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brass plugs, inserted at a, b, c, d, e, g, h, k. Putting plugs at h and e, and connecting the poles of a galvanic cup at the bindingscrews A and C, and the current runs successively through the tro coils R, each causing deflection in the same direction. Let R represent the resistance of one coil of the galvanometer, then the resistance of the galvanometer will be 2R. This arrangement is used in working with ordinary galvanic currents.
If instead of the former connections, plugs be put at a, d, g, and h, the wires from the source of electricity being connected at E and F, then the galvanometer resistance becomes JR. This arrangement is to be used with circuits of small resistance, such as thermo-currents. For this kind of work the instrument is thoroughly adapted.
This instrument can also be used as a differential galvanometer. To do this, put the positive pole of the battery at E. Plug a and c. Divide the negative wire into two equal branches wbich are to be connected at B and D. The circuit being thus closed, the needle evidently remains at zero. Introducing any wire the resistance of which is to be determined, into one branch, bring the needle to zero again by introducing known resistances into the other, and the unknown resistance is readily determined. In measuring fractions of an ohm, a rheochord is, all things considered, the best. The contacts are good, and an audience obtains a better idea of what is meant by electrical resistance than when a resistance box alone is used. Using platinum wire weighing 7:37 grams per meter, the resistance of which is one ohm to 192.9 cm. of wire (which is 96.45 cm. on the instrument scale), and thousandths of an ohm can be measured direct.
If ground connections are made the negative pole of the battery is sent to ground direct, and the branches of the current from B and D are sent to ground through the unknown resistance and the resistance box respectively.
Shunts may be introduced into either of the half circuits. This may be done by introducing coils of resistance ;R or 7'R, between the binding screws A, B or C, D. These wires may also be wound upon metallic plugs, which have been split lengthwise, the parts being insulated and each being connected with one extremity of the wire. Permanent shunts may be introduced by connecting one extremity with plates A or D, the other extremity being attached to an insulated plate, to be put in contact with B or C by means of a solid metallic plug. These shunts are used in Latimer Clark's differential galvanometer, and their use in measuring resistance is too well known to need further explanation.
The advantages possessed by this galvanometer are:
1. It is easily adjusted to any vertical lantern, from which it can be removed in a moment if desired.
2. The distance between the deflecting coils being readily varied, it can be adjusted to currents of various intensity.
3 The resistance of the galvanometer is quickly varied from one-balf, to twice the resistance of one of the galvanometer coils.
4. The coils may be replaced by others when desired.
5. It can instantly be converted into a differential galvanometer and used in measuring resistance.
6. It can be constructed in any work-shop at a very small expense.
St. Louis, Oct. 25, 1875.
Art. XI.— On a new occurrence of Tartronic Acid, with some
remarks on the Molecular Structure of Glyceric Acid ; by Sam. UEL P. SADTLER.
(Read before the American Philosophical Society, September 17, 1875.)
In the Propyl series, nine normally formed acids are possi.
do.OH, and the following are the acids considered as having the molecular structure just given :
I, propionic acid ; II, lactic acid (of fermentation); III, pyruvic or pyro-racemic acid; IV, ethylene lactic acid; V, glyceric acid; VI, carbacetoxylic acid; VII, malonic acid; VÍII, tartronic acid; IX, mesoxalic acid.
In one or two of these cases, however, there is still a difference of opinion as to whether the acid named is the one possessing the normal molecular structure given above, or is only an isomer of it, having its carbon atoms differently united. Notably with glyceric acid is this yet an open question. Some results lately obtained in the course of a study of this acid appear to me to be of value for the solution of this question.
The other view of the molecular structure of glyceric acid makes it unsymmetrical, two of the carbon atoms being doubly united. The formula given is
CH,OH ;d.oh Oi
Prof. Wislicenus, however, frankly gives one experiment made by bimself, the result of which tends the other way. He reduced the ß iodo-propionic acid by sodium amalgam and obtained what appeared to be the normal propionic acid, showing the regular molecular structure.
In favor, moreover, of the normal structure for the molecule of glyceric acid is the formation of pyruvic or pyroracemic acid.
Co.oH from glyceric acid upon heating this to 140°, explained by the following reaction:
Co.OH CO.OH. The structure of this pyruvic acid is known from the fact that acted upon by nascent hydrogen it gives normal lactic acid.
A strong additional argument would be had, if we could show a connection between glyceric acid,
Co.oh and tartronic acid,
Co.oH. Hitherto tartronic acid had not been formed from glyceric acid, but only in an indirect way, by the spontaneous decomposition of nitra-tartaric acid, according to the following reaction :
Co.oH However this mode of formation was interesting as tending to show its symmetry of structure. For that matter a dibasic, triatomic acid could hardly exist, except by the assumption of two carboxyl groups.
I have been fortunate enough to find tartronic acid associ. ated with glyceric acid in the oxydation products of glycerine. The preparation of the two acids was as follows: One part by weight of glycerine is mixed with one part of water, and to the mixture is added, by means of a long funnel tube reaching to the bottom of the cylinder, about one and a quarter parts of
red fuming nitric acid. After allowing them to rest until all gas evolution has ceased (which usually takes some six days), the solution is evaporated down at a gentle beat until the fumes of nitric acid are no longer perceptible. It is then very thick and syrupy. It is now diluted with water, and plumbic carbonate is added in excess. The oxalate and undissolved carbonate are filtered off, and the solution slightly concentrated and allowed to crystallize. The glycerate of lead deposits in thick crystalline crusts. These are separated from the motherliquor, dissolved, and the lead precipitated out from the solution by sulphuretted hydrogen.
The colorless or ligbt straw-colored filtrate is somewhat concentrated, and calcic carbonate is added to neutralization. The solution is filtered, if necessary, and to the filtrate is added 95 per cent alcohol. The calcium salts present are all precipitated, in greater part at once, and completely on standing twelve hours.
If the solution bad been very concentrated the calcium salt is precipitated in a granular condition. If, on the other hand, it was more dilute, the salt only separates gradually, and has a beautiful micaceous and scaly appearance.
I had at first considered this precipitate to be pure calcium glycerate, but found on dissolving it in water, in order to free it from the lime and obtain the glyceric acid, that while the greater portion dissolved readily in warm water, a considerable portion, although not more than one-tepth of the whole amount, remained and dissolved only on continued boiling. This, when tiltered off and washed in cold water, appeared as a dull white, almost impalpable powder, contrasting in appearance with the crystalline glycerate.
It was dried carefully at 100° until constant weight was obtained.
Calcium deterininations were first made. Weighed portions were ignited in a platinum crucible once or twice with excess of concentrated sulphuric acid until the weight remained constant.
-5755 grms. salt yielded .4925 grms. Caso, equal to 25°22 per cent Ca.
•1759 grms. salt yielded •1505 grms. Caso, equal to 20:16 per cent Ca.
The theoretical per cent of calcium in calcium tartronate is 25-32, wbile in calcium glycerate, allowing for two molecules of water of crystallization, it is 13.99.
I had analyzed the micaceous preparation of calcium gly. cerate about the same time, and had gotten in two determinations, 14:03, 14:07 per cent of calcium respectively. The difference was so great that I could not understand it. On