The life of the artificial cell may also be prolonged by maintaining around it an appropriate medium. If we prematurely dry such a preparation of artificial cells, the molecular currents will cease, to recur again when we restore the necessary humidity to the preparation. This to my mind gives us a most vivid picture of the conditions of latent life in seeds and many rotifera.
These artificial cells, like living organisms, have an evolutionary existence. The first stage corresponds to the process of organization, the gelatine representing the blastema, and the drop the nucleus. Thus the cell becomes organized, forming its own cytoplasm and its own enveloping membrane.
The second stage in the life of this artificial cell is the period during which the metabolism of the cell is active and tends to equalize the concentration of the liquid in the cell and in the surrounding medium.
The third stage is the period of decline. The double molecular current gradually slows down as the difference of concentration decreases between the cell contents and its entourage. When this equality of concentration has become complete the molecular currents cease, the cell has terminated its existence; it is dead. The currents of substance and of energy have ceased to flow--the form only remains.
These artificial cells are sensible to most of the influences which affect living organisms. Like living cells they are influenced both in their organization and in their development by humidity, dryness, acidity, or alkalinity. They are also greatly affected by the addition of minute quant.i.ties of chemical substances either to the gelatinous blastema or to the drops which represent the primary nuclei. We may in this way obtain endless varieties, nuclei which are opaque or transparent, with or without a nucleolus, and cells containing h.o.m.ogeneous cytoplasm without a nucleus.
We may also obtain cells with cytoplasm filling the whole of the cellular cavity or separated from the cell-membrane. We may obtain {65} cells imitating all the natural tissues, cells without a membranous envelope, cells with thick walls adhering to one another, or cells with wide intracellular s.p.a.ces.
[Ill.u.s.tration: FIG. 10.--Artificial liquid cells, formed by coloured drops of concentrated salt solution in a less concentrated salt solution.]
The forms of these artificial cells depend on the number and relative position of the drops which represent the nuclei, and on the molecular concentration or osmotic tension of the solution. The number of the cellular polyhedra is determined by the number of centres of diffusion. The magnitude of the dihedral angles, from which radiate three and occasionally four walls, depends on the position of the hypertonic poles of diffusion.
The curvature of a surface is determined by the differences of concentration on either side. Between isotonic solutions the surface is plane, whilst it is curved between solutions of different osmotic pressures, the convexity being directed towards the hypertonic solution.
[Ill.u.s.tration: FIG. 11.--Liquid cells with a fringe of cilia, obtained by sowing coloured drops of concentrated salt solution in a weaker salt solution. The contents of the cells have undergone segmentation.]
The time required for these artificial cells to grow varies from two to twenty-four hours, according to the concentration of the gelatine, the growth being most rapid in dilute solutions.
Similar cells may be produced in water. If we pour a thin layer of water on a horizontal plate, and with a pipette {66} sow in it a number of drops of salt water coloured with Indian ink, we may obtain artificial cells composed entirely of liquid, having the same characters as those produced in a gelatinous solution.
It is possible by liquid diffusion to produce not only ordinary cells but ciliated cells. If we spread a layer of salt water on a horizontal gla.s.s plate, and sow in it drops of Indian ink, artificial cells are produced by diffusion. At the edge of the preparation there is often to be seen a sort of fringe, a.n.a.logous to the cilia of living cells (Fig. 11).
These tissues of artificial cells demonstrate the fact that inorganic matter is able to organize itself into forms and structures a.n.a.logous to those of living organisms under the action of the simple physical forces of osmotic pressure and diffusion. The structures thus produced have functions which are also a.n.a.logous to those of living beings, a double current of diffusion, an evolutionary existence, and a latent vitality when desiccated or congealed.
{67}
CHAPTER VI
PERIODICITY
_Periodic Precipitation._--A phenomenon is said to be periodic when it varies in time and s.p.a.ce and is identically reproduced at equal intervals.
We are surrounded on all sides by periodic phenomena; summer and winter, day and night, sleep and waking, rhythm and rhyme, flux and reflux, the movements of respiration and the beating of the heart, all are periodic.
Our first sorrows were appeased by the periodic rhythm of the cradle, and in our later years the periodic swing of the rocking-chair and the hammock still soothe the infirmities of old age.
Sound is a periodic movement of the atmosphere which brings to us harmony and melody. Light consists of periodic undulations of the ether which convey to us the beauty of form and colour. Periodic ethereal waves waft to us the wireless message through terrestrial s.p.a.ce and the radiant energy of the sun and stars.
It is therefore not to be wondered at that the phenomena of diffusion are also periodic. According to Professor Quinke of Heidelberg, the first mention of the periodic formation of chemical precipitates must be attributed to Runge in 1885. Since that time these precipitates have been studied by a number of authors, and particularly by R. Liesegang of Dusseldorf, who in 1907 published a work on the subject, ent.i.tled _On Stratification by Diffusion_.
In 1901 I presented to the Congress of Ajaccio a number of preparations showing concentric rings, alternately transparent and opaque, obtained by diffusing a drop of pota.s.sium ferrocyanide solution in gelatine containing a trace of ferric {68} sulphate. At the Congress of Rheims in 1907 I exhibited the result of some further experiments on the same subject.
These periodic precipitates may be obtained from a great number of different chemical substances. The following is the best method of demonstrating the phenomenon. A gla.s.s lantern slide is carefully cleaned and placed absolutely level. We then take 5 c.c. of a 10 per cent. solution of gelatine and add to it one drop of a concentrated solution of sodium a.r.s.enate. This is poured over the gla.s.s plate whilst hot, and as soon as it is quite set, but before it can dry, we allow a drop of silver nitrate solution containing a trace of nitric acid to fall on it from a pipette.
The drop slowly spreads in the gelatine, and we thus obtain magnificent rings of periodic precipitates of a.r.s.enate of silver, with which any one may easily repeat the experiments detailed in this chapter.
[Ill.u.s.tration: FIG. 12.--Lines of diffusion precipitate, showing the simultaneous propagation of "undulations of different wave-length.]
_Circular Waves of Precipitation._--The wave-front of the periodic rings of precipitates is always perpendicular to the rays of diffusion. The distance between the rings depends on the concentration of the diffusing solution.
The greater the fall of concentration, the less is the interval between the rings. Each ring represents an equipotential line in the field of diffusion. These equipotential lines of diffusion give us the best and most concrete reproduction of the mode of propagation of periodic waves in s.p.a.ce. They are, in fact, a visible diagram of the propagation of the waves of light and sound. Occasionally we may observe in the gelatine the simultaneous propagation of undulations of different wave-length, just as we have them in the ether and the air. These diffusion wavelets {69} give us a very beautiful representation of the simultaneous propagation of undulations of different wave-length in the same medium.
[Ill.u.s.tration: FIG. 13.--Waves of diffusion refracted at a plane surface on pa.s.sing from a less concentrated into a more concentrated solution. The refracted wave-front is flattened, the wave-length being less in the denser medium.]
Like waves of light and sound, these waves of diffusion are refracted when they pa.s.s from one medium into another of a different density, where they have a different velocity. When, for instance, a diffusion wave pa.s.ses from a 5 per cent. solution of gelatine into a 10 per cent. solution, the wave-front is r.e.t.a.r.ded, the r.e.t.a.r.dation being proportional to the length of the path through the denser medium. Hence the wave-front is flattened, the curvature of the refracted wave being less than that of the original wave of diffusion. The contrary is the case when the wave-front pa.s.ses into a medium where its velocity is greater. The middle of the wave-front now travels faster than the flanks, and the curvature is increased.
[Ill.u.s.tration: FIG. 14.--Transformation of a spherical wave-front into a plane wave-front by a convergent diopter.]
These diffusion rings furnish us with most excellent diagrams of refraction at a "diopter," _i.e._ a spherical surface separating two media of different densities. Fig. 14 shows the refraction at a convergent diopter, _i.e_. a surface where the denser medium is convex. The diffusion waves in this case emanate from the princ.i.p.al focus of the diopter, and therefore become plane on pa.s.sing through the convex surface of the denser gelatine.
These periodic diffusion rings also ill.u.s.trate the phenomena of colour diffraction. Diffusion waves of different {70} wavelength are unequally refracted by a gelatine lens. Hence rings of different wave-length which, originating at the same spot, are at first concentric, are no longer parallel after pa.s.sing through a gelatine lens. A convergent lens which will change the long spherical incident waves into shorter plane waves, will transform the short incident waves into concave waves whose curvature is opposite to that of the original waves, _i.e._ it will transform a divergent into a convergent beam. This is an ill.u.s.tration of what is called the aberration of refrangibility.
In the same way we may demonstrate the course of diffusion waves through a gelatine prism, showing the refraction on their incidence and again on emergence. The prism is made of a stronger gelatine solution, which is more refractive than the gelatine around it. The waves of diffusion whilst traversing the prism are r.e.t.a.r.ded, and this r.e.t.a.r.dation is greatest at the base where the pa.s.sage is longer. Hence the wave-front is tilted towards the base of the prism, and this tilting is repeated when the wave-front leaves the prism.
If we examine diffusion waves of different wave-length on their emergence from the gelatine prism, we shall see that they cut one another. With a dense prism, the wave-front of the shorter waves is more tilted towards the base than the wave-front of the longer waves. For diffusion as for light the shorter waves are the most refracted. Both refraction and dispersion are due to the unequal resistances of the medium to undulatory movements of different periodicity.
[Ill.u.s.tration: FIG. 15.--Diffraction of diffusion waves on pa.s.sing through a narrow aperture.]
_Diffraction._--When light traverses a minute orifice, instead {71} of pa.s.sing on in a straight line, it spreads out like a fan, forming a diverging cone of light, just as if the orifice were itself a luminous point. This is the phenomenon of diffraction which has. .h.i.therto been considered incompatible with the emission theory of light. Diffusion waves may also be made to pa.s.s through a narrow orifice, when they will behave exactly like the waves of light. The new waves radiate from the orifice like a fan, instead of giving a cone of waves bounded by lines pa.s.sing through the circ.u.mference of the orifice and the original centre of radiation. Thus on pa.s.sing through a small orifice diffusion waves exhibit the phenomenon of diffraction just as light waves do.
[Ill.u.s.tration: FIG. 16.--Interference of diffusion waves.]
_Interference._--The phenomenon of interference may also be ill.u.s.trated by waves of diffusion. If on a gelatine plate we produce two series of diffusion waves from two separate centres, we get at certain points an appearance corresponding to the interference of two sets of light waves.
This appearance is best shown by sowing on the gelatine film a straight row of drops equidistant from one another. It should be remarked that this phenomenon of the production of circles of precipitate separated by transparent s.p.a.ces, although periodic, is not of necessity vibratory or undulatory. It would thus appear that periodic phenomena may be propagated through s.p.a.ce without vibratory or oscillatory motion. If we submit to a critical examination the various experiments which have established the undulatory theory of light, we find that they do indeed demonstrate the periodic nature of light, but in no wise prove that light is a vibratory movement of the ether. {72} On the contrary, the hypothesis that light is propagated by vibratory movements is open to many objections. Even the Zeeman effect, although it may tend to establish the fact that light is produced by vibratory movement, by no means proves that it is propagated in the same manner. When the theory was accepted that the transmission of light was periodic it was supposed that this periodic transmission could only be vibratory or undulatory in character, since waves or vibrations were the only periodic phenomena known at that time. We now know that there are other means of periodic transmission which are apparently not undulatory. The periodic precipitates produced by diffusion show us the transmission of spherical waves through s.p.a.ce, which follow the laws of light, although the periodic phenomenon is apparently emissive rather than vibratory.
It will be remembered that Newton considered light to be produced by projectile-like particles emanating from a centre, and proceeding in straight lines in all directions. This emission theory of light was abandoned in favour of Huygens" undulatory theory.
It was said that the phenomena of interference and diffraction could not be explained by the theory of emission, while the undulatory theory gave a simple explanation. The scientific mind was unable to conceive the idea of emission and periodicity as taking part in the same phenomenon. The savants and thinkers who have meditated on this question have always considered the theory of emission and that of periodicity as incompatible. Nevertheless, we are here in presence of a phenomenon in which emission and periodicity exist simultaneously. The molecules emanating from our drop are diffused in straight radiating lines, and yet produce periodic precipitates which are subject to interference and diffraction like the undulations of Huygens.
The phenomena a.s.sociated with the pressure of light, the {73} discovery of the cathode rays and the radiations of radium, together with the introduction of the electron theory of electricity, all seem to have brought again into greater prominence Newton"s original conception of the emissionary nature of light.
Some of the phenomena of radiation can be explained only by the emission theory, and others by the undulatory theory of light. All these difficulties would be solved if we admitted the hypothesis that radiating bodies project electrons, which produce in the ether periodic waves similar to those formed in our gelatine films by the molecules of diffusion.
These diffusion films are of the greatest possible service in the practical teaching of optics. They place before the eye of the student a working model as it were of the undulations of light. When projected on the screen, they give excellent pictures of the phenomena of refraction, diffraction, and interference, and the simultaneous propagation of undulation of different wave-lengths, and they show in a visible manner the changes of wave-length in media of different densities.
Diffusion waves differ greatly in length, varying from several millimetres to 2 [mu]. Many are even shorter than this, too short to be separately distinguished even under the highest power of the microscope, when they give the effect of moire or mother-of-pearl.
It is easy to construct a spectroscopic grating in this way with fine lines whose distance apart is of the order of a micron, separated by clear s.p.a.ces. Every physical laboratory may thus produce its own spectroscopic gratings, rectilinear, circular, or of any desired form.
The most beautiful colour effects may be produced with these diffusion gratings, as we have shown at the Congress of Rheims in 1907. We have a considerable collection of these diffusion gratings, some with very fine lines, giving a very extended spectrum, and others with coa.r.s.er striations which give a large number of small spectra.
This study of periodic precipitates is of the highest interest when we come to investigate the production of colour in natural objects, such as the wings of insects or the plumage of {74} birds. Many tissues have this lined or striated structure and exhibit interference colours like those of the periodic precipitates, their structure showing alternate transparent and opaque lines, whose width is of the order of a micron. This is the structure of muscle, and to this striated surface is also attributable many of the most beautiful colours of nature, the gleam of tendon and aponeurosis, the fire of scarab and beetle, the colours of the peac.o.c.k, and the iridescence of the mollusc and the pearl. The study of liquid diffusion has given us an idea of the physical mechanism by which these striated tissues are produced, a mechanism which up to the present time has not been even suspected. Our experiments show how readily such striped or ruled structures may be produced in a colloidal solution by the simple diffusion of salts such as are found in every living organism.
[Ill.u.s.tration: FIG. 17.--Photomicrograph of striated structure of a periodic precipitate of carbonate and phosphate of lime (magnified 500 times).]
To make a spectroscopic grating by diffusion we proceed as follows. We take 5 c.c. of a 10 per cent. solution of gelatine, and add to it one drop of a concentrated solution {75} of calcium nitrate. We spread the gelatine evenly over a plain gla.s.s lantern slide and allow it to set. After it is set, but before it dries, we place in the centre of the slide a drop of concentrated solution containing two parts of sodium carbonate (Na_2CO_3) to one of dibasic sodium phosphate (Na_2HPO_4). Tribasic sodium phosphate alone without the addition of the carbonate will also give good results. If the phosphate solution is placed on the gelatine in the form of a drop, we obtain circular periodic precipitates. If it is desired to make a rectilineal grating, we deposit the phosphate solution on the gelatine in a straight line by means of two parallel gla.s.s plates. In this way we may obtain lines of periodic precipitation to the number of 500 to 1000 per millimetre, forming gratings which produce most beautiful spectra.
Pearls and mother-of-pearl both owe their iridescence to a similar ruled structure, which is developed in the living tissue of a mollusc. They are, in fact, periodic precipitates of phosphate and carbonate of lime deposited in the colloidal organic substance of the mollusc. They have the same structure and the same chemical composition; they have the same physical properties, the glow, the fire, and the brilliancy of our spectroscopic gratings. In these experiments, indeed, we have realized the synthesis of the pearl, not only a chemical synthesis, but the synthesis of its structure and organism.