226 SELECTED ARTICLES. 
To be slightly compressed: 
Burdock, Gentian, 
Borage, Galls, 
Polygala, Parsley root, 
Saponaria, &c. 
Not to be compressed: 
Poppy flowers, Rhubarb, 
Red roses, Saffron. 
Poppy capsules cannot be lixiviated with water, and this ope- 
ration is difficult with gentian, and more especially with rhu- 
barb. This latter should be in a very coarse powder, and 
should be previously moistened with an equal weight of 
water; and even then the process succeeds only in very skil- 
ful hands. 
When water is passed through a layer of vegetable matter, 
the first portions are highly charged with the soluble portions, 
but it gradually becomes less so : more water should be used than 
is indicated by theory; first, because the soluble matter being 
contained in the cells of the vegetable, the fluid cannot act on 
it at once; secondly, because the different strata of fluid readily 
mix with each other. This has been observed by M. Baudri- 
mont in operating on the pulp of beets, and M. Guillermond 
has proved the same fact by passing a solution of an extract 
through an inert powder, and endeavouring to displace this 
with cold water. 
In lixiviation, water flows through much slower than alco- 
holic fluids, because it dilates the organic tissue and also forms 
viscous solutions with the mucous principles. It also adheres 
more firmly to the surfaces of the grains of the powder, so that 
whilst the upper strata of water readily force downwards the 
fluid interposed in the spaces between the particles of the pow- 
der, it with difficulty detaches those portions which adhere 
to the surfaces of this powder. This was also observed by M. 
Boullay with regard to oil, water forcing out every drop of this 
substance that was interposed between the grains of powder, 
but leaving all those which adhered to the surfaces of the par- 
ticles. 
