III. EUTECTIC METALLIC ALLOYS.
Although many fusible alloys have been long known, I believe no true eutectic metallic alloy had been studied until Dr. Guthrie[6] worked at the subject, employing the same methods as with his cryohydrates. It is found if two metals are fused together and the mixture allowed to cool, that the temperature falls until a point is reached at which that metal which is present in a proportion greater than is required to form the eutectic alloy begins to separate. If this solid be removed as it forms, the temperature gradually falls until a fixed point is reached, at which the eutectic alloy solidifies. Here the thermometer remains stationary until the whole has become solid, and, on remelting, this temperature is found to be quite fixed. In addition to the di-eutectic alloys, we have also tri- and tetra-eutectic alloys, and as an example of the latter we may take the bismuth-tin-lead-cadmium eutectic alloy, melting at 71°.
We have already seen with salt eutectics that, given the curve of melting-points of a mixture in various proportions, we may predict the existence, composition, and melting-point of the eutectic alloy. As a matter of course, the same thing holds good for metallic eutectics. An interesting example of this is furnished by the tin-lead alloys, the melting-points of which have been determined by Pillichody.[7] From these determinations we obtain the curve given in Fig. 2, and from this curve, since it dips below a horizontal line passing through the melting-point of the more fusible constituent, we are at once able to predict a eutectic alloy. We should further expect this to have a constitution between PbSn3 and PbSn4 and a melting-point somewhat below 181°. On melting together tin and lead, and allowing the alloy to cool, we find our expectation justified; for by pouring off the fluid portion which remains after solidification has commenced, and repeating this several times with the portion so removed, we at length obtain an alloy which solidifies at the constant temperature of 180°, when the melting-point of tin is taken as 228°. On analysis 1.064 grm. of this alloy gave 0.885 grm. SnO2, which corresponds to Sn 65.43 per cent., or PbSn3.3. This, therefore, is the composition of the eutectic alloy, and it finds its place naturally on the curve given in Fig. 2.
Fig. 2.
It will be seen that the subject of eutexia embraces many points of practical importance and of theoretical interest. Thus it has been shown by Dr. Guthrie that the desilverizing of lead in Pattinson's process is but a case of eutexia, the separation of lead on cooling a bath of argentiferous lead poor in silver being analogous to the separation of ice from a salt solution. Dr. Guthrie has also shown that eutexia may reasonably be supposed to have played an important part in the production and separation of many rock-forming minerals.
It is with considerable diffidence that I suggest the following as an explanation of the multitude of facts to which previous reference has been made.
In a mixture of two substances, A and B, we have the following forces active, tending to produce solidification:
1. The cohesion between the particles of A.
2. The cohesion between the particles of B.
3. The cohesion between the particles of A and the particles of B.
With regard to this last factor, it will be seen that there are three cases possible:
1. The cohesion of the mixture A B may be greater than the cohesion of A + the cohesion of B.
2. The cohesion of A B may be equal to the cohesion of A + the cohesion of B.
3. The cohesion of A B may be less than the cohesion of A + the cohesion of B.
Now, since cohesion tends to produce solidification, we should in the first case expect to find the melting-point of the mixture higher than the mean of the melting-points of its constituents, or the curve of melting-points would be of the form given in a, Fig. 3. Here no eutectic mixture is possible.
Fig. 3.
In the second case, where cohesionA B = cohesion A + B, we should obtain melting-points for the mixture which would agree with the mean of the melting-points of the constituents, the curve of melting-points would be a straight line, and again no eutectic mixture would be possible.
In the third case, however, where cohesionA B is less than cohesion A + B, we should find the melting-points of the mixture lower than the mean of the melting-points of its constituents, and the curve of melting-points would be of the form given in e, Fig. 3. Here, in those cases where the difference of cohesion on mixture is considerable, the curve of melting-points may dip below the line e f. This is the only case in which a eutectic mixture is possible, and it is, of course, found at the lowest point of the curve.
If it be true, as above suggested, that the force of cohesion is at its minimum in the eutectic alloy, we should expect to find, in preparing a eutectic substance, either that actual expansion took place, or that the molecular volume would gradually increase in passing along our curve of melting-points, from either end, for each molecule added, and that it would obtain its greatest value at the point corresponding to the eutectic alloy.
Of this I have no direct evidence as yet, but it is a point of considerable interest, and I may possibly return to it at some future time.—Chemical News.
Read before the Birmingham Philosophical Society, January 22, 1885.
Guthrie, Phil. Mag. [5], xvii., p. 462.
Guthrie, Phil. Mag., 4th Series, xlix., pp. 1, 206, 266; 5th Series, i., pp. 49, 354, 446, vi., p. 35.
F. Guthrie, Phil. Mag. [5], xvii., p. 469; F.B. Guthrie, Journ. Chem. Soc,. 1885, p. 94.
Comptes Rendus, 1883, 2, p. 45.
Phil. Mag., 5th Series, xvii., p. 462.
Dingler's Polyt. Jour., 162, p. 217; Jahresberichte, 1861, p. 279.
CHINOLINE.
Dr. Conrad Berens, of the University of Pennsylvania, reaches the following:
1. Chinoline tartrate is a powerful agent, producing death by asphyxia.
2. The drug increases the force and frequency of the respirations by stimulating the vagus roots in the lung.
3. It paralyzes respiration finally by a secondary depressant action upon the respiratory center.
4. It does not cause convulsions.
5. It lessens and finally abolishes reflex action by a direct action upon the cord, and by a slight action upon the muscles and nerves.
6. It diminishes or abolishes muscular contractility respectively when applied through the circulation or directly.
7. It coagulates myosin and albumen.
8. It causes insalivation by paralysis of the secretory fibers of the chorda tympani; increases the flow of bile; has no action upon the spleen.
9. It lowers blood-pressure by paralyzing the vaso-motor centers and by a direct depressant action upon the heart muscle.
10. It diminishes the pulse rate by direct action upon the heart.
11. It lowers the temperature by increasing the loss of heat.
12. It is a powerful antiseptic; and, finally,
13. Its paths of elimination are not known.
METHOD FOR RAPID ESTIMATION OF UREA.
Being called upon to make a good many brief and rapid analyses of urine on "clinic days" of our medical department, I devised the following modification of Knop's method of estimating urea; and after using it for a year with perfectly satisfactory results, venture to describe and recommend it as especially adapted for physicians' use, by reason of simplicity, cheapness, and accuracy. In perfecting and testing it I was assisted greatly by J. Torrey, Jr., then working with me.
The apparatus consists of the glass tube, A, which is about 8 cm. long and 2½ cm. in diameter, joined to the tube, B, which is about 25 or 30 cm. in length in its longer arm and 8 or 10 in its shorter, and has a diameter of about 5 mm. Near the bend is an outlet tube, c, provided with "ball valve" or pinch cock. d, e, f, g, are marks upon the tubes. C is a rubber cork with two holes through which the bent tube, D, passes. D is of such size and length as to hold about 1 c.c., and one of its ends may be a trifle longer than the other.
The apparatus is used as follows: Remove the cork and pour in mercury until it stands at e and g, then fill up to the mark, f, with sodium or potassium hypobromite (made by shaking up bromine with a strong solution of sodium or potassium hydroxide). Next carefully fill the tube in the cork with the urine, being careful especially not to run it over or leave air bubbles in it. This can easily be done by using a small pipette, but if accidentally a little runs over, it should be wiped off the end of the cork with blotting paper. The cork is then to be inserted closely into the tube; the urine tube being so small, the urine will not run out in so doing. The mercury is then drawn out through c till it stands in B at d. Its level in A will of course not be changed greatly. Now, incline the apparatus till the surface of the hypobromite touches the urine in the longer part of the urine tube, and then bring it upright again. The urine will thus be discharged into the hypobromite, which will of course decompose the urea, liberating nitrogen, which will cause the mercury to rise in B. Shake until no further change of level is seen, and mark the level of mercury in B with a rubber band, then remove the cork, draw out the liquid with a pipette, dry out the tube above the mercury with scrap of blotting paper, pour back the mercury drawn out, and repeat the process to be sure that no error was made.
If now two or three marks have been made upon the tube, B, indicating the height of the mercury when solutions containing known per cents. of urea are used, an accurate opinion can be at once formed as to the condition of the urine as regards urea.
As is well known, normal urine contains about 2.5-3 per cent. of urea, so that graduations representing 2, 2.5, 3, and 4 per cent. are usually all that are needed, though of course many more can be easily made.
The results obtained with this apparatus have been repeatedly compared with those of more elaborate ones, and no practical difference observed. Evidently the same apparatus, differently graduated, might be employed to determine the carbonate present in such a substance as crude soda ash or other similar mixture. In such a case the weighed material would be put upon the mercury with water and the small tube filled with acid.
Bowdoin College Chemical Laboratory.
—F.C. Robinson, in Amer. Chem. Jour.
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