On the Laws of Absorption of Liquids by Porous Substances. 57 
of incandescent solid particles of a sodium compound has no 
very marked absorptive action on light of its own colour; but 
that to give rise to this kind of opacity it 1s necessary that the 
sodium compound should be in the state of vapour. It appears, 
moreover, to prove that it is not necessary for this vapour to be 
in the metallic state; for it could hardly be supposed that so 
highly combustible a vapour as that of metallic sodium could be 
present in that part of the flame which is seen to possess this 
great opacity. ‘T'hat soda salts are easily volatile at the tempera- 
ture of flame, is a fact abundantly proved by Bunsen*. The 
reason why the opacity is only exhibited by that part of the outer 
shell of vapour which is situated at the edge of the flame, and 
not by its entire extent, is owing to its thickness being insuffi- 
cient to produce sensible absorption on rays which traverse it 
perpendicularly ; an appreciable action taking place only when 
they pass as a tangent to the edge of the flame, and thus traverse 
a considerable extent of absorbing medium. 
VII. Experimental Researches on the Laws of Absorption of 
Lnquids by Porous Substances. By Tuomas Tate, Esq. 
[Continued from vol. xx. p. 510.] 
Il. On the Filtration of Liquids through different Porous 
Substances. 
ILTRATION is in general produced by the action of two 
forces, viz. by the force of absorption and that of pressure. 
Filters may be divided into two classes. The first class com- 
prises those substances which are highly porous, and which 
undergo little or no change during the process of filtration. 
The second class comprises “those substances with close pores, 
which under certain circumstances undergo a decided change 
during the process of filtration. The following laws (with cer- 
tain limitations) apply to both kinds of filters :— | 
1. The rate of filtration, other things being the same, varies 
directly as the area of the surface of the filter in contact with 
the liquid, and inversely as the thickness. 
2. The rate of filtration, other things being the same, increases 
in a high ratio with the increase of temperature. 
3. The rate of filtration, other things being the same, varies 
as the depth of the column of liquid upon the filter. 
And so on to other laws which will be hereafter illustrated. 
These experiments were, for the most part, made with the 
apparatus represented in the annexed diagram. 
* Phil. Mag. S. 4. vol. xviii. p. 513. 
