534 
SUMMARY OF CURRENT RESEARCHES RELATING TO 
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 conclusive, such experiment should be carried out in 
four different directions: — 
(1) 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 sapientium. 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. B e-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 osmic acid, all the other tests act in precisely the 
same way ; even Moll’s, preferred to all others by some of our Conti- 
nental confreres , being as unsatisfactory as the rest. But sooner or 
later its characteristic 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 its absence from the same tissues 
when acted upon by ordinary tannin reagents, recourse was again had 
to epidermis. The experiment succeeded in all cases ; among these may 
be cited Fatsia japonica, wallflower, 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 solutions 
of tannin. Moreover, with gallic acid a grey-green one is thrown 
down, thus affording an easy means of distinguishing between these 
bodies. 
