The analogy arrowroot has to potato starch, has induced many persons to adulterate the former substance with it; and not only has this been done, but I have known instances in which potato starch alone has been sold for the genuine foreign article.  There is no harm in this, to a certain extent; but it certainly is a very great fraud upon the public (and one for which the perpetrators ought to be most severely punished), to sell so cheap an article at the same price as one which is comparatively costly.  There is, moreover, in potato starch, a peculiar taste, bringing to mind that of raw potatoes, from which the genuine arrowroot is entirely free.  This fraud, however, can be readily detected; arrowroot is not quite so white as potato starch, and its grains are smaller, and have a pearly and very brilliant lustre; and further, it always contains peculiar clotted masses, more or less large, which have been formed by the adhesion of a multitude of grains during the drying.  These masses crush very readily when pressed between the fingers, and as before stated, arrowroot is free from that peculiar odor due to potato starch.  This may be most readily developed by mixing the suspected sample with hot water; if it be genuine arrowroot, the mixture is inodorous, if potato starch, the smell of raw potatoes is immediately developed.  If a mixture of arrowroot and potato starch be minutely observed by means of a good microscope, the grains of arrowroot may be readily detected; they are very small and exceedingly regular in shape, whilst those of potato starch are much larger, and very irregular in shape.  But the most convenient and delicate test of all, is that proposed by Dr. Scharling, of Copenhagen.  After mentioning the test by the microscope, he goes on to state that he has obtained more favorable results by employing diluted nitric acid; and that, if arrowroot or potato starch be mixed with about two parts of concentrated nitric acid, both will immediately assume a tough gelatinous state.  This mass, when potato starch is employed, is almost transparent, and when arrowroot is used, is nearly opaque, as in the case above mentioned, in which hydrochloric acid is substituted.  A mixture of nitric acid and water, however, operates very differently on these two kinds of starch.  The glutinous mass yielded by the potato starch, becomes in a very brief period so tough that the pestle employed for stirring the mixture is sufficiently agglutinated to the mortar, that the latter may be lifted from the table by its means.  Arrowroot, on the other hand, requires from twenty-five to thirty minutes to acquire a like tenacity.