THE ANALYST. 7 ON THE LOGWOOD TEST FOIt SLUU. By W. W. STODDART, F.I.C., &c. IN the rear 1858 Xr. Hadow, when demonstrator of chemistry in King’s College proposed ae a test for the presencc of alum in bread the decoction of logwood, with the mode of preparation and method of applying it, as an easy and effectual test. B e affirmed that he could, without difficulty, detect 5.7 grains of alum iu each pound of bread.About three or four years afterwarda Mr. Horsley, of Cheltenham, published a pcculiar method of application, so that a much smaller quantity of alum may be made evident. With or without alum the bread is coloured a strong and distinct purple colour, but when the bread is afterwards exposed on a porcelain capsule to the atmosphere for a few hours, if alum is absent the beautiful colour disappears, and a dull brown takes its place, but if alum has been mixed with the bread a conspicuous and beautiful blue is the permanent result.I have, for some years, been in the habit of employiDg the logwood test, in the manner recommended by Mr. Horsley, invariably with a mccessful and reliable result, so much so that I was astonished at hearing and reading that several analysts had found the test useless.Dr. Hassall, in the last edition of “ Food and its Adulterations,” Bass, I ‘ The logwood test is fallacious. We hare frequently emplogerl this test in cases in which alum has been present without obtaining the slightest violet coloration.” So invariably have I found the reverse happen that 1: have often wondered why so many good chemists hare failed.I think the cause of disappointment is in the logwood itself. The wood must be freshly cut and kept in a stoppered bottle, secure from the atmosphere. The decoction or alcoholic tincture also must be recently prepared. If this precaution be not strictly observed, the logwood test is quite useless. I have frequently found that, wbere the tincture has been made for some months, that even strong ammonia or solution of potass has not had any effect in altering the colour of the logwood solution.The fact is that the usefulness of this test is altogether dependent on the presence of hcemateate of ammonia, which is instantly formed when ammonia is added to hcematoxylin. I f a few crystals of fresh hcematoxylin be used instead of the wood itaelf, the aame reaction is produced. The solution on exposure to air changes to a brown, but if a trace of alum be present, a beautiful and striking b h e is left instead. My experience has been so invariable and distinctive, that if this singular reaction is wanting I do not proceed further in the analysis. Of coumc I need not say, I always perform the chemical separation of alumina and sulphuric acid before evidence of adulteration is shewn in a court of justice, although I really think the logwood test, if properly made, ie quite deoisive.