calories, which, in comparison with the reliability of some of the other data used, is perhaps not important. Francis3 erroneously compares his value for methane (gas, 1 atmosphere) with Parks and Kelley's value for methane (liquid, 1 atmosphere) which was derived from Lewis and Randall's6 value for methane (gas, 1 atmosphere). The vapor pressure of (hypothetical) liquid methane a t 25" C. and 1 atmosphere would, of course, not be 1 atmosphere. Francis' statement that "practical pressures will not usually overcome an unfavorable free-energy change of more than 5000 calories" is rather loose. The effect of pressure depends upon the magnitude of the decrease in volume accompanying the ...

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