a column of mercury a metre in length and a millimetre square in section). Taking the same unit as a basis, the electromotive force of a Bunsen's element is equal to 21, and of a Daniell's equal to 12. Hence the electromotive force of a Leclanché's element is only 0.896 that of a Daniell's, while from Leclanché's determinations* it is said to be 1.38 as great as that of a Daniell's. This difference (1.38 against 0.896) may be easily explained. Without galvanic polarization the electromotive force of such an element should be equal to that of a Bunsen's cell (compare my Lehrbuch der Physik, 7th edit. vol. ii. p. 263). But the degree of polarization depends on the strength of the current which the cell furnishes, and therefore on the magnitude of the resistance which is interposed in the circuit. In my experiments the resistance was very small, and hence there was a powerful polarization; while in Leclanché's ex-. periments the current was not so powerful, and the electromotive force was therefore not so greatly weakened as in my experiments. Leclanché found the mean resistance of a manganese-cell of mean size (porous cell 15 centims. in height and 6 centims. in diameter) to be equal to 550, taking as unit of resistance an iron wire 4 millims. in diameter and a metre in length. Referred to Siemens's unit, this resistance is where n is the resistance of iron as compared with mercury (that is, 0.12), and l=550, r≈4; while I found r=1.89. The cells with which Leclanché experimented were doubtless somewhat larger than mine. I was concerned to ascertain, if possible, the part which the manganese plays in this. Leclanché's statements on this point are inadequate; for he says (Dingler's Journal) that the manganese rapidly and uniformly absorbs the hydrogen. If by this it is meant that the hydrogen liberated at the negative pole is immediately oxidized, the statement is manifestly incorrect; for then the galvanic polarization would not exist, and the electromotive force would be 21 (that is, equal to that of a Bunsen's cell). But whether the manganese does generally exert an influence on the electromotive force can only be decided by investigating a cell which has just the same structure as a Leclanché's, but with the difference that the mixture of manganese and carbon is replaced by pieces of carbon (without manganese). For such a cell I found the electromotive force e'=6·16, considerably less, therefore, than the electromotive force of a manganese-cell. Hence the voltaic polarization is not entirely removed by the carbon being partly surrounded by manganese, although it is materially lessened. The manganese manifestly gives up some oxygen, although it is not sufficient to oxidize all the liberated hydrogen. With this agrees the experience, that in Leclanché's cells which had been for some time in use the manganese had lost its activity. Poggendorff's Annalen, June 1870. * Dingler's Polytechn. Journal, vol. clxxxviii. p. 97. ON THE MELTING OF LEADEN PROJECTILES BY THEIR IMPACT UPON AN IRON PLATE. BY EDUARD HAGENBACH. At the commencement of the present year experiments were made at Basle with the view of using targets of iron instead of wood in practice with firearms. Strong plates of iron were, on this occasion, fired at from the short distance of 100 paces. Conical bullets by their impact against the iron plate produced a scarcely perceptible indentation, and fell down near the target; at the same time the lead projectile was melted to a very considerable extent. This could be recognized by the fact that, around the point where the ball had struck, the plate was spattered with lead in the form of a white star, that, moreover, the melted lead was found in the vicinity, and that, of the original bullet, which weighed 40 grammes, only the comparatively small portion of 13 grammes remained. This residual part exhibited a very peculiar kind of deformation and inversion, as may be seen in figs. 1 and 2. Fig. 1 gives the section of the original projectile, and fig. 2 the section of the residue; the concave surface a bed, in consequence of the pressure resulting from the impact, was transformed into the convex surface a, b, c, d. The phenomenon in question is obviously interesting with regard to the mechanical theory of heat, inasmuch as we have here a very distinct example of the transformation of the impetus of the motion of a body into the impetus of molecular motion. We will inquire how far, with the help of this theory, we are in a position to account for the matter in question. According to the statement of a competent military man, the velocity of the projectile, under the circumstances in question, may be mv2 assumed to be equal to 320 metres; hence the impetus, of the , 2 movement of the body is equal to 209 kilogrammetres *. Assuming 424 kilogrammetres as the mechanical equivalent of the heat, this gives us 0.49 thermal unit. Let us now inquire how much heat is necessary to produce the melting described. The entire projectile (40 grammes) had to be raised to the temperature of the melting-point of lead, or near it; and then 27 grammes had to be melted. Assuming 100° for the initial temperature of the projectile, which must have been somewhat warmed by the heat of combustion of the powder and by * In this we neglect (what must in any case be very small) the impetus due to the velocity of rotation of the projectile. the friction against the barrel, taking the melting-point of lead as 335°, its specific heat as 0.031, and its latent heat of fusion as 5'37, we find necessary (1) That the mechanical theory of heat sufficiently accounts for the operation. (2) Almost all the impetus of the motion of the body is transformed into heat,-a result which was indeed to be expected, seeing that the iron plate was very slightly deformed, and the projectile rebounded but little. (3) By far the greater part of the heat was used in heating and in melting the lead. This also is readily intelligible; for the short time within which the entire process was effected could give rise to but little loss by conduction and radiation.-Poggendorff's Annalen, No. 7, 1870. AN EXPERIMENT ON THE BOILING IN CONJUNCTION OF TWO LIQUIDS WHICH DO NOT MIX. BY AUGUST KUNDT. Magnus, and after him Regnault†, have shown that the vapours of liquids which do not mix obey Dalton's law of diffusion. The common tension of the vapours of two non-miscible liquids (e. g. bisulphide of carbon and water) in a state of saturation is equal to the sum of the tensions which would correspond to the state of saturation of the individual vapours for the temperature in question. Two such liquids boil, therefore, when together, at a temperature which is lower than the boiling-point of the most volatile. Magnus, however, in describing his experiments, remarks that the temperature of the boiling liquid is somewhat higher than that of the most volatile when the latter is underneath the less volatile one. Regnault remarks that in the boiling of two liquids which do not mix it is very difficult to preserve constant temperatures in the vapour and in the liquid; the temperature varied materially with the heating and with the formation of bubbles. I have found that the anomaly observed by Magnus (that is, the difference in temperature of the liquid and of the vapour) may be completely avoided, and the experiment be so arranged that the liquid during boiling retains exactly, and without variations, the temperature which corresponds to Dalton's law. For this purpose I do not heat the liquids, such as bisulphide of carbon and water, together in a vessel by direct heat, but heat one by the vapour of the other. Magnus once used this method to show that a concentrated saline solution can be heated by vapour from pure water to the boiling-point of the solution in question. The method is applicable both to miscible and to non-miscible liquids. If into a vessel (and best of all a glass cylinder) which is Poggendorff's Annalen, vols. xxxviii. and xciii. * † Comptes Rendus, vol. xxxix.; Relat. des Expér. vol. ii. about one-third filled with CS2, steam from a flask with distilled water be passed continuously by means of a tube which goes to the bottom, the liquid (that is, the mixture of bisulphide and water) which is traversed by aqueous vapour has the same temperature as the mixed vapours. Both the liquid and the vapour indicated a temperature of 42°.6, a temperature which of course varies with the purity of the CS2 used (boiling-temperature 46°·6) and the barometric height. The temperature once obtained is kept perfectly constant as long as there is a small quantity of CS2 in the cylinder. The same temperature of 42°.6 is maintained constantly in the liquid and the vapour when the experiment is inverted, and water is poured into the cylinder, and the latter heated by having bisulphide vapour passed in. I made the same experiments with water and benzole, with water and oil of cloves, and several other liquids, and in all cases with the same result. When, for instance, aqueous vapour was passed into oil of cloves, the mixed liquids and also the vapour showed very nearly 99°. More accurate numbers and a few remarks which naturally arise out of them will be published subsequently. For the present it is my purpose to describe an experiment to which I have been led by that above related; for it elucidates in a very clear manner, and one especially suited for lectures, that two liquids which do not mix boil when together at a lower temperature than the most volatile. As far as I am aware, the experiment has not hitherto been described. If CS2 boils when alone at 46°.6, and CS2 and water when together at nearly 43°, it is clear that boiling must occur when both liquids are heated separately to a temperature between 43° and 46°·6, about 45°, and are then brought together. Experiment confirms this completely. Into a glass vessel about a foot in height and foot in diameter let water be brought whose temperature is not quite 46°.6, let a test-tube about inch in diameter be half filled with CS2 and immersed in the water until the temperature of the bisulphide has risen to about 45°. If then the bisulphide be poured into the water, a brisk ebullition is set up, which, with an adequate quantity of water, is maintained for some time. If after a while the ebullition becomes weak or even entirely ceases, stirring with a glass rod starts it again and keeps it in fresh ebullition. By stirring, other particles of water are brought into contact with the CS2, which have not yet been cooled down by parting with the heat necessary for evaporation. Even when the entire mass has already been cooled below 42°, solitary bubbles rise, though there is no longer a continuous ebullition. The tension of the bisulphide is then only sufficient between the bisulphide and the water, especially if the former does not cover the entire base, but forms detached drops, to form a bubble (as Quincke has also observed *), which, when it attains sufficient magnitude, can detach itself on shaking or stirring and ascends to the surface. Proper continuous boiling only sets in at a temperature of about 43°.-Poggendorff's Annalen, No. 7, 1870. * Pogg. Ann. vol. cxxxix. p. 19. 465 INDEX TO VOL. XL. Ball (J.) on the cause of the descent Barlow (W. H.) on the cause and theoretic value of the resistance Bezold (Prof. Von) on the electrical Bleekrode (Dr. L.) on the influence of heat on electromotive force, 310. Books, new:-Tyndall's Researches Chemistry, on statical and dynamical Cinchonæ, experiments on living, Clausius (Prof. R.) on a mechanical Comets, on a theory of, 300. Croll (J.) on the cause of the motion of glaciers, 153; on the physical Davis (J. E.) on deep-sea thermome- Dawson (Dr. J. W.) on the structure rotatory polarization of liquids, 393. 53. Delaunay (M.) on the late Mr. Hop- kins's method of determining the Douglas (J. C.) on a new optometer, Duncan (Dr. P. M.) on the physical Earth, on the method of determining Phil. Mag. S. 4. Vol. 40. No. 269. Dec. 1870. |