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And it is seen that the greatest abasement of temperature occurs when there is about 40-60 of salt to 160–140 of water (say 25 per cent. of salt). This result is what we should expect, remembering that the saturated solution contains 26•27 per cent. of salt. Table II.- Maximum Cold produced on mixing Salt and Water

in different proportions by weight.

Temp. of salt = 20°.2 C.

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$ 7. Specific Gravity of Brines of various strengths. The question as to the source of cold when brines are diluted can be conveniently approached from an examination of the density of brines of different strengths, and the comparison between the observed specific gravities and the theoretical specific gravities, supposing no change of volume to take place. The brine was found to have a specific gravity of 1.2011 at 26° C., the temperature at which the determinations were made. In Table III. the columns 1 and 2 give the weights in grams of the quantities of water and saturated brine. Columns 3 and 4 give the percentage of the water and NaCl in the resulting brines. Column 5 gives the specific gravity found, and column 6 gives the specific gravity calculated under the hypothesis that no alteration of volume ensues.

TABLE III.-Specific Gravity of Brines of various strengths.

(1)
(2)

(3) (4) (5) (6) Water, in Saturated brine, Water, Naci, Observed Calculated grams.

per cent. per cent. spec. grav., spec. grav.

in grams.

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There is accordingly an increase of volume when a solution of salt is diluted ; and this is of course connected with the absorption of heat examined in $ 4, Table I. When the original temperatures are restored, a mixture of strong brine and water may have a volume two hundredths greater than the sum of the volumes of its constituents. And when the brine contains as little as 4 per cent. of salt, its specific gravity is sensibly smaller than if such a proportion had been the result of the mixture without contraction of the strongest brine and water.

$ 8. Refraction of Light by Brines of different strengths.-It seemed, from a consideration of the numbers in Table III., that at or near the point of saturation a definite hydrate of salt exists; and that this is not merely diluted, but also actually decomposed on the addition of water, so that expansion takes place. This I imagined might be tested by the change in the refractive index of the brine. Accordingly brines of various strengths were placed in a hollow prism of 60°. The refraction was measured on a goniometer (Babinet's) table provided with telescope and collimator. An alcohol-flame containing sodium was employed as a source of light shining through a very fine slit. The minimum refraction being obtained, the D line was split by the spiderthread. In Table IV. the empty prism is considered as giving a displacement of 0°. The angular displacements are alone here given, as they perfectly suffice for the detection of singular values.

TABLE IV.
Refraction of Light by Brines of various strengths.

Temp. = 22° C.

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These numbers show a singular value at about the 10- to 11. per-cent. solutions.

§ 9. Another method of attacking the question is offered by the examination of the boiling-points of various brines. Accordingly I took solutions made by mixing saturated brine with water; and waiting for a day on which the barometer stood nearly at the mean, I determined the boiling-points of the brines. The vessels were tall copper cylinders. In the Table V. the temperature of the steam, as well as that of the brine itself, is given.. The boiling was, of course, only continued a short time, to avoid the error of strengthening the brine.

TABLE V. Boiling-points of Brines of various strengths in copper cylinder.

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We have bere again a singular value about the 10. to 1l-percent. solutions.

§ 10. Freezing-points of Brines of various strengths.--The molecular separation of water from salt when a brine is boiling has to some extent a counterpart in the separation of ice from brine when the latter is subjected to cold. If a weak brine such as 9 of water to 1 of saturated brine (that is, 1 of salt to 242 of water) is subjected to cold, pure ice begins to be formed in this case at — 10.5. The temperature gradually sivks; but, as has been shown, the solid part consists of ice, which may be completely freed from salt by mere pressure. This formation of ice continues, and the temperature sinks until the inferior limit attainable by an ice-salt freezing-mixture is reached (-22° C.). The brine, of course, becomes richer and richer in salt. In other words, brines richer in salt yield up ice at lower temperatures. Thus, in Table VI. The same brines are examined as were examined in the preceding Tables. The various brines were examined in succession, being contained in small beakerglasses in an ice-salt freezing-mixture. It is noteworthy that all

the brines are supersaturable in regard to ice, so that the tempe-
ratures are most exactly determined by two observations. “A
good deal of ice is allowed to form ; and this is then suffered
nearly entirely to disappear ; the temperature is observed at
which the ice begins to increase in quantity when the brine is
again subjected to cold.

TABLE VI.
Temperatures at which Brines of various strengths give up Ice.

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(3)

(4)

Temperature
Per cent. of

at which icc
NaCl.

is first formed.

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100
110
120
130
140
150
160
180
185
190
195
200

140
130
120
110
100
90
80
70
60
50
40
20
15
10
5
0

18.389
17-075
15.762
14448
13.130
11.821
10-508
9.194
7.881
6 567
5.254
2.627
1.970
1.313
( 656
0.000

- 150
-- 12.4
-11:1

9-4

7.7
- 77

6.7
5.4
4:)
3.4
2.1
1:5

1:9 (?)

1.5 0.0

Again the brines which contain 10 to 11 per cent. of salt have singular behaviour in regard to the temperature at which they yield ice. These are the very solutions, it will be remembered, which behave singularly in respect of their refractive indices and also of their boiling points. On comparing with Table III., this singularity does not manifest itself in regard to the specific gravities.

§ 11. Effect of cooling saturated Brine. It is seen from Table VI. that, as far as the strengths of brine there examined extend, the stronger the brine the greater the cold required to separate ice from it. The strongest brine there examined contains 18-389 per cent. of NaCl. If saturated brine (containing 26.27 per cent. of NaCl) be cooled, quite a different class of phenomena ensues. Down to 0° C. no solidification whatever ensues either of ice or of salt. At — 7° crystals of the bihydrate are observed to fall (NaCl + 2 H2O). These present a beautiful appearance of iridescent scales heavier than the mother-brine. Their composition has been examined by Löwitz, Fuchs, Nölle,

per cent.

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and Mitscherlich, whose analyses, however, are not in good
accord, that of Löwitz being as much as 10 per cent. different
from the calculated percentage of the bihydrate. Ehrenberg
and Frankenheim state that the same hydrate is produced when
a dilute solution of chloride of sodium evaporates at 15° C., and
that it suddenly converts itself into the cubical anhydrous salt
and water. At -22° C. the whole of this hydrate appears to be
removed. The gradual impoverishment of the mother-brine
was tested by maintaining a brine, at first saturated, for half an
hour at the successive temperatures – 10°, -16°, and -21° to
-22° C. The brine had been previously kept at 0° for an hour.
The original brine contained 26-2724 of NaCl per cent. It was
the mother-brine from each crop of crystals which was subjected
to the lower temperature.
Saturated brine at 0° contained

26-2724 of NaCl. Mother-liquor after keeping at-10°

24-6528 -10°

24 6187 -16°

24:1182

-21° to 22° 23.8874 It is clear, therefore, that down to -21° the solidification impoverishes the brine,—a result which might be due to the formation of the bihydrate or the precipitation of the anhydrous salt, but which is inconsistent with the formation of ice alone. The determinations were made by weighing 10 or 12 grammes of the brines, when at the atmospheric temperature, into longnecked flasks, evaporating and heating to about 300° C.

$ 12. A quantity of brine which had been thus impoverished by being kept for an hour at -21° to - 22° was decanted into another flask and further cooled by contact with solid carbonic acid and ether. The whole (3 or 4 ounces) solidified; and the temperature remained perfectly constant at -22° to -23° (say -23) until the last drop had frozen at — 23°; it then sank rapidly.

If the vessel be continually shaken during crystallization, the form of the crystals may be very clearly seen. While the anhydrous salt crystallizes in the well-known cubes, the bihydrate separates as iridescent scales, and the body we are now considering solidifies in acicular bundles radiating from nuclei, and much resembling in appearance the supersaturated solution of sulphate of sodium when solidifying. It is, however, of a more than pearly whiteness, and finally of complete opacity.

8 13. To see whether we have here indeed a definite hydrate, the whole was remelted by the warmth of the hand and successively partially refrozen under continual agitation. Each mother

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