wave were passing through a medium with a small number of free ions, the effect of these ions would be rather to affect the velocity of propagation than to produce any great absorption. In the case of hydrogen at atmospheric pressures we have seen that Go is of the order 100; in this case p would have to be larger than 100 to make the effects depending on du, /dt large compared with those depending on Gapu. We could not by discharging Leyden jars get electrical vibrations of this rapidity, but at the pressure of Too of an atmosphere G20 would only be of the order 10%, and we could easily get electrical vibrations sufficiently rapid to make p large compared with this quantity, and thus to make the effects depend chiefly upon the term du1/dt, that is, upon the inertia of the ions.

The preceding experiments are, I think, sufficient to show the close analogies existing between the phenomena of chemical combination and of the electric discharge, and give hopes that the study of the passage of electricity through gases may be the means of throwing light on the mechanism of chemical combination. The work of chemists and physicists inay be compared to that of two sets of engineers boring a tunnel from opposite ends-they have not met yet, but they have got so near together that they can hear the sounds of each other's works and appreciate the importance of each other's advances.

On the Electrification of Molecules and Chemical Change.
By H. BRERETON BAKER.

[Ordered by the General Committee to be printed in extenso.]

MORE than twenty years ago a striking fact was discovered by Dr. Wanklyn, that dried sodium could be melted in dried chlorine without the production of the bright flame usual under the circumstances. The action of chlorine on other metals in absence of moisture was investigated by Dr. Cowper in 1876. He showed that in many cases the same result was obtained as that of Dr. Wanklyn in the case of sodium. About this time Professor Dixon, who was working on the rate of chemical change in a mixture of carbon monoxide, hydrogen, and oxygen, was led to suspect the great influence of the presence of moisture on the combustion of the former gas, and he succeeded, by drying a mixture of carbon monoxide and oxygen as completely as possible, in passing a stream of electric sparks in the mixture without any explosion taking place. It was this experiment which first led to the great interest taken by chemists in the influence of moisture on chemical action. Many chemists have investigated different chemical actions, and a large number of changes have been shown to be dependent on the presence of moisture. A list of them will be found in a paper on this subject in the 'Chemical Society's Journal' of July last. It seemed at one time to a chemist who was studying these actions, that no chemical action could take place without the presence of moisture. As action after action was investigated, and as new methods of purification were introduced, further additions could be made to the list. I have recently been engaged in studying several decompositions, however, and I believe that although, in some cases, no breaking up of the molecules takes place, as in the very interesting case of the action of heat on dried ammonium chloride, in which no dissociation occurs, yet in some cases action does take place. Potassium chlorate and silver oxide do decompose, and give not atomic but molecular oxygen. Carbon bisulphide burns

in dried oxygen, although the elements of which it is composed do not. Oxygen forms ozone under the influence of the electric discharge as rapidly when dry as when moisture is present. It may be that the substances have not been sufficiently purified, but I believe it may be due to another cause. In many of these cases in which water-vapour appears to play no part, we are dealing not with molecules but with atoms. If an atom of oxygen will not unite with another atom of oxygen, then by the decomposition of dried potassium chlorate we should perhaps get a gas composed partly of atoms and partly of molecules, and by the decomposition of silver oxide, if its molecular formula is Ag2O, the gas evolved might be composed only of atoms. In these cases, however, molecules of oxygen only are obtained. It may be, therefore, that, whatever be the state of dryness of the substance, atoms will always combine. Similarly with regard to the combustion of carbon bisulphide, though I used all possible care in its purification, yet it always burnt in dried oxygen. It was noticed, however, that the decomposition point of carbon bisulphide, when heated in a neutral gas like nitrogen, was a little below the point of ignition when heated in oxygen. Therefore, in the latter case, I was dealing not with carbon bisulphide and oxygen but with carbon bisulphide in a decomposing state, carbon and sulphur being set free not in their ordinary state but in a condition in which they would combine with dried oxygen. This fact is most easily explained by supposing that when carbon bisulphide is heated it splits up into atoms of carbon and sulphur, and that these then combine with oxygen in absence of moisture.

With regard to the explanation of the effect of moisture on chemical actions in general, several hypotheses have been suggested. The first, that of Professor Dixon, is that the water molecules present undergo an actual decomposition. In the combustion of carbon monoxide, for instance, the gas takes up oxygen from the water, liberating hydrogen, which then combines with the free oxygen, re-forming water. I venture to think that this hypothesis, although I believe it explains all the known facts, is open to one or two objections. For instance, if we accept Berthelot's law of maximum work, there seems to be no reason why water should be decomposed by red-hot carbon rather than oxygen molecules, since the direct action on the oxygen liberates a far greater amount of energy. Dr. Traube has suggested that the explanation is dependent on the oxidation of water rather than on its reduction; that hydrogen peroxide is first formed by direct union of water-vapour with oxygen, the peroxide again being reduced to water, giving up its extra atom of oxygen to the combustible. This hypothesis seems to be inadequate in many respects, since many actions in which water plays an important part, e.g., the action of sodium on chlorine, or the combination of ammonia and hydrogen chloride, free oxygen is not present, and, therefore, hydrogen peroxide could not be formed.

Mr. Harcourt first suggested, in 1886, that the explanation of the action was to be sought from a physical, rather than from a chemical, point of view. Dr. Armstrong proposed in the same year a hypothesis, which he calls that of reversed electrolysis,' which supposes that no chemical action can take place without the presence of a third body, which must be an electrolyte. With regard to this theory I hope to be able to say more later. I am engaged upon an investigation whose object is to find out what substances can replace water in chemical action, and it may be found to be the case that all such substances are electrolytes.

I have been engaged during the last two years in an effort to investi

gate the question whether the union of elements and compounds was in any way connected with electrical discharge. The facts of electrolysis point strongly in this direction. I gave up at once the idea of directly investigating the question whether molecules of gases in contact were electrically charged, but in an indirect way I have obtained some evidence on the question. Taking a tube divided in the middle by a tap, and filling it with a mixture of dried ammonia and hydrogen chloride gases in equal proportions, I introduced at the two ends platinum plates which were oppositely charged from the terminal of a Wimshurst machine. After half an hour the tap was closed, and the gases in the two parts of the tube were drawn through a solution of litmus. On admitting moisture white fumes of ammonium chloride were produced, but there was found to be residual gas in both halves of the tube. The gas from the part which had contained the negatively charged plate turned litmus blue, and the gas from the other part reddened the litmus. The experiment was often repeated, and the result was the same. The separation was never very great, but the evidence of some separation seemed conclusive. With other gases there was also evidence of separation. With air dried by sulphuric acid there was found to be 1.8 per cent. more oxygen in the part containing the positive plate than there was in that containing the negative plate. Using a mixture of hydrogen and oxygen dried by phosphorus pentoxide, analysis showed an excess of 2.3 per cent. of oxygen in the part of the tube containing the positively charged plate.

It is possible, therefore, that the molecules of gases which may under certain circumstances combine together, may have an electrical charge. I hope to extend these observations to the case of other gases, and to find out, if possible, whether the charge on the molecules, if it exists, is theirs intrinsically, or if it only exists when molecules of a different nature are in contact with each other.

With the object of finding out if electric discharge bears any analogy to the process of chemical combination, I undertook some experiments a year ago to see if electric discharge in air was affected by drying the air as completely as possible. It was found that sparks from a Ruhmkorff's coil, when the discharge was very feeble, would leap across a space of moist air, but that none would pass in the dried air though the sparking distance was very much less. If, however, a strong discharge was used, sparks were obtained in the dried air, and, more than this, the feeble discharge would then easily pass in the dried air. These results entirely agree with those published by Professor J. J. Thomson.1

I am inclined to think, however, that there may be another interpretation of the latter part of the phenomenon besides that offered by Professor Thomson. It may be that the strong discharge splits the molecules into atoms, and that these can then carry on the feeble discharge. If this is proved to be true, it will serve as yet another point of analogy between chemical combination and electric discharge, for, as I have tried to show above, chemical combination does take place between atoms, whatever the state of dryness of their environment.

With regard to this analogy there may be found some evidence in the study of the action of actinic light on chemical combination and electric discharge. We find that when a mixture of chlorine and hydrogen gases is exposed to light, combination takes place only after a certain interval, and that the interval is not shortened by exposing the gases separately to the action of light. If the interval is spent in breaking up molecules into 1 Phil. Mag., October 1893.

atoms we should expect that there would be an increase in volume. This increase in volume has been observed by Pringsheim with the mixed gases, though he ascribes it to another cause. I have undertaken some experi

ments with unmixed chlorine, and I found that in this case there is absolutely no increase in volume.

Now with regard to the analogy of chemical action and electric discharge we find that light, and light of the same kind as that which brings about chemical action, has a remarkable influence on the passage of the electric discharge. This phenomenon was first noticed by the late Dr. Hertz, who found that when the negative pole of a spark gap was illuminated by an actinic light a discharge would pass which would not pass if the spark gap were unilluminated. It may be that the action of light is to split up molecules into atoms, and that the ready passage of the discharge is thus explainable. However this may be, I think that the analogy of the phenomena of electric discharge and chemical combination may be of importance. I have attempted to point out the analogy which exists when we work with substances which are in an exceptional state of dryness. Whether the analogy holds for other influences which affect chemical combination, and whether the analogies which I have indicated are only superficial and not real, can only be decided by a long series of experiments to which I hope to devote myself.

Report on Planimeters. By Professor O. HENRICI, F.R.S.
[Ordered by the General Committee to be printed in extenso.]

SEVERAL classifications of planimeters have been used by different writers, and different names have been used to distinguish them.

Mr. Boys distinguishes three types, which he calls Radius Machines, Sine or Cosine Machines, and Tangent Machines.

Professor Hele-Shaw has two classes only, according as the recording apparatus does or does not show slipping.

A more fundamental classification seems to be got on considering first of all the geometrical generation of the area by the motion of a line. This gives us three types: the first follows the generation of an area, as in the Integral Calculus, by rectangular co-ordinates; the second by polar coordinates; whilst the third is based on purely geometrical considerations due to Amsler.

In the following Report the classification adopted is :

Type I. Orthogonal planimeters.

II. Polar co-ordinate planimeters.

III. Planimeters of the Amsler type.

Instruments of either type may have a recording or integrating apparatus with or without slipping, and this would give rise to subdivisions, which, however, it is not necessary to elaborate here.

This Report covers only a comparatively small part of the subject of integrators; it deals only with planimeters proper. Integrators for such purposes as the continuous registration of work done, integraphs and instruments for the integration of differential equations, harmonic analysers, &c., will not be considered. As it is in these more complicated instruments that the injurious effect of slipping is chiefly noticeable, a description of the various integrating apparatus will not be given. For

the same reason the work of Maxwell, Lord Kelvin, Boys, Abdank-Abakanowicz, and Hele-Shaw will remain practically unnoticed.

With regard to the integrating apparatus in particular, there is the wellknown paper on 'Mechanical Integrators,' by Professor Hele-Shaw, in the 'Proceedings' of the Institute of Civil Engineers for 1885, in which, besides, many descriptions with figures are given of instruments here treated of.

In this Report there is first given the geometrical theory of generating areas, together with simple descriptions of planimeters based on it. Then follows a historical sketch up to the invention of Amsler's planimeter. Next, this instrument is considered, its errors are discussed, together with those more modern planimeters which have been constructed with a view to avoid these errors. Lastly, some planimeters are described which have recently been introduced.

The object of a planimeter is to measure an area; it has, therefore, to solve a geometrical problem by mechanical means.

FIG. 1.

B

D

To give at once an idea how this is possible, consider a very simple case. If a line AB (fig. 1) of finite length 7 moves parallel to itself to CD, where AB is perpendicular to AC, then it will sweep over the area of a rectangle which will have the value lw if w=AC. Let the line be replaced by a material rod QT, and let a wheel W be mounted on it. On placing this apparatus on the paper above the line AB, the wheel resting on the paper, and moving this rod along to CD, it will describe the same area. At the same time the wheel will w turn and the arc of its circumference, which comes in contact with the paper, will have the length w. If the circumference is

A

graduated and a fixed index is provided, say, at the highest point, the length of this arc can at once be read off.

This arc, as read off at the index, may be called conveniently the 'roll' of the wheel (Macfarlane Gray).

This instrument may be considered as a simple planimeter, which, however, measures only the areas of rectangles with fixed altitude, and is, therefore, practically of no use. Nevertheless it will serve to elucidate a great many properties common to nearly all planimeters.

First we have a geometrical generation of an area by aid of a moving line, and secondly the 'recording apparatus' represented by the wheel, with its graduation and index. It is advisable always to keep these two ideas quite separate. The one is geometrical, the other kinematic. The former of these will first of all engage our attention.

GEOMETRICAL GENERATION OF AREAS.

Our instrument teaches us, if the 'rod' QT is moved one way, the 'roll' of the wheel will increase, whilst it will decrease when moved in the opposite sense. Hence we must consider the 'sense' in which the motion takes place; we shall call the one motion positive, the other negative. the same time we shall call the area generated positive or negative. It will be seen that in this case alone will the area always be measured by the 'roll.'

At

If the rod QT is turned round so that Q is above B, and T above A, 1894.

K K