92 ABSORPTION AND EXHALATION. L.ECT. IV. 



observed, that in some cases it is effected by a kind of fil- 

 tration. A liquid which contains in suspension insoluble 

 particles, is separated by filtration into two portions : the 

 liquid part, which is imbibed by the substance of the 

 filter, and is strained ; and the solid part, which remains on 

 the filter. Anatomists know that by injecting into the veins 

 or arteries a solution of gelatine coloured by very finely 

 powdered vermilion, the solution becomes colourless when 

 it percolates through the coats of the vessels. Every con- 

 tusion of the skin produces a spot, the centre of which is 

 of a dark blue, and the circumference of a green colour 

 surrounded by yellow. In this case, the extravasated clot 

 of blood is separated from the serum, which is imbibed by 

 the neighbouring tissues. 



Do not forget the fact which has been demonstrated to 

 you respecting imbibition. Salt water becomes fresh by 

 traversing a bed of sand ; while a solution of carbonate of 

 soda, filtered under the same conditions, becomes denser. 

 Imbibition, capillarity, and the simple play of molecular 

 attraction, can overcome affinities. The old notion, there- 

 fore, according to which the secreting organs were consi- 

 dered as mere filtering apparatus, is not entirely without 

 foundation. 



In another lecture we shall find, that membranes, and in 

 general all organized tissue, are capable of being traversed 

 by gaseous bodies. Fodera was the first to prove, that 

 sulphuretted hydrogen, enclosed in a portion of the intes- 

 tinal tube, diffuses itself throughout the body of the animal 

 and occasions death. 



Absorption in Vegetables. A few words, also, on absorp- 

 tion in plants. Here, in these small glasses, are a great 

 number of plants, more or less immersed in a very dilute 

 solution of acetate of iron. Some of them are haricots 

 [kidney-beans, Phaseolus vulgaris ;] others beans [Vicia 



