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The number 5622 Mr. Capron has taken by mistake from the wrong series of numbers-from spectrum No. I. instead of from spectrum No. II. The other two numbers, 5189 and 4829, I am not able to find in the • Index of Spectra’ at all. The wave-lengths actually given for the three lines of the tube-spectrum are as follows:

Spectrum No. II. j 58.

5602 k 74.

5195 1 92.

4834

I have satisfied myself that these lines are due to a compound of carbon and oxygen ; and their production in tubes supposed to contain oxygen is not at all surprising. The results obtained by the use of vacuum-tubes must always be received with caution, since it is almost impossible to form any certain conclusion as to the real nature of the small quantity of gas remaining in the tube, even when one has filled the tube one's self; and in the case of tubes purchased from makers of such apparatus, the nature of the contents is simply a matter of conjecture.

I am, &c.,

W. MARSHALL WATTS. Giggleswick Grammar School,

Ap-il 14, 1875.

EXPERIMENTS WITH THE HOLTZ MACHINE. BY F. ROSSETTI*.

In a recent series of experiments upon the lIoltz machine, first pattern, M. Rossetti sought to determine in what measure the intensity of the current produced depends on the velocity of rotation of the machine, on the work expended, and on the humidity of the air, to estimate its electromotive force, internal resistance, &c. The results at which he arrived on these points are the following.

In one and the same series of experiments, the intensity of the current is very sensibly, but not exactly, proportional to the velocity of rotation of the plate; the intensity increases a little more rapidly than the velocity of rotation.

The effect is modified by the humidity of the air, the velocity necessary to produce a certain intensity being greater in wet than in dry weather.

The work expended for the production of the electricity is exactly proportional to the intensity of the current. It was measured by the difference of the weights necessary to impress a certain velocity on the plate, according to whether the machine was charged or not.

* Nuovo Cimento, Ser. 2, vol. xii. p. 89.

The ratio between the work expended and the intensity of the current diminishes when the humidity augments—in such a manner that, to obtain a current of given intensity, a greater velocity of rotation is requisite in wet than in dry weather, but a less expenditure of work. The Holtz machine is therefore more economical in wet weather than in dry.

The distance between the two disks of the machine has also some influence on the intensity of the current: the less the distance the stronger the current, and the greater the amount of the work.

The Holtz machine, like the voltaic couples, possesses electromotive force and internal resistance. The electromotive force is independent of the velocity of rotation ; but it diminishes as the degree of humidity increases. The effective motor weight (difference between the weights necessary to turn the machine charged and not charged) is proportional to the electromotive force produced. This is very great in comparison with the electromotive forces of the most energetic voltaic couples ; in fact it has been found to amount to 433000 Siemens units with a relative humidity equal to 0.69, to 599000 with the humidity 0.35—the Daniell couple giving E=11:57, and that of Grove E 19.98. The electromotive force of the Holtz machine is therefore about 50000 times as great as that of the Daniell couple, and 30000 times as great as that of the Grove couple.

The internal resistance of the Holtz machine is independent of the hygrometric state, but varies with the rotation-velocity, diminishing more rapidly than the velocity increases. It is very great: the lowest resistance (which corresponds to the greatest velocity attainable, or 8 turns per second) is equal to 570 million Siemens units ; for a velocity of 2 turns per second it is 2810 millions of the same units.

Under these conditions a resistance inserted in the outer circuit must be very considerable in order to exert any sensible influence on the intensity of the current. It was because the resistance employed by Poggendorff was too feeble that he could not verify an effect of this kind. M. Rossetti, on the contrary, by interposing in the circuit a column of distilled water of greater or less length, has ascertained that the current of the Holtz machine is susceptible of being very notably weakened by augmentation of the external resistance, following Ohm's law in this equally well with the ordinary galvanic currents.

The author has deduced from his experiments a measure of the mechanical equivalent of heat, by comparing the serviceable work expended for the production of the electricity with the total heat which could be developed by the current obtained. He found for that equivalent the number 428.-Bibliothèque Universelle, Archives des Sciences, March 15, 1875, pp. 250–252.

ON THE SOLUTION OF HYDROGEN IN METALS, AND THE DECOM

POSITION OF WATER BY IRON. BY MM. L. TROOST AND P. HAUTEFEUILLE.

In previous researches on the alloys formed by hydrogen *, we have pointed out the characters by which these definite combinations may be distinguished from the solutions of hydrogen in the metals. We have seen that besides potassium, sodium, and palladium, which can combine with hydrogen, there are other metals which simply dissolve this gas. The number of those which possess this last property appears to be considerable.

We shall see that iron, nickel, cobalt, and manganese, which are united by the analogy of their chemical properties into a natural group, present great similarities in their behaviour in the presence of hydrogen at various temperatures. As the facility with which they absorb or give out hydrogen depends largely on their physical state, it is necessary, in order to account for the differences observed, to investigate these metals successively in ingots, in thin plates, and in the pulverulent condition.

1. Nickel.-An ingot of pure nickel, cast in lime, was submitted for twenty-four hours, at a red heat, to the action of a current of hydrogen gas, and then cooled slowly in the gas. The volume of hydrogen extracted from it at a red heat in vacuo was one fifth of the volume of the metal.

Some laminæ of nickel, obtained by decomposing with the pile the double sulphate of nickel and ammonia, were heated in a vacuum to 200° C. ; they gave out forty times their volume of hydrogent. On afterwards being heated to near 200° in a current of hydrogen and slowly cooled in this gas, they absorbed sixteen times their volume of it, which they gave up in vacuo at 200°. The same laminæ, placed for twenty-four hours at the negative pole of a voltameter, absorbed about ten times their volume of hydrogenț.

The pulverulent nickel was obtained by reducing the oxide or a mixture of oxide of nickel and alumina by means of hydrogen at 300°. Nickel thus prepared is pyrophoric, as Magnus has shown g. In a vacuum it gives up a certain quantity of hydrogen at the ordinary temperature; but to expel this gas entirely a dull red heat is requisite. The total volume of gas discharged is about one hundred

* Comptes Rendus de l'Acad. des Sciences, vol. lxxviii. pp. 686, 807. Phil. Mag. 8. 4. vol. xlvii. p. 397.

† The gas analyzed did not give any perceptible quantity of nitrogen. Some laminæ prepared in the same way, then washed and dissolved in chlorhydric acid, gave traces of ammonia.

I M. Raoult states (Comptes Rendus, tol. Ixix. p. 826) that the impure porous nickel cubes of commerce, when placed at the negative electrode of á voltameter, absorb 165 volumes of hydrogen, which they gradually disengage at the ordinary temperature. The same cubes electroplated with pure nickel did not appear to him to disengage any appreciable quantity Š Annales de Chimie et de Physique, 2 série, vol. xxx: p. 103. Phil. Mag. S. 4. Vol. 49. No. 326. May 1875. 2 F

of gas.

times that of the metal. Submitted, at a dull-red heat, to the action of a current of hydrogen, it reabsorbs & volume sensibly equal to that which it had discharged. The metal is again pyrophoric after the hydrogen is expelled.

II. Cobalt.-An ingot, cast in lime, of pure cobalt was submitted to a red heat for twenty-four hours in a current of hydrogen, and then cooled slowly in that gas. The volume of hydrogen extracted from it in vacuo at a red heat was only one tenth of that of the metal.

Laminæ of cobalt, obtained by galvanic decomposition of the double sulphate of cobalt and ammonia, were heated in vacuo to 200°. They gave up thirty-five times their volume of hydrogen* Heated subsequently in a current of hydrogen to about 200°, and again cooled slowly in the same gas, they absorbed twenty-four times their volume of it, which they set free again in vacuo at 200°. The same laminæ, placed during twenty-four hours at the negative pole of a voltameter, absorbed seven times their volume of hydrogen.

Pyrophoric cobalt loses its hydrogen in vacuo still more readily than nickel. Instead of making a vacuum, the condensed gas can be expelled by putting the metal into a small balloon furnished with a discharge-tube and filled with water exhausted of air. Heated to 100°, all the gas is disengaged in a few hours. The volume of the gas thus collected is about one hundred times that of the metal; the cobalt, too, is again pyrophoric after complete expulsion of the hydrogen. Submitted at a dull red heat to the action of a current of hydrogen, it reabsorbs a volume equal to that which it has set free.

III. Iron. We have previously proved t that 1 kilogramme of soft iron in the form of an ingot can dissolve at about 800°, and afterwards set free in vacuo at the same temperature, 20 cubic centims. of hydrogen, or one sixth of its own volume. Under the same conditions 1 kilogramme of grey pig-iron, wood-cast, dissolves 88 cubic centims. of hydrogen, or more than the half of its volume .

It is known that the iron obtained in decomposing by the pile chloride of iron in the presence of sal ammoniac, when plunged into hot water disengages hydrogen and at the same time a small quantity of ammonia, as was proved by MM. Meidinger and Kroemerll. M. Cailletet I has recently obtained in this way a volume of hydrogen equal to 260 times that of the metal.

* Analysis of the gas did not show any sensible quantity of nitrogen. Some laminæ prepared in the same manner, then washed and dissolved in chlorhydric acid, gave, like the nickel, traces of ammonia.

Comptes Rendus, vol. lxxvi. p. 562.

* We have since ascertained that iron wire, hardening slightly by steeping, dissolves at a red heat nearly one fourth of its volume of hydrogen; the same wire, after cémentation, dissolved one third of its volume of the gas. The solubility of hydrogen in steel increases, therefore, with the amount of carbon contained in the latter.

§ Dingl. Polytech. Journ. vol. clxjii. p. 283. li Arch. Pharm. 2nd Series, vol. cv. p. 284.

Comptes Rendus, vol. lxxx. p. 319.

Pyrophoric iron obtained by reducing at a low temperature either the sesquioxide alone or a combination of oxide of iron and alumina (precipitated from their chlorides by ammonia), gives up, like pyrophoric nickel and cobalt, all its hydrogen in vacuo, and, like them, retains the property of ignition at a low temperature in air. As to the quantity of hydrogen which can be fixed by pyrophoric iron, its determination presents special difficulties. The cold metal loses in vacuo a portion of the gas which it absorbed. Boiling water, by the use of which we succeeded in obtaining the hydrogen dissolved in pyrophoric nickel or cobalt, gave with iron entirely different results. In fact, the pyrophoric iron which comes from the reduction of the combined oxides of iron and aluminium, put with air-exhausted water into a small balloon furnished with a discharge-tube, gave a continuous disengagement of hydrogen when heated: thus 1 grm. of iron liberated 10 cubic centims. of gas per hour; and the liberation went on until the iron was almost completely oxidated. The water was therefore decomposed at about 99° by the minutely divided iron. Pyrophoric iron from the reduction at a low temperature of the hydrate of the sesquioxide alone, decomposes water with a rapidity nearly equal to that of the metal combined with alumina.

With respect to the pulverulent iron less minutely divided which is obtained on reducing by hydrogen the sesquioxide resulting from the calcination of the nitrate, it also decomposes water at about 99°, but the decomposition is much slower. The reduced iron of commerce and the spongy iron obtained by the galvanic pile behave in the same manner*.

Not being able to determine by immersion in boiling water the volume of the gas condensed in pyrophoric iron, we essayed to determine it by keeping the iron in cold water; but here again we had to recognize the decomposition, though slower, of the water: 1 gramme of pyrophoric iron, kept in water deprived of air and at 150, liberated hydrogen regularly during two months.

In brief, iron, nickel, and cobalt directly absorb hydrogen; but we cannot affirm that combination takes place: this is what we have already proved for lithium and thallium. Pyrophoric iron, nickel, and cobalt condense a greater quantity of gas than the compact metals; but the whole of the gas is liberated below a red heat, and the metals deprived of hydrogen continue to be pyrophoric: this property therefore does not depend on the presence of condensed hydrogen. Finally, iron in a state of minute division exhibits a property which is not found in either nickel or cobalt; it decomposes water slowly at ordinary temperatures, and rapidly at about 100°: in this respect it approaches manganese, of which we shall shortly make known some new properties.- Comptes Rendus de l'Acad. des Sciences, vol. lxxx. pp. 788-791.

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* The vapour of water, under the tensions comprised between 5 and 25 millims. is likewise decomposed by iron at the temperature of 100°, as results from experiments by H. Sainte-Claire Deville.

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