Page images

them. In the case of two metals joined metallically, the potentials of the metals will be equalised, a portion of the negative charge going through the connection between the two metals, and thence to the film.

In the experiments, a pair of metals were taken, each of which was carefully cleaned, and the difference of their potential determined immediately; into the apparatus, a gas which acts chemically on the metals was then introduced, and the difference of potential determined after various intervals of time. The gas causes an immediate reversal of the potential; this is compared with a similar reversal which occurs when a solution of the same gas is added to water in which the same pair of metals is immersed. Experiments were made with copper-iron in ammonia and hydrogen sulphide, silver-iron in hydrogen sulphide, and copper-nickel in ammonia and hydrochloric acid gases, and the results compared with those obtained by adding solution of ammonia, hydrochloric acid, and potassium sulphide to water, in which these several pairs of metals were immersed. It would appear from these results that no contact experiment with clean metals has as yet been made, since they are cleaned whilst exposed to the atmosphere, and are thereby covered with a film of condensed water containing dissolved gases. Experiments are quoted to show that the difference of potential of a copper-zinc pair in air is reduced by the introduction of dehydrating agents, such as sodium and phosphoric anhydride. It was further found possible to join the films only on the two metals, without bringing the metals themselves in contact, and thus to produce an electromotive force from metals apparently dry; the thickness of the environing film was determined by means of a micrometer screw.

From the above experiments, it follows that difference of potential in a pair of metals is not an intrinsic property of the metals in themselves, but results from a chemical action induced by their exposure to the atmosphere; this action proceeds until terminated by its own effects.

Thus, in the case of a copper-zinc pair in air, the oxygen of the moisture combines with the zinc, and thus is produced a configuration of oppositely electrified atoms, forming a double layer. This idea has been put forward by Helmholtz in illustration of the phenomena of electrolysis, and successfully applied to explain polarisation.

V. H. V.

Electric Accumulators. By D. DRAKE and J. M. GRAHAM (Dingl. polyt. J., 262, 382).-From the results of a series of experiments on the permanency of accumulators, the authors draw the following conclusions:-(1.) The duration of lead plates or conductors and the retention of their structure is not dependent on the amount of current condensed in and discharged from the cells. (2.) New cells or cells which have been out of use for some time should be charged to the maximum extent. (3.) The cells should not be completely exhausted; moreover, it is proposed not to discharge them beyond the point at which the E. M. F. begins to be sensibly reduced. (4.) The film of dioxide formed during the condensation of the current is said to protect the plate from the injurious effects of overcharging and

local action. (5.) A certain (small) amount of sulphate is necessary to hold the active material together; an excess, however, causes the oxide to separate from the conductor.

D. B.


Determination of Atomic Weight from Specific Heat. G. JANEČEK (Chem. Centr., 1887, 3-4).--The author denies, on theoretical and experimental grounds, the correctness of the law of Dulong and Petit. Of the 70 known elements, there are only 16 whose atomic heats can be shown to undoubtedly lie between 6·028 and 6-849, whilst there are two (S and P) much below 6, one (Si) between 4 and 5, one (Be) about 3.8, one (B) about 2:5, one (C) about 2, and five (Al, Cr, Cu, Fe, and Ga) between 104 and 126. For all the remaining elements, the atomic weight has simply been adjusted to the law of Dulong and Petit. The atomic heats of the gaseous elements in their solid condition have been deduced almost exclusively from such arbitrary values. Of the values so obtained, only those of two elements (Cl and N) correspond in some degree with the law of Dulong and Petit, whilst those of the others (F, Ö, and H) do not at all agree. Weber's researches on the influence of temperature on the specific heat of the solid elements afford no support to the law. Weber's numbers for carbon, boron, and silicon do not refer to the absolute specific heat, but rather show the entire absorption of heat under different conditions. It is possible to calculate the atomic heats from the differences in the observed values, and the numbers obtained have been used to confirm the law of Dulong and Petit. The conclusions, however, which are applicable to these three elements may also be applied to all the other elements, the atomic heats of which have been calculated from specific heat determinations made at an arbitrarily selected temperature. When this temperature is departed from for the specific heat determinations, the agreement of the atomic heat with the law of Dulong and Petit entirely disappears. G. H. M.

Specific Heats of Liquids. By M. LANGLOIS (Compt. rend., 104, 420-422).—The author has defined the terms enveloping molecule and secondary molecule in a previous communication. His experiments on the heat of vaporisation show that the enveloping molecule, considered by itself, behaves as a true liquid molecule. The secondary molecules, on the other hand, behave like gaseous molecules, and consequently in calculating the specific heat of a liquid, it is merely necessary to calculate the heat which they would absorb in the gaseous state when expanding under constant pressure. The variations in the attraction on the molecular surface are so small within a comparatively narrow range of temperature, that they may be neglected. When the temperature of an enveloping molecule is raised, it experiences an alteration in the force of atomic translation, and this results in an oscillating motion with absorption of a quantity of heat, which the author defines as the heat of oscillation. From these considerations, the author has calculated the specific heats of water, carbon bisulphide, chloroform, carbon tetrachloride, ether, ethyl alcohol, and acetone, and has obtained numbers which agree closely with the actual determinations of Regnault.

C. H. B.

Isomerism of Position. By A. COLSON (Compt. rend., 104, 428 -430). The author has determined the specific heats and the heats of fusion of the dibromo-, dichloro-, and tetrachloro-xylenes :

[merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][ocr errors][merged small][merged small]

The last column contains the values obtained by multiplying the molecular weight into the number obtained by dividing the latent heat by the absolute temperature of fusion. The higher values obtained for the bromine-derivatives may be due to their greater instability, or to the fact that the mean specific heat (0·199) used in calculating the numbers is too high. If the results can be generalised, it follows that at the melting points the difference between the entropy of the liquid and solid is constant for all isomerides of position. When the results are calculated to the molecular weights, it is found that this difference will not be sensibly altered by the substitution of chlorine for hydrogen. The heat of fusion L is connected with the temperature t, pressure p, and contraction v'· -v by the equation of Clausius and Clapeyron:

[blocks in formation]

From the author's results, it follows that for isomerides of this kind the first term in this equation is constant, and hence

[blocks in formation]

or the temperature of fusion increases proportionally with the pressure. C. H. B.

Determination of the Constitution of Carbon-compounds from Thermochemical Data. By H. E. ARMSTRONG (Phil. Mag. [5], 23, 73—109).—The author quotes the chief results obtained by Thomsen (Thermochem. Unters., Bd. iv) in the determination of the heats of combustion of a large number of carbon-compounds, and criticises the views there put forward with reference to their constitution. In Thomsen's work, all the heats of combustion are based

on the assumption that the substance burned is in the state of gas at 18°, and that the products are gaseous carbonic anhydride and liquid water at that temperature; before, however, these data can be applied to the determination of the constitution of the compounds themselves, it is necessary to deduce the heat of combustion of gaseous atomic carbon, a value denoted by the symbol f(C1). The argument by which this value, ƒ(C1), is estimated by Thomsen is as follows:The heat of combustion of carbon in its compounds is greater than that of amorphous carbon, and the average value obtained by the comparison of the heats of combustion of compounds differing in constitution by one or more atoms of carbon is 121090 units; these comparisons are instituted between a saturated and an unsaturated compound, the latter being formed from the former by the addition of a carbon-atom, which, it is usually said, becomes doubly linked with another carbon-atom. Now, if carbonic anhydride were capable of combining with an atom of carbon, it is to be supposed that it would form an unsaturated compound CO: CO, bearing the same relation to it that ethylene bears to methane, and that the heat of combustion of this compound would exceed that of carbon dioxide by 121090 units; whence it follows that the heat of combustion of carbonic anhydride being nil, the heat of combustion of the product in question should be 121090 units. But in point of fact, 2 mols. of carbonic oxide are produced by assimilating an atom of carbon with a molecule of carbonic anhydride, the double linkage being annulled whilst the volume is doubled. In the act of expansion 580 units are absorbed; hence the heat of combustion of the product of the union of an atom of carbon with a molecule of carbonic anhydride exceeds 121090 units by the amount absorbed in the separation of doubly-linked carbon-atoms (v2), plus 580 units, or ƒ(C1) = 121090 + v2 = 135920 — 580 = 135340 units; whence also V2 = 14250 units.

Very important conclusions are drawn by Thomsen with reference to the heat developed in the combinaton of carbon-atoms by one, two, or three affinities of each, that is, in the way in which they are assumed to be associated in the paraffins, in ethylene, and in acetylene respectively-these values are severally denoted by v1, v2, and v3. These values can be deduced from the heat of formation of compounds containing carbon-atoms united by one, two, or three affinities, as the case may be, by employing as a constant the heat developed by the union of 1 gram-molecule of hydrogen with gaseous atomic carbon. This constant (2r) is evaluated thus: the heat of formation of methane, CH, at constant volume, calculated on the assumption that it results from the combination of ordinary hydrogen with gaseous atomic carbon-of 2 hydrogen molecules with 1 carbon-atom-is 59550 units; halving this number, 2r is found to be 29775 units. With regard to the value of v1, the heat developed in the formation of ethane, C2H (104160 units), results from the combination of 3 hydrogen molecules with 2 carbon-atoms, and of these carbon-atoms with each other by single affinities; hence, (2C,3H2) = 3.2r + v1 = 104160 units ..v1 = 14835 units. The value of v2 has already been determined to be 14250 units, whilst that of vs can be obtained from the heats of

formation of acetylene (28990 units), allylene (74610 units), and dipropargyl (133080 units), and is found to be 81 units, a value so small that it can be neglected. From these values, it follows that the same amount of heat is developed in the combination of carbon-atoms, whether they become singly or doubly linked-in other words, there is no difference between these modes of union-whilst the so-called treble linking of carbon-atoms is unattended with the development of heat.

In addition to his already published views on the constitution of benzene (Abstr., 1881, 89), pyridine, and thiophen (Abstr., 1885, 1126), the following are among the more curious conclusions having reference to the constitution of carbon-compounds arrived at by Thomsen by the use of these values of v1, v2, and v: phenol is ranked with primary alcohols; ethylene oxide is in reality methylene oxide, CHO CH2; aldehydes are unsaturated substances of the general formula R·C(OH); methylal and methyl orthoformate contain hydroxyl and are analogous to alcohols in constitution; cyanogen has the formula NCC:N, and the amines are compounds of pentad nitrogen: methylamine, for example, is H2C NH,, dimethylamine, H2C: NH,Me, and trimethylamine, H2C: NHMe,, although aniline is found to be Ph-NH2; the nitro-paraffins are nitroso-alcohols, nitro-methane being formulated CH, (NO) OH; and finally pyridine is not analogous in constitution to benzene.

The author recalling some of these results, and noting the conclusion that v1 = v2-in other words that there are no such things as double bonds-and that vs = 0, namely, that in acetylene there is not even a single bond between the carbon-atoms, adds that it cannot be admitted by chemists that ethylene oxide is methylene oxide, whilst Thomsen's formule for the amines are altogether improbable. The method of proof that the carbon-atoms are united in ethylene oxide and that the amines have the constitution usually assigned to them, is the method by which the constitution of compounds generally is arrived at, and results obtained by this method would have to be retained in some instances but rejected in others were Thomsen's views adopted. The explanation of these anomalous results is most probably to be found in the view that the true heat of combustion of gaseous atomic carbon is greater than 135340 units. Thomsen's determination of this value involves the assumption that the addition of the first and second atom of oxygen to a carbon-atom involves the development of the same amount of heat, but consideration of the properties of carbonic oxide (such as its tendency to combine with but a limited number of other substances, and, as a rule, only under special conditions) appears to favour the contrary view that, of the total heat developed in the formation of carbonic anhydride, the larger proportion is evolved in the combination of the carbon-atom with a single oxygen-atom-in other words, ƒ(C1) = 135340 + æ, where & has probably a high value.

Among the results independent of the value of f(C.), Thomsen finds that in the formation of haloïd derivatives of hydrocarbons, the addition of a single atom of chlorine to methane, for example, is attended with a heat-evolution of only 13500 units; that of two,

« PreviousContinue »