118 =Mr. T, Tate’s Experimental Researches on the 
direction of the current of filtration, the filter had to a consider- 
able extent regained its original power. 
Precisely similar results were obtained with filters of wood- 
charcoal and unsized paper. 
As it is scarcely possible to obtain filters, especially of char- 
coal and coke, of precisely the same internal structure, different 
filters, even of the same substance, vary yery much with respect 
to the value of the ratio expressing the change in their filtermg 
power. The rate of change of the filter in the foregoing ex- 
periment was unusually great. 
The results recorded in Experiment XXI. were no doubt to 
some extent affected by the progressive deterioration of the 
filterimg power of the-coke; but it must be observed that this 
filter, having been previously tested, underwent an exceedingly 
small change during the process. 
The following experiments were made to determine the analogy 
subsisting between the filtration of liquids through porous sub- 
stances, and the discharge of liquids through small perforations 
made in thin plates. 
The results of these experiments showed that the velocity of 
discharge through the orifices of ‘thin plates varies according to 
a certain power of the column of liquid pressure, that is, v ac A”, 
where the exponent z 1s constant for orifices of the same dia- 
meter, but for orifices of different diameters it varies between the 
limits n= and n=1. 
For orifices less than +,th of aninch diameter, n=4 ; that is, 
the velocity of discharge, in this case, varies as the square root 
of the column of liquid pressure. 
For orifices about ;45th of an inch diameter, n=1, that is, 
the velocity, in this case, varies as the depth of the column of 
liquid pressure. 
Moreover it was found that the velocity of discharge increases 
in a high ratio with the increase of temperature, especially for 
the smallest orifices, and also that it varies with the adhesiveness, 
and even, in some cases, with the chemical composition of the 
liquids. 
Hence it would appear that the laws of filtration are, in some 
respects, analogous to the discharge of liquids through minute 
perforations not exceeding ;4,,th of an inch diameter. 
Experiment XXVII. 
In this experiment the liquid was discharged from a small 
orifice z,th of an inch in diameter, made in a thin plate cemented 
to the bottom of the filter-tube of the foregoing experiments. 
