586 



NA TURE 



\_Apru 24, 1890 



Nessler's test is made, as all the world knows, by saturating a 

 solution of potassium iodide with mercuric iadide, and adding 

 an excess of caustic potash. Ammonia gives with this a reddish 

 precipitate ; tannin a brown, and when in considerable quantity 

 a deep black one ; but if little tannin be present, the brown may 

 tend towards purple. It goes without saying that much experiment 

 must be undertaken before one can be sure of the substance 

 giving the brown precipitate being really tannin. To be con- 

 clusive, such experiment should be carried out in four different 

 directions : — 



(i) The reaction ought to be given in all cases when the 

 ordinary reagents make their presence immediately felt. 



(2) Cells which will not immediately give the tannin reaction 

 with ordinary tests, but which will do so with Nessler's test, 

 must also do so under the former conditions if time be allowed. 



(3) Tissues which will not yield the reaction with Nessler's 

 test, must not give it with any other reagent even after the lapse 

 of some time. 



(4) Solutions of tannin must give a brown precipitate with 

 Nessler's test. 



Under the first of these headings may be mentioned growing 

 shoots of the garden rose. On laying a radial longitudinal or a 

 tangential section of this in Nessler's fluid a copious black-brown 

 precipitate is obtained, and the same thing occurs with the 

 beautiful tannin-sacs of Musa sapienttim. In all other instances 

 where tannin has betrayed its presence by the use of ordinary 

 reagents, the brown colour has been obtained upon treatment 

 with Nessler's test. 



The primrose leaf may be again cited as an example of the 

 time sometimes necessary to show up tannin with the usual 

 reagents, of which it must here suffice to particularize ammonium 

 molybdate. On laying in the molybdate a small piece of epidermis 

 torn off the lower side of the leaf, one first sees a cell here and 

 there coloured the characteristic and beautiful yellow given by 

 this test : these coloured cells are usually situated among the 

 elongated more or less rectangular cells overlying the vascular 

 bundles. Re-examination after half an hour or so shows 

 several more of the cells similarly coloured, but it is usually not 

 till after a couple of hours that one can safely declare all 

 the tannin-containing cells to have been stained. With variations 

 in respect of time, and with the sole exception of osmicacid, all 

 the other tests act in precisely the same way ; even Moll's, pre- 

 ferred to all others by some of our Continental confreres, being 

 as unsatisfactory as the rest. But sooner or later its charac- 

 teristic colour is imparted to these cells by every reagent, thus 

 proving tannin to be present. 



For the negative experiment — the absence of the brown colour 

 from tissues treated with Nessler's fluid, and iLs absence from the 

 same tissues when acted upon by ordinary tannin reagents — re- 

 course was again had to epidermis. The experiment succeeded 

 in all cases : among these may be cited Fatsia japonica, wall- 

 flower, box, Stellaria media, and Pelargonium zonale. In none 

 of these did tannin show up, although twenty-four hours were 

 allowed to elapse before the preparations were destroyed. 



Lastly, Nessler's fluid gives a rich brown precipitate with solu- 

 tions of tannin. Moreover, with gallic acid a grey-green one is 

 thrown down, thus affording an easy means of distinguishing 

 between these bodies. 



For these reasons, therefore, viz. the rapidity, certainty, and 

 distinctness of its action ; the ease with which it can be made ; 

 its permanence when made ; and lastly, the difference in its be- 

 haviour towards tannin and towards gallic acid — for these reasons 

 I am bold enough to anticipate the time when, to adapt a 

 hackneyed expression, Nessler's fluid will be regarded as a reagent 

 which no botanical laboratory should be without. 



Spencer Moore. 



The Moon in London. 



Some years ago a weekly paper represented a young rustic 

 asking his mother, " Be that the same moon they have up to 

 Lunnon ? " to which question the mother evasively replied, 

 "You leave the moon alone and go to bed." The boy was 

 satisfied by retorting, "I baint a touching on it." But his 

 question is this month brought once more to the front by the 

 following passage, which will be found in one of our most im- 

 portant monthly magazines. " But if," says the writer, "there 

 is an abuse of the deductive method of reasoning, there is also 

 an abuse of the inductive method. One who refused to believe 

 that a new moon would in a month become full, and, disre- 



garding observations accumulated throughout the past, insisted 

 on watching the successive phases before he was convinced, 

 would be considered inductive in an irrational degree." We 

 cannot, of course, presume to dictate to or for the moon "up 

 to Lunnon," but here in the country the new moon becomes full 

 inhalf a month, and we have convinced ourselves by watching 

 the successive phases that a new moon will in a month become 

 a new moon again. Nevertheless we willingly admit that life is 

 far too short and too encumbered to allow of any man's repeat- 

 ing more than a small fraction of the accumulated observations 

 on which his scientific beliefs are founded. Yet, on the other 

 hand, taking things for granted is probably the source of nine- 

 tenths of the errors that fill our minds, while the men of genius 

 seem to be just those who know best what and how to observe 

 for themselves, and how much to trust in the observations of 

 others. T. R. R. Stebking. 



Tunbridge Wells. 



Foreign Substances attached to Crabs. 



There is, of course, no analogy between whiffing for 

 mackerel with red flannel, and fishing for cod on the bottom 

 with any kind of bait. 



If Actinians are offensive to fish, it is a singular fact that, 

 when a cod-line is baited with mussels, herring, sand-eels, and 

 anemones (viz. /. crassicornis and A. mesembryanthemum), the 

 latter prove by far the most successful baits. 



Impalement on a hook by no means kills an anemone, whose 

 powers of offence are, perhaps, little lessened thereby ; and 

 under natural conditions the tentacles are not always expanded. 

 Though the full-grown cod does not affect the tidal waters of 

 the coast, yet the "rock" cod, by no means the youngest of its 

 species, ventures close inshore ; and the largest cod abound 

 amongst the tidal waters of the Bell Rock. 



The cuidas of an anemone seem very efficient weapons against 

 a soft-skinned Cephalopod, but they are not necessarily so 

 against a tough-skinned fish. 



Prof. Mcintosh, in the work referred to in a previous letter, 

 records Icalia and Peackia from the stomach of the cod, and 

 Edzvardsia (in swarms) from that of the flounder. He also in- 

 forms me that he has found Stomphia in the stomach of the cod. 

 I may add that the practice of baiting here with anemones is 

 much more recent than the work referred to. 



Of all British Coelenterates, Cyanaa is, perhaps, the most 

 deadly ; yet many trustworthy observers have found young cod 

 sheltering themselves beneath its umbrella — a fact which seems 

 to indicate that they hold its stinging powers in some contempt ; 

 and Dr. Collingwood, in "A Naturalist's Rambles in the 

 China Seas" (p. 150), has recorded the discovery of an immense 

 fish-sheltering anemone. Ernest W. L. Holt. 



St. Andrews Marine Laboratory. 



The Relative Prevalence of North-east and South-west 

 Winds. 



In a note at p. 470 (Nature, March 20), attention is drawn 

 to the statement by Mr. Prince contained in his meteorological 

 summary of observations taken at Crowborough, Sussex, in 

 1889, concerning the greater prevalence of north-east as com- 

 pared with south-west winds which he finds to exist in recent 

 years. The writer of the note mentions that this is not borne 

 out by the Greenwich observations, but some definite statistics 

 as regards Greenwich, and distinct comparison with the Crow- 

 borough numbers, may perhaps not be unacceptable to your 

 meteorological readers. 



Mr, Prince remarks that in previous years he finds only two 

 years in which north-east winds have been in excess of south- 

 west. In the first, 1864, the days of north-east wind were 104, 

 of south-west wind 89 ; in the second instance, 1870, the days 

 of north-east wind were 107, of south-west wind 88. The 

 corresponding Greenwich numbers were, in 1864, 43 and 108 ; 

 and in 1870, 65 and 96. 



On the average of the years 1859 to 1883 Mr. Prince gives 

 north-east wind on 63 days, south-west wind on 99 days. The 

 corresponding Greenwich values are 43 and iii respectively. 

 For the years 1885 to 1889 he gives the average frequency of 

 different winds as follows, to which I have added the values for 

 Greenwich. C. indicates Crowborough, and G. Greenwich. 

 N. N.E. E. S.E. S. S.W. W. N.W. Calm. 

 C. 41 102 21 22 38 72 50 17 — days. 

 G. 49 52 35 23 37 100 40 19 10 days. 



