221 ON THE DISCRIMINATION OF STARCHES BY POLARIZED LIGHT, By JOHN W. TRIPE, M.D., M.R.C.P., Edin., &c. Read before the Society of Public Analysts, on 19th Nor., 1879. SOME time since the Inspeotor appointed to obtain samples under the Sale of Food end Drugs Act, brought to me several packets containing arrowroot, pea-meal, ground lentils, maizena, (Indim corn flour) and ground ginger, all of which were found to be genuine.During the course of examination, I felt dissatisfied with the means ordinarily adopted, and therefore employed selenite plates with the polmiscope. The plates used were red and green, and blue and yellow, when a beautiful play of colour was exhibited by the arrow-roots, md even in a more marked degree by potato stamh222 THE ANALYST.md tous Zes 9120is. The colours shown by Bermuda arrowroot were not so brilliant as by those from St. Vincent and Natal. The starches of wheat, barley, oats, and rice scarcely showed any colour, and the little that was exhibited was equally diffused over the whole granule instead of being more brilliant in certain parts as obtained in the preceding class. Maizena starch presented considerably more colour than that of wheat, whilst that of lentils, peas, and beans, became less coloured than maizena, and, like wheat, did not show green colouration. The amount of colour varied considerably, according to the plate used, being usually much brighter with the green and red than with the blue and yellow, although in sago there was little 3r no colour with the former, whilst a band of a somewhat sober shade was visible with the latter across the narrow end, and the starch of ginger gave a slight irridescence with the red and yellow, but not with the other plate, the colour being spread over the whole surface of the starch, as in wheat, &c.The prismatic colours of tous Zes rnois and potato starch were very brilliant with the green and red plate, the green being more vivid in the potato than in the arrowroot, so that in case of doubt or when a mixed sample is examined, the comparative soberness of the colour of arrowroot will readily serve to distinguish one from another.A little care in using the analyser is necessary in order to bring this out strongly, but a comparison between the two without moving the analyser will show this most satisfactorily, the analyser being used in such a way as to give a red or green ground, the red being the best.The starches examined evidently belonged to two groups, viz., those which showed but little colour with the selenites, and the others which exhibited brilliant and various colours. It is evident that there must be some structural difference between the two, for 8s ti rule the starches obtained from seeds gave but little colour, whilst those obtained from tubers and rhizomes afforded much colour.Of course there are exceptions, as stated, as regards ginger, &c. The use, however, of this plan of diagnosis need not be confined to the starches, as the spiral ducts of some plants colour more than others, and the woody fibre and cells differ sufficiently in colour to assist in the discrimination of the substance examined; for instance, coffee shows numerous coloured pieces of tissue which are absent in chicory.The best results seem to be obtained by using low objectives, half-an-inch with an A eye-piece being decidedly preferable in my opinion to higher powers, and I may mention that the solution employed was one part of glycerine and three of water.One great advantage of this method of diagnosing starches consists in the ease and rapidity with which the starches of cereals can be distinguished from those of potato and arrowroots, when compared with the ordinary methods of examination, in which considerable time is often occupied in measuring the size of the granules, or in bringing out their markings.I also find that I can see the markings much more plainly with the selenites than without them, which, however, may only arise from a peculiarity of my sight, which is not only near but we&. I have not worked out this subject by any means to my satisfaction, for want of time, and therefore hope that some one else mag m&e ti more extended investigation of the whole matter.THE ANALYST. 228 The members present then inspected samples through the microscope lent by Messrs. Murray & Heath. Dr. DuprPl said anything which would make the microscope work more readily would be a considerable advantage ; a clumsy microscopist would not mistake wheaten starch for arrowroot, but with a, middling sized potato starch it required considerable experience to distinguish it. Mr. Wigner was convinced that all the assertions made by Dr. Tripe were correct, and it would, he thought, be a most useful process ; he had not yet, however, carried it beyond the starches which Dr. Tripe had mentioned.