DRAPER’S AND MRS. SOMERVILLE’S EXPERIMENTS. 
455 
water, and increases in proportion to the 
strength of the solution. By itself, however, 
oxalic acid passes more slowly through ani- 
mal membrane than water. When a solution 
of tartaric acid exceeds the specific of 1‘05, 
the endosmose takes place from the water to 
the acid ; if it is lighter than 1‘05, the pro- 
cess is reversed. The same happens with 
citric acid. 
Dutrochet terms the passage of the oxalic 
acid to the water, inverse endosmose. The 
mineral acids do not exhibit this phenome- 
non ; phosphoric acid, however, exhibits it 
for an instant, when reduced by the addition 
of water, to the specific gravity of 1*085. 
Change of temperature affects the passage of 
the acid through the membrane, as it does its 
specific gravity. 
This agrees with the experiments of Girard, 
upon the flowing of pure water, and water 
containing nitrate of potash, through capil- 
lary glass tubes. A solution of one part nitrate 
of potash in three parts water, at the tempera* 
ture of 40®, flows more readily through a ca- 
pillary tube than pure water ; while above 
40® the reverse happens. Dutrochet has found 
these observations to hold only with animal 
membranes ; not with vegetable, or thin inor- 
ganic porous p\zXes.—{Pharmaceutisches Ceir 
tral-hlatt, Feb. 1836, 92) 
2. Jerichau,*of Copenhagen, has obtained 
some interesting results on this subject, A 
forked glass tube, 1| line in diameter, was 
closed at one extremity with sealing wax, and 
then the closed leg was filled with water, the 
covered portion with mercury, and the open 
leg partly with an aqueous solution of sugar. 
'1 he tube was placed in a vertical position. 
In the course of a week the mercury in the 
closed leg rose a line, which might lead to the 
supposition that the wax hadnot been so close- 
ly applied to the glass as to prevent the exist- 
ence of any capillary opening between the 
wax and glass. To determine this point ano- 
ther equally curved tube was taken, fused at 
one extremity, and filled as before. To as- 
certain the smallest rise, a small mirror, with 
a transverse scratch upon it, was placed be- 
tween the legs of the tube, close to the bent 
legj so that the scratch, when it cut the re- 
flexion of the eye over the pupil, appeared 
as a tangent to the upper surface of the mer- 
cury in the tube. 
On a small rise of the mercury, the eye 
must be brought forward, when the scratch 
appears still to be the tangent of the mercury, 
and passes immediately through the middle of 
the reflexion of the eye. 
After trial, it was found that a tube fused 
at one end, and about a line in diameter, an- 
swered the desired purpose. This tube was 
filled with water, and then a globule of mer- 
cury, which occupied 0’7 line of the tube, was 
placed in it | some pulverized sugar was dis- 
solved in the water. The tube was then made 
fast to the mirror, and placed horizontally, in 
order that the mercury might not be pressed 
down by its own weight. The drop slowly 
progressed towards the closed end of the tube. 
After the lapse of a month, it had advanced 
about a line. The water which was expelled, 
mixed itself with the sugar solution ; and this 
was concentrated by evaporation, without de- 
positing any crystals. A solution of gum, sub- 
stituted for that of the sugar, gave the same 
result. In ten days, the drop of mercury ad- 
vanced 0*2 lines towards the closed end of the 
tube. 
The converse of this experiment was tried 
will) a portion of a straight tube, which was 
fused at one extremity. A dense solution of 
sugar was placed between the closed end and 
a drop of mercury ; and before the drop some 
water was introduced. If the latter evapo- 
rated, it was renewed. The drop advanced 
towards the open end of the tube, while the 
saccharine solution increased in volume by 
the absorption of water. 
Extending his researches in this way, with 
different liquids, Jerichau draws six inferen- 
ces from his experiments. 
1. That liquids, separated by porous plates, 
reciprocally penetrate in part through these 
plates. This, however, has been previously 
stated. 
2. The proportion of the volume, which 
passes from both solutions in equal time, de- 
pends on the nature of the solutions and par- 
tition as well as the temperature. It is not, 
however, a necessary condition, that a greater 
volume should pass through the plate from 
the one solution than from the other; or, that 
on one side of these plates a greater volume 
should enter, as Dutrochet erroneously 
thinks. 
3. When the diffusion is terminated, tha 
volume remaining on each side of the parti- 
tion may be calculated from the original vo- 
lume, being inversely, as the square root of its 
density ; as Graham has shewn with regard to 
gases. If equal volumes of a saturated solu- 
tion of common salt, and solution of sugar of 
the specific gravity 1'078, are separated by a 
bladder, the first increases in volume at first 
but diminishes in specific gravity, in a great- 
er degree than would take place by a regular 
mixture. 
4. The height to which solutions ascend in 
capillary tubes, is often proportionate to the 
quantity of liquid diffused. Thus, some solu- 
tions, which rise highest in capillary tubes, 
are conveyed in strongest streams, but there 
are many exceptions to this rule. 
5. Liquids are not only conveyed through 
solid porous plates, but also through a short 
canal between mercury and glass. 
6. By the chemical action of electricity, 
the proportion of the liquid which passes may 
be increased ; but this cause only operates, 
in so far as it separates acids, alkalies, and 
salts, 
OPTIC^. 
EFFECT OF LIGHT IN MAGNETIZING 
NEEDLES. — Mr. Draper* has repeated the 
experiment of Mrs. Somerville, which consist- 
• Journal of tlie Franklin Institute, Februa- 
ry, 1835. 
• Poggendorff’s Annalen, xxxiv. 613. 
