378 
Rhubarb. 
The same process may be successfully employed in the 
case of rhubarb, and as small a quantity of turmeric as one 
tenth per cent, may be detected in no more than two and half 
grains of rhubarb. There is not the least difficulty in dis- 
tinguishing by this means inferior pale rhubarbs, coloured by 
turmeric so as to resemble those of the best quality. 
The detection of small quantities of gamboge, added to 
increase the purgative action of these inferior kinds, is not 
so easy, and requires some care, especially in the presence of 
turmeric. The suspected powder should be placed on a small 
filter, and so little alcohol added as will give a few drops of 
clear solution, which would contain a comparatively large 
amount of the very soluble gamboge, and but little of the less 
soluble colour of the rhubarb itself. This alcoholic solution 
should then be agitated with bisulphide of carbon, and the 
supernatant alcoholic solution removed by means of a pipette 
and blotting-paper, and the solution in the bisulphide eva- 
porated to dryness. This would contain a colour peculiar to 
the different kinds of rhubarb, and, perhaps, also turmeric 
and gamboge. If a small quantity dissolved in benzol gives 
the green fluorescent solution, turmeric is present ; and in 
order to detect the gamboge it would then be requisite to re- 
dissolve in alcohol, and add a little nitric acid, which so changes 
the colour of the turmeric that it is no longer removed by 
agitating with bisulphide of carbon ; whilst the gamboge 
remains unchanged and is abstracted by it from the alcoholic 
solution. After separating this, evaporating the bisulphide 
to dryness, and redissolving in alcohol, a small quantity of 
an alcoholic solution of iodine should be added, and then 
excess of ammonia, a little sulphite of soda, and a drop of 
Avater to make it dissolve. This ensures the complete removal 
of any free iodine that may remain after the addition of 
ammonia. 
in the case of pure rhubarb this solution, when sufficiently 
but not too strong, has a decided pink colour, giving no 
well-marked absorption band in any part of the spectrum, 
but transmitting the blue rays very considerably better than 
the green ; whereas, when gamboge is present to the extent 
of two per cent, or upwards, the yellow colour due to it cuts 
off the blue light, and the solution is orange colour. The 
spectra are as follows : 
Pure rhubarb . . . . . 4 ... 8 9 10 — 11 — 
Rhubarb, with 2 per cent, of gamboge . 4 ... 9 — 10 — 
„ „ 4 „ „ 4... 8— 9- 
Gambogc alone . . . . .7 •• 8 — 9 — 
