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Патент USA US2116958

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May 10, 1938.
K. P. BRACE Er AL
2,116,958
MEANS FOR CIRCULATING FLUIDS
Filed April 19, 1935
2 Sheets-Sheet l
.
31;
‘
3nventor4
ffemperPBrace
ZRObertBPQr'a 0rd
5 W(Ittomeg
“If
May 10, 1938.
K, p_ BRA¢E ET AL
2,116,958
1 MEANS FOR 'CIRCULATING FLUIDS
Filed April 19, 1935
2 Sheets-Sheet 2
i
m
\.
M
Mu
ffemperPBrace
3nnentoud
Z?’ober?i POmwfard
015W
Gttorneg
Patented May 10, 1938
2,116,958
‘ um'rao ,S'l'A'i‘ES PATENT OFFICE
'
2,116,958
'
MEANS FOR CIROULATING FLUIDS _
Kemper l‘. Brace and Robert B. P. ‘Crawford,
'
-
Washington, D. 0.
Application April 19, 1935, Serial No. 17,368
10mm. (Cl. 62-1195)
This invention deals with the forced circulation
of ?uids and more particularly with the propul
a non-electrolyte, the osmotic pressure of a solute
pressure without the employment of pumps or
other mechanical moving parts. An object of
lowering caused by the solute in the solution. The
equation connecting the lowering of the vapor
sion of one liquid into another under a higher _ in a solution is a function of the vapor pressure
the invention is to provide a method and ap
pressure and the osmotic pressure is as follows:
_R-T P°"'P -_1_(p0—p)’ 1(p0—p)‘
‘ paratus for continuously concentrating a solu
l0
tion which is being continuously vdiluted and for
continuously diluting a solution which is being
continuously concentrated thereby maintaining
P_TS( p0 +2
a method and apparatus for pumping the liquid
in an absorberv type of refrigerator from the ab
20 sorber into the boiler without the use of mechani
cal pumps and without the use 01' hydrogen or
other gases commonly used to balance the pres
sures in systems of this kind.
10
S=Speci?c volume of the solvent at P
=Vapor pressure of the pure solvent
p=Vapor pressure of the solution.
A further more speci?c object is to provide
a method and means for. supplying liquid to a
chanical moving mechanism.
A still further more speci?c object is to provide
'
T=Absolute temperature
‘
16 boiler without the use of a pump or other me
po
R=Gas constant=82.07
the degree of concentration substantially con-.
stant.
p0 +3
where P=0smotic pressure in atmospheres
From the (above formula it can be readily seen 15
that the osmotic pressure of any solution which
is of su?icient concentration to lower the vapor
pressure appreciably of the solvent is very high.
For example, the. osmotic pressure of a six molar
(67%) sugar solution is 232 atmospheres or 3410 20
lbs. per square inch whereas the vapor pressure
lowering of a six molar sugar solution
Further objects and advantages will become
LIL'LP)
pc
25 apparent as‘ the description proceeds. _
Referring to the accompanying drawings which
25
is only 20% at 70 degrees F.
, ‘
are made a part hereof and on which similar ‘
‘The
fact
that
the
osmotic
pressure
of
a
six
reference characters are used to denote the same _
molar sugar solution is equal to 3410 lbs. per
> part thruout,
30
Figure 1 is a view in elevation of one of the
simplest forms of apparatus embodying the in
vention,
.
'
Figure 21s a view in elevation of a refrigerat
ing system of thevabsorber type showing the ap
35 plication of the invention,
Figures 3 and 4 are modi?ed forms of refrig
erating apparatus showing the invention.
It is well known that if a pure solvent is sepa
rated from a solution of a solute in the same
solvent by a semi-permeable membrane, there will
be a ?ow of solvent thru the membrane into the
solution. This ?ow will occur even when the
pressure upon the solution is much greater than
the pressure upon the solvent.
The pressure
45 necessary to be applied to the solution to prevent.
this ?ow is called the osmotic pressure of the solu
As would be expected, the‘ osmotic pres
square inch means that if a semi~permeable mem
brane separates the solution from pure solvent, 30
that is, frompure water, a pressure of 3410 lbs.
per square inch would have to be applied- to the
solution to prevent water from passing thru the
membrane into the solution.
11' instead of separating a solution from a pure 35
solvent, a solution is separated from a stronger
solution by meansoi' a semi-permeable mem
brane, there will be a ?ow of pure solvent from
the weaker‘solution to the stronger and the pres
sure causing the ?ow is equal to the difference 40
between the osmotic vpressures of the two solu
tions. The ?ow will continue until the concen
tration of the two solutions is the same, that is
until both solutions have the same osmotic pres
sure.
‘
45
Now if the stronger solution is continually con
sure of any solution depends upon the co'ncen- v centrated by boiling oil? the solvent and the
weaker solution is continually diluted by the ad
tration of the solution, that. is, the higher the
50 concentration, the higher the osmotic pressure. dition of pure solvent, then the ?ow of pure
n For solutions, of non-electrolytes such as cane - solvent" thru the membrane from the weaker solu 50
sugar, the osmotic pressure is equal to the pres-’v tion to the strong solution will continue as long as
sure which an equivalent gram molecular volume there is a difference in concentration.
Figure 1 shows one of the simplest embodi
of the solute would exert if con?ned to the space
55 occupied by the solution. Due to the e?fect of ments of the invention in the form of apparatus
ionization, the osmotic pressure of electrolytes for continuously concentrating a solution which 55
is being continuously diluted and for diluting a
such as NaCl, NaOH, LiC'l, etc., is as a rule con
‘ tion.
, siderably higher than that of non-electrolytes of
the same concentration. It hasibeen found that
for any substance, whether it is an electrolyte or
solution which‘ is being continuously concen
trated. _Vessels Ill and l l contain weak solution
I 2- and strong solution l3 respectively of some
non-volatile solute in a suitable solvent, such for 60
2,1 18,958
2
example as cane sugar in water. The solutions
are separated by a semi-permeable membrane 15.
Pure solvent may be boiled o? from the strong
solution I3 by heating element 14, thereby con
centrating the solution and pure solveht'may be
supplied by pipe [8 to the solution 12 to dilute
this weak solution. So long as the concentration
of the solution 13 is greater than the concentra
tion of the solution 12 there will be a ?ow of pure
10 solvent thru the semi-permeable membrane from
chamber 18 into. chamber ll.
-
We may assume that the solution 12 is a 1.0
molar solution of cane sugar in water and that the
solution 13 is a six molar solution of cane sugar
15 in water. The osmotic pressure of a six molar
with the solutions and solvent, all air and other
foreign gases are evacuated from the system and
the system closed. The evaporator will be sur
rounded by or in heat exchange relation with
the media to be cooled. Heat extracted from this
medium will evaporate the refrigerant. Now the
vapor pressure above the pure solvent 28 is
greater than the vapor pressure above the weak
solution 28.
There will therefore be a ?ow of
vapor from chamber 18 thru channel 8| into 10
chamber 19, this vapor being absorbed into the
solution 28. This evaporation of vapor from the
solvent ‘body 28 and absorption into 28 will cause
the temperature in 18 to fall and the temperature
in IS to rise. The added heat in solution 28 may
solution is 3410 lbs. per square inch. The osmotic be carried o? by'the cooling coil 25. The addi
pressure of a 1.0 molar solution is 397 lbs. per tion of pure solvent to the weak solution 28 will
square inch. Therefore, the force driving pure tend to dilute this solution. But the solution in
solvent (water) from the weak solution 12 into the boiler 20 is stronger than the solution 28.
There will, therefore, be a flow of pure solvent
20 the strong solution 13 is 3410 minus 397 lbs. per thru the semi-permeable membrane 82 from the
square inch or‘3013 lbs. per square inch, and
nothing will stop the ?ow of solvent from the body of solution 28 into the solution 21. The
weak into the strongsolution except an external heater 24 continuously boiling of! pure solvent
pressure on the strong solution of 3013 lbs. per from the solution 21 continuously concentrates
this solution. The addition of pure solvent from
25 square inch. It is apparent from the foregoing absorber 19 will tend to keep the solution at the
that a semi-permeable membrane provides
means for causing a solvent to ?ow from a weak same degree of concentration.
The rate of flow of pure solvent from I8 to 28
solution under a low pressure to a strong solution
is directly proportional to the di?erenoe between
under a. high pressure without the aid of a me
the osmotic pressures of the solutions 21 and 28
30 chanical pump. Pressure may be built up in the minus the difference in total pressure in vessels
boiler by providing a throttling valve 11.
i9 and 211. Thus if the solution 28 is a 1.0 molar
From‘ the structure described it will be ap
sugar solution having an‘ osmotic pressure of 397
parent that the device may have many possibili
ties in its adaptation to useful purposes. For lbs. per square inch and solution 21 is a 6 molar
sugar solution having an osmotic pressure of 3410
35 example, if the vapor under pressure from the lbs. per square inch and if the pressure above the
boiler is conducted to a ?uid motor the device
two liquids be ignored for the present, there will
may serve as a boiler for an engine in which the
boiler is fed without the use of mechanical pumps be a difference in pressures of 3410 minus 397, or
3013 lbs. per square inch and this will represent
or injectors.
the force driving the pure solvent from solution
Figure 2 shows the invention adapted to a re
40
frigerating system of the absorber type. In this 28 into- solution 21.
For practical purposes a solution of sodium by
form of refrigerator a vapor is absorbed into a
solution. As here shown numeral 18 indicates an droxide (NaOH) . and water is preferable to the ‘
cane sugar solution. Let us assume, therefore,
_ evaporator, 19 an absorber, 20 a boiler, 2i a con
. that water and NaOH are the‘ solvent and 'the
denser,
22
a
vapor
conducting
connectionfrom
45
the ‘boiler to the condenser and 123 a connection . solute-respectively. Let us assume that the tem
from the condenser‘ to the evaporator. v Vapor is perature desired in the evaporator is 40 degrees
boiled o? by a heater 241. A cooling coil 25 cools F. and that the ‘temperature of ‘the tap water for
the liquid in the absorber._ This cooling coil may cooling coil 25 is '10 degrees. Now the vapor pres;
be supplied with cooling water which passes thru .. sure of water at 40 degrees is 6.4 mm. of Hg'and
so 'pipe
26 to the condenser after leaving coils 25. If therefore in order for the-water 29_ in the evapo
preferred both the cooler 25 and the condenser rator 18 to boil at 40°, the vapor pressure of the
may be air cooled; :such cooling systems are well solution 28 in the-absorber 19 must be less'than'
known and therefore need ‘not be here described; 6.4 mm. of Hg at a temperature slightly in ex
‘ Before starting, the boiler will be partially ?lled "cess of 70 degrees (say perhaps 80 degrees). Ex
perimental data showithat the vapor pressure of
_ with a suitablesolntion. . .This may be a solution
a 40% solution of NaOH in water exerts a vapor
of cane ‘sugar and water asstated in the descrip
tion of the system shown hf‘Figure 1, or a solution . pressure of 5.3 mm. of Hg. at 80°. Y Therefore, we
of any suitable non-volatile solute, such as NaOH, can assimie that the concentration of the solu
tion 28 is 40% and that the driving force causing
v60 .LiCl, H2804, Lil, ZnCl, etc., in a solvent.
Numeral 21 indicates the strong solution in the solvent vapor (water) to pass from l8 to'l9 as
boiler, 28 a weak solution in the absorber and 29 6.4-5.3,‘that is as 1.1 mm. of mercury. Withan
the pure solvent in the evaporator. There will unrestricted passage 31 between the chamber 18
be a body of solvent'vapor 30 above the body .of and‘ the chamber 19 water vapor will pass at a
rapid rate from 29 to 28 and the temperature of
65 solvent in the evaporator. The upper portion of
29 will be maintained at 40 degrees as long as the
‘the evaporator communicates with the upper por
temperature of the solution 28 is kept at 80 de
tion of the absorber thru a passage 3|. A semi
permeable membrane 32 separates the strong grees and the concentration at 40%‘.
In‘ order to maintain the concentration at 40%
solution 21 in the boiler from the weak solution
pure solvent must pass thru the semi-permeable 70
28
in"
the
absorber.‘
Suitable
examples
of
such
70
membrane as fast as solvent vapor passes from
membranes are collodion, parchment, animal in
testine, animal bladder, ?sh skin, rubber,-copper 29 to 28. In order to maintain this osmotic ?ow
ferrocyanide precipitated in porous earthenware, of pure solvent from weak solution" into strong
solution 21 the concentration of 21 must be much
porcelain, ground glass, gelatine, etc.v
After ?lling the system to the desired levels greater than'28. Let us assume that the concen- 7‘
75
2,116,958
tration of 21 is 50% by weight. By the formula
given, the osmotic pressure of a 40% solution 28
at 80 degrees F. is 28,300 lbs. per square inch.
The osmotic pressure of a 50% solution 21 at 165
degrees F. (B. P. at 36 mm. Hg) is 35,750 lbs.
per square inch. The di?'erence- between the
osmotic pressure of solution 21 and solution 28 is
7,450 lbs. per square inch. Experimental data
indicate that the amount of semi-permeable
10 membrane required for a 100 lb. ice box is five
square feet.
Figure 3 shows- a form of the invention in
which an interchanger 40 is positioned between
the concentrator and the absorber. It will be
noted that there is a temperature difference of
about 85 degrees between the concentrator and
the absorber. It is advisable to prevent a loss of
heat from the concentrator to the absorber. The
interchanger is designed to reduce the heat loss
20 as far as possible, and thereby increase the em
ciency of the system. Numerals 4|, 42, 48 and
44 denote the condenser, the evaporator, the ag
3
ammonia in the condenser at a temperature of
110 degrees.
"
Now the vapor pressure of pure ammonia 45
in the evaporator 42 at 15 degrees F. is 2100 mm.
of Hg. In order to have a ?ow of vapor from
the evaporator to the absorber the vapor pressure
of the weak absorbing solution must be less than
the vapor pressure of the pure solution. The
vapor pressure of a 60% solution of NH‘CNS in
ammonia at 100 degrees F. is approximately 2000 10
mm. of Hg. This gives us a vapor pressure diifer
ence‘ between the evaporator and the absorber
of 100 mm. of Hg. We may therefore assume
that the weak solution 46 is a 60% solution of
NH4CNS in NH3. According to the formula the 15
osmotic pressure of a 60% solution of NH4CNS
in NH; at 100 degrees F. is 40,719 lbs. per square
inch. Assume that the strong solution 41
in the boiler is an 80% solution of NH4CNS
in‘NHa. From the formula we note‘that the 20
osmotic pressure of an 80% solution of NHiCNS in
NH: at 100 degrees F. is 49,677 lbs. per square
T e
inch. It should be noted that the strong solu
evaporator contains pure solvent 45, the absorber tion which is circulated from the boiler is cooled
contains weak solution 46 and the boiler a strong by the jacket 64 and by the body of weak solu
solution 41. Solvent is boiled off by heater 48. tion at 100 degrees temperature which surrounds
Cooling water for the cooler 48 is supplied thru the chamber 58.
pipe 50 the water leaving thru pipe 5! after pass
Now as we have stated above, the vapor pres
ing thru the condenser 4| to condense the vapor sure above the ‘weak solution 46 is 2000 mm. of
from the boiler. Both cooler 48 and condenser Hg or about 38 lbs. per square inch. The vapor 30
4| may be air cooled if desired.
pressure above the strong solution is the‘ vapor
The upper portion of the evaporator chamber pressure of pure ammonia at condenser tempera
is connected by pipe 52 with a pipe 58 which ex
.ture, which as we-have stated is 110 degrees F.
tends down into the weak solution 46 and ter
This vapor pressurelis about 250 lbs. per square
minates in a header 54 which is perforated to inch. The force tending to drive pure ammonia 35
permit vapor to pass into the solution 46 and be liquid from the weak solution into the strong is
sorber and the concentrator or boiler.
absorbed therein. The absorber is connected by
pipes 55 and 56 with a chamber 51 thru which
the weak solution is circulated by convection.
40 A chamber 58 is ‘positioned within the chamber
51 and its walls are formed partially from semi
permeable membraneous material 59. The
strong solution in the boiler is circulated thru
the chamber 58 passing from the boiler thru pipe
45 63 thence thru the interchanger 40, pipe 60, pipe
61, back thru the interchanger 40 and pipe 62
into the boiler. A water jacket 64 may be placed
about the pipe 60 to cool the strong solution be
50' fore it enters the chamber 58.
This jacket may
be supplied with water from the coil or water
cooling jacket 48.‘
'
The heater may include a ?ue 61 which passes
up thru the boiler to effect better heating.
Refrigerant vapor driven oi the boiler passes
thru pipe '65, condenser 4| ‘and into receiver 66.
In order to insure that only liquid refrigerant
shall pass from the receiver to the evaporator
a valve 68 operated by a ?oat 69 controls passage
of refrigerant to the evaporator.
‘
Any suitable refrigerant may be used. For
domestic refrigerators requiring evaporator tem
‘ perature of 20 degrees or lower it is believed the
most satisfactory solution ‘would be a solution
of ammonium thiocyanate (NHiCNS) in liquid
ammonia (NH:).
v
‘
Let us assume that the temperature desired in
the evaporator is 15 degrees F. and that the tem
perature of the tap water or cooling air, when air
is used, is 90 degrees. We will assume that the
cooler 48 is large enough to maintain a tempera
ture of 100 degrees in the weak solution 46. We
will assume also that the cooling water in passing
thru the cooler 48 is heated to approximately‘ 100
degrees and that it is e?ective to condense the
(49,677+38) —(40,719+250) =8746 lbs. per square
-
inch
»
,
It is therefore apparent that in spite of the pres
sure difference of 212 lbs. in the boiler and the
absorber pure liquid will ?ow from the absorber
into the boiler at a rapid rate thru the semi
permeable membrane due to osmotic force.
Figure 4 shows another embodiment of the in
vention. Numerals 10, 1| and 12 represent the 45
evaporator, the boiler and the absorber. The ab
sorber consists of a plurality of tubes having
walls of semi-permeable membraneous material
connecting headers‘!!! and 82. This absorber is
positioned in a chamber 13 where it is enveloped
in vapor passing from the evaporator thru pas
sage 14. The solution circulating by convection '
thru the absorber is cooled by air cooler 15 of
any suitable sort. Pure solvent vapor in chamber 55
13 will be absorbed thru the walls of the tubes
and will tend to dilute the solution therein. To
maintain the proper degree of concentration in
the tubes, a portion of the solution is circulated
thru the boiler thru pipes 11,18 and interchanger
18. This circulating solution ‘is cooled by air
cooler 16. This pipe 18 may be coiled around the
?ue 80 to effect better heating.
The boiler will be partially ?lled with a suit
able solution. This may be a 60% solution of 65
NHACNS in NHa. The pure solvent in the evapo
rator may be liquid ammonia 82. Ammonia
vapor boiled o? from the boiler will pass thru
pipe 93, be condensed by air condenser 83 and
delivered to receiver 84. A valve 85 controlled 70
by ‘bellows 86 will control passage of liquid from
pipe 81 to pipe 88 so .as to prevent the passage
of gas to the evaporator- Pressure in the bellows
and pressure below the valve will be equal since
both are subject to the pressure in the condenser
9,118,958
4
causing osmotic ?ow of pure solvent from the
weaker to the stronger solution.
3. A refrigerator of the absorber type compris
thru pipes 81 and 88 so that'the valve 85 will
be closed by gravity. When liquid accumulates
in the leg 88 of the U-tube this will unbalance
the valve and permit liquid refrigerant to pass
thru the valve into the line 89.
ing a boiler, an absorber, an evaporator, a con
denser and heater, the boiler having therein a
strong solution of a solvent and a non-volatile
solute, the absorber having therein a weaker
solution of the same solvent and solute, means
Instead of the pressure operated valve Just de
scribed we may use the ?oat controlled valve
shown in Figure 3.
Assume that a temperature of 15 degrees F. is
10 desired in the evaporator and that the cooling
for conducting vapor from the evaporator into
the weaker solution to have it absorbed therein, 10
means for circulating said weak solution and for
media for the absorber is at a 90 degree tempera
ture and that the coolers are suf?ciently large to
maintain a temperature of 100 degrees in the
absorption solution and a temperature of 110 de
15 grees in the condenser. We will assume, as stated
above, that the solution Si is a 60% solution of
. NHlCNS in NIB.
cooling it, a semi-permeable membrane separat
ing the weak from the strong solution, means
for circulating said strong solution in surface
contact with said membrane to circulate the pure
solvent absorbed from the weaker solution and
bring it into the boiler to dilute the solution
therein which is being continuously concen
trated by boiling oif pure solvent therefrom.
4. A refrigerator of the absorber type compris
Now, as we have stated, the
vapor pressure of pure ammonia at 15 degrees is
‘2100 mm. of Hg.
The vapor pressure of a 60%
solution of NH4CNS in NH: at 100 degrees F. is ing. a boiler, an evaporator, an absorber, a plu
2000 mm. of Hg. The osmotic pressure of a 60%
rality of tubes positioned in the said absorber, said
solution of NHrCNS in NH: is 40,719 lbs. per tubes being ?lled with a solution of a solute and
square inch. Since there is a pressure in the a solvent, said tubes being exposed to solvent
absorber chamber 13 outside the tubes of about ' vapor from the evaporator, said tubes having
'38 lbs. per square inch the total force tending to walls of semi-permeable membranes to cause the
drive the pure solvent into the solution thru the solvent to flow by osmosis from the absorber to
semi-permeable membrane is I 40,757 lbs. per the solution, means for carrying. of! heat from
square inch. But the vapor pressure of the pure the solution, and means for circulating said so
ammonia in the condenser is the vapor pressure lution thru the boiler, and means for boiling of!
at a temperature» of 110 degrees which is about pure solvent from the solution, for condensing it
250 lbs. per square inch. Taking this from the and returning it in liquid form to the evaporator.
pressure indicated above, since this is a pressure
5. A refrigerating system of the absorber type
acting against the flow of solvent into the strong
comprising a still, an absorber, an evaporator, a
solution, we have a total pressure acting to drive
condenser, and a semi-permeable membrane sep
arating vapor of the pure solvent in the evapo
rator from solution circulated from the still, a
liquid\trap between the condenser and evapo
solvent vapor into the solution of 40,507 lbs. per
square inch.
,
8
From the foregoing it will be apparent that we
have a system in which the solvent will be auto
matically supplied to the boiler and that the sys
tem will operate continuously without. the neces
rator, and a pressure controlled balanced valve
operable upon a rise of liquid in the trap for re 40
turning only liquid refrigerant to the evaporator.
sity of a pump or the presence of an inert gas to
.6. A device of the kind described comprising
tion without departing from the spirit thereof.
We, therefore, do not limit ourselves to the in
a pair of chambers, one containing a solution of
a volatile solvent and a non-volatile solute, the
other containing a body of the solvent substan-,
tially free of the solute and under a pressure less
than the pressure in the ?rst named chamber, a
vention as shown in the drawings and- as de
semi-permeableymembrane separating the solu
scribed in the speci?cation but only asset forth
tion from vapor of the body of pure solvent, and
means for vaporizing solvent from the solution 50
balance pressures.
,
'
It will be obvious to those skilled in the art
that various changes may be made in the inven
in the appended claims.
>
in the ?rst named chamber, condensing it, and
1. A device of the kind described comprising a
vessel having a compartment therein having a
solution of a pure solvent and a nonvolatile solute
dissolved therein, a second compartment having
a body of pure-solvent therein under a pressure
less than that in the ?rst named compartment,
delivering it to the body of solvent in the other
What we claim is:
.
chamber, the said membrane causing pure solvent
to flow by osmosis from the body ofpure solvent
to the, solution.
.
r
55
'1. A refrigerating system of the kind described
comprising a boiler containing a solution of a
a semi-permeable membrane separating the two . solute and a volatile solvent, a chamber, having
bodies of liquid to-cause a ?ow of solvent by
osmosis into the solution, means for boiling o?
walls of semi-permeable membraneous material,
in communication with the solution in the boiler,
solvent from the solution and means for supply
ing pure liquid solvent to the body of pure sol
thermal means for circulating the solution‘ from
vent.
,
2. A device of the kind described comprising av
vessel divided into two chambers by a semi-per
65
meable membrane, a concentrated solution of, a
the boiler thru the said chamber, an evaporator,
means for boiling off vapor from the solution, con
densing it and delivering it to the‘ said evaporator,
means for conducting refrigerantvapor from said 65
evaporator into surface contact with said semi
solvent and a nonvolatile solute in one chamber,
permeable membraneous walls while maintaining
a less concentrated solution of the composition in
said vapor under pressure appreciably lower than
the pressure in said chamber, the solvent vapor
being caused to ?ow by osmosis only thru the
semi-permeable membraneous walls of the said
chamber into the solution in said‘ chamber.
KEMPER P. BRACE.
the other chamber, and a third chamber vhaving
a body of-pure solvent therein, the space above
70
said last named solvent being open‘to the space
»_above the weaker solution to permit passage of
vapor from said solvent to the said solution ‘to bev
absorbed therein, the semi-permeable membrane
ROBERT B. P. CRAWFORD.
70
.
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