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If, for instance, a platinum wire of 1.5 metre in length and 0.5 millimetre in diameter is to be heated to redness, how many Bunsen's cups, of the electro motive force of 800 and the resistance of 10, must be used and how are they to be combined?
The resistance to conduction of a copper wire 1 millimetre in diameter and 1.5 metre in length is 1.5; that of a like platinum wire is 5.1,5=7,5. But the resistance of a wire of one-half the diameter is four times as great, viz : 30. This would be the resistance at the usual temperature; but when the wire is red hot it is at least twice as great, viz :
If we suppose that the resistance of the other part of the closing circuit is comparatively so little that it may be neglected, we have
60, and, for our case, a 172.0,5 = 86. Therefore,
86, consequently, n= 12,9
from which it follows that a battery of 12 double elements has to be employed.
It is evident from this example that in the above mentioned experiments the arrangement was not the most advantageous.
If a copper wire 1 millimetre in diameter and 0.5 metre in length is to be heated to redness, its resistance would be 1, supposing it to be twice as great at a red heat as it is at the usual temperature. If the resistance of the rest of the closing circuit is also equal to 1, its total will be equal to 2; but a in this case is 433, and therefore
2.16, m= 10.8. We have, therefore, to use a battery of two elements, each of which consists of 11 cups.
A more accurate knowledge of the resistance to conduction of metals at a red heat would be necessary to give a greater degree of exactness to these calculations.
In general more cups in a series will be required for producing ignition if the wires are bad conductors and of greater length, and more cups, side by side in each element, if they are good conductors and of greater diameter.
$ 60. Ignition of metallic wires in different gases.-Grove has made the remarkable observation that platinum wire heated to redness by the voltaic current in atmospheric air, is apparently extinguished when covered with a bell-glass, filled with hydrogen.—(Phil. Transact., 1847, pt. 1 ; Pog. Ann., LXXI, 196.) Since the resistance to conduction is greater in a wire intensely ignited than in one the heat of which is less intense, it was to be expected that, cæteris paribus, the same wire when in hydrogen would conduct a stronger current than in atmospheric air.
Grove proved the correctness of this conclusion in the following
manner : In the circuit of a constant
battery besides a platinum wire, which could conveniently be surrounded by an atmosphere of different gases, a voltameter was inserted. The intensity of ignition in the platinum wire was found to be very different in the different gases, but, at the same time, the rate of the decomposition of water in the voltameter was also changed, so that in equal times the quantity of detonatiog gas obtained was greater as the heat evolved by the wire was less. The following quantities of detonating gas were obtained per minute in the voltameter when the platinum was immersed in the gases enumeated: In hydrogen
7.7 cubic inches. olefiant gas....
7.0 carbonic oxide......
6.6 carbonic acid.
6.4 atmospheric air....
6.4 do. condensed
6.5 do. rarified ......
6.3 chlorine ..
6.1 With the appearance of light in the wire, the heat produced in it also is greater, as is demonstrated by the following experiment: The bulb of a thermometer was placed at a certain distance from a coil of wire, which was heated to redness by a battery of 4 cells. When the coil remained in atmospheric air the thermometer rose 15° in five minutes, but when it was immersed in hydrogen the rise, during the same interval, was only 7.5o.
Poggendorf, in a note, expresses the opinion that this phenomenon may be connected with the observation formerly made by Dulong and Petit, that a heated body is more rapidly cooled in hydrogen than in atmospheric air. To me this view seems inadmissible, for if the wire in hydrogen gives out more rapidly the heat developed in it, the thermometer ought to rise more rapidly when the wire is placed in this gas, provided the quancity of heat produced in the wire by the galvanic current is always the same in whatever gas it is placed.
This experiment, however, is not yet decisive; but another one, described by Grove in a later memoir on the same subject, beyond á doubt refutes the above explanation of Poggendorf.—(Phil. Magazine, XXXV, 114 ; Pog. Ann. LXXVIII, 366.) Two glass tubes, A and Fig. 50.
B, fig. 50, 1.5 inch in length and 0.3 inch interior diameter, were closed at both ends with corks, which
were penetrated by copper wires, connected inside of the tube by a spiral of platinum wire o inch in diameter and 3.7
inches in length. The tube A was filled with oxygen, B with hydrogen, and the tubes were then placed in separate vessels, similar in every respect, and containing
about 3 ounces of water. A thermometer was immersed in the water of each of the vessels, and the copper wires were so connected that they formed part of the closing circuit of a constant zinc-platinum battery of 8 cells, each of 8 square inches acting surface.
When the battery was closed the wire in the oxygen became incandescent, while that in the hydrogen was not visibly ignited. The temperature of the water, which was 60° F. in both vessels at the beginning of the experiment, rose within 5 minutes to 70° in that around the hydrogen tube, and to 81° in that around the one containing the oxygen. When both the tubes were filled with the same kind of
the temperature in both vessels rose to the same degree.
This experiment decidedly proves that the appearance of less heat in the wire immersed in hydrogen, with perfectly identical strength of current, cannot be caused by a more rapid absorption of heat by the hydrogen, because then, on the contrary, the water surrounding the hydrogen tube ought to be heated sooner. All this indicates that, in fact, a less production of heat takes place in the wire when surrounded by hydrogen.
Grove has proved that this phenomenon is not caused by a small amount of conduction of electricity by the hydrogen; he has also demonstrated that it cannot be brought into any connexion with the other physical properties of the gases, their density, specific heat, &c.
As to the explanation of this peculiar fact, Grove endeavored in vain to find a tolerable one, and in the course of his somewhat dilated and obscure discussion arrives himself quite inconceivably at the conjecture that the difference of the gases might have a similar effect to a difference in the condition of the surfaces. This would essentially coincide with Poggendorf's above mentioned opinion, which was propounded, however, before the experiment with the two glass tubes of fig. 50, which in the most distinct manner refutes such a view, was known to him. But Grove gives his consent to it immediately after he has himself made and described the experiment, which proves that this basis of explanation is inadmissible, and that the phenomenon cannot be deduced from differences in conduction and radiation of heat.
In my opinion the phenomenon is still entirely isolated and unexplained. I do not think it profitable in such cases to cover up our want of knowledge with dilated disquisition, in which the physical scape-goat of our days, molecular action, has to play the principal part.
$ 61. Effect of ignited platinum wires on different gases. It is a known fact that some of the compound gases suffer decomposition in red hot tubes. Grove has produced similar effects upon these gases by the action of ignited platinum wires.—(Phil. Trans., 1847, pt. 1; Pogg. Ann. LXXI, 194.) The following is the apparatus he used for this purpose :
Into the upper end of an eudiometer tube, fig. 51, a curved platinum wire was fused, from whose extremities copper wires conducted to the two mercury cups which connected them with the poles of the battery. The gas to be examined was confined over water, and, to prevent the glass from becoming too much heated, the whole eudiome
ter tube was immersed in a wider vessel filled with water. Sometimes the water was covered with a layer of oil one inch in depth.
When the gases had to be confined over mercury, or when a longer continuation of the ignition was necessary, the apparatus of fig. 52 was used. Here the eudiometer tube is bent, and its closed end, containing the platinum wire, immersed in a vessel filled with water or oil ; the open end dipping into another vessel containing the water or mercury, used for confining the gases. With this apparatus the following results were obtained :
Nitric oxide, over distilled water, contracted in varying proportions to the heat. (The volume, of course, was not measured before the apparatus had entirely cooled.) In the best experiments the contraction amounted to one-third of the original volume. The remainiag gas was nitrogen, and nitric acid was found dissolved in the water.
Nitrous oxide was decomposed into nitrogen and oxygen ; the volume increased by 0.35 of the original. The full equivalent proportion or 0.5 could not be obtained.
Carbonic acid did not show any perceptible change.
Ammonia increased to double its original volume; the gas could no longer be absorbed by water, and consisted of 3 vol. of hydrogen and 1 vol. of nitrogen.
Olefiant gas contracted a little, and deposited carbon. The remainder was hydrogen and olefiant gas; the greater the heat the more hydrogen was formed.
Nitrogen remained unchanged.
Oxygen contracted but very little, about one-fiftieth of its volume; it might, perhaps, have contained a minute quantity of hydrogen.
Chlorine over water gave white fumes, and a grayish-yellow insoluble powder collected on the sides of the tube, near the platinum wire; this was afterwards found to be chloride of platinum. The greatest part of the chlorine combined with the hydrogen of the aqueous vapor, and the muriatic acid formed was absorbed by the water. When the experiment was finished the volume of gas was reduced to about onehalf, and the remainder was oxygen.
With bromine and iodide of chlorine oxygen was evolved, (how the experiments with these bodies were performed I could not perfectly
understand.) The residue could not be examined, because it acted both upon the platinum and upon the glass.
Hydrogen contracted very much, sometimes to one-tenth of the original volume. The cause of this contraction was a small quantity of oxygen, with which hydrogen gas is nearly always contaminated. Phosphorus brought in to the most carefully prepared hydrogen emits vapors of phosphorous acid, shines in the dark, and produces a slight contraction. But even after this, the ignited wire produces a further contraction. The phosphorus, therefore, cannot remove all the oxygen from the hydrogen.
After this experience Grove doubts the correctness of the values ascertained for the atomic weight of hydrogen.
According to these experiments it seems that it would be more advantageous to use the platinum wire ignited by the galvanic current, than the electrical spark in eudiometric experiments.
Hydrogen and carbonic acid mixed in equal volumes were easily affected by the ignited wire. They contracted to 0.48 of the original volume; the residue was carbonic oxide. One equivalent of oxygen and 1 of hydrogen had, therefore, combined together.
Carbonic oxide exhibited a remarkable phenomenon. Carefully purified from any carbonic acid, it was exposed to the action of the ignited wire over distilled water, and its volume increased from onefifth to one-third, according to the intensity of ignition.
When the gas was dry and confined over mercury, this increase of volume did not take place; it must have been dependent, therefore, upon the presence of aqueous vapor; and, in fact, the increase of volume was found to be caused by the formation of carbonic acid. By agitation with caustic potash or lime water the gas was reduced to exactly its former volume; but then it was found to be mixed with a volume of hydrogen equal to that of the carbonic acid absorbed. This is explained in the following manner: “Half a volume or one equivalent of oxygen derived from the vapor of the water had combined with one volume or equivalent of carbonic oxide, and formed one volume or equivalent of carbonic acid, leaving in place of the carbonic oxide, with which it had combined, the one volume or equivalent of hydrogen with which it had been originally associated.
On comparing this experiment with the previous one, the singular inversion of affinity under circumstances so nearly similar will appear surprising; in the former case hydrogen abstracted oxygen from carbonic acid in order to form water, leaving carbonic oxide, while in the latter the carbonic oxide takes the oxygen from the aqueous vapor to form carbonic acid and leaves hydrogen.
A more exact idea of the nature of these reactions has not yet been obtained. By the latter experiment, in which a decomposition of aqueous vapor also took place, Grove was led to the idea that it might be possible to decompose aqueous vapor and produce detonating gas simply by means of the ignited wire. He succeeded in this as will
be seen in the following:
§ 62. Decomposition of aqueous vapor by ignited platinum wire.Grove discusses the decomposition of aqueous vapor into its elements in the same memoir in which he treats of the action of the ignited