Davy — On a Neiv Chemical Test for Carbolic Acid. 159 



Another useful application of this test is, that it ailords a very 

 ready means of distinguishing creasote from carbolic acid, which is a 

 matter of some commercial importance, much of what is sold as creasote 

 being, as is well known to chemists and those in the drug trade, 

 little else than carbolic acid ; for these two substances, though obtained 

 from different sources — true creasote being procured from the distilla- 

 tion of wood tar, whilst carbolic acid is got from that of coal tar — 

 and though they differ likewise from each other in chemical composi- 

 tion, still so closely resemble each other in several of their proper- 

 ties, that the cheaper substance, impure carbolic acid, is in whole 

 or in part frequently sold to the public for the dearer article creasote. 

 If, however, we take a drop or two of each, and agitate them well 

 wdth about a quarter of a fluid ounce of distilled water, and, having 

 filtered the liquid, test a drop or two with the molybdic solution as 

 already described, we will get in the case of pure creasote only a 

 brown or reddish-brown reaction, which on standing or warming 

 slightly becomes fainter, passing to a light-yellowish brown : whereas 

 in the case of carbolic acid the brown passing to a maroon soon deve- 

 lops a more or less intense purple colour. This treatment will be 

 sufficient to distinguish creasote from carbolic acid, and also to detecfc 

 the presence of that acid in creasote, where it occurs in considerable 

 proportion ; for if, on the addition of the molybdic test solution, the 

 mixture, instead of fading away to a light-yellowish brown on stand- 

 ing a short time, or on gently heating, passes to a reddish brown 

 or to a maroon, it is an indication that carbolic acid is present. But 

 I have found that the following very simple proceeding gave more 

 satisfactory results, especially where small quantities of carbolic 

 acid had been added to a large proportion of creasote. From five to 

 ten drops of the liquid under examination are taken, and agitated 

 briskly with about half an ounce of distilled water for a few minutes, 

 so as to dissolve out the carbolic acid ; the mixture is then filtered, and 

 the filtrate is put into a little flask furnished with a close-fltting cork, 

 through which passes a small glass tube about ten or twelve inches 

 long, and bent above the cork at a little more than a right angle. 

 The contents of the flask are then heated, and, when the liquid boils, 

 the first portions which distil over will be found to present a more or 

 less turbid appearance as they pass down the tube, from their contain- 

 ing minute globules of creasote ; and a drop or two having been 

 collected and tested with the molybdic reagent will give only the 

 brown reaction of creasote : but by continuing the boiling, that sub- 

 stance will be more or less completely expelled, and then it will be 

 found that a drop or two of the later portions of the liquid which 

 distil over will give the purple reaction of carbolic acid. I may 

 here observe that as it is only a drop or two of the distillate which is 

 required each time for testing, it is not necessary to use any condens- 

 ing arrangement, for the vapour passing through the tube itself is 

 -cooled sufficiently to furnish the small quantity required for each 

 trial; but when it is desirable to collect in larger quantities the 



R. I. A. PROC, SER. II. J VOL. III. — SCIENCE. N 



