The extreme values of /V experimentally obtained were x/V10 in series 3 and a/V=16 in series 4. Hence the values n=1.5 x 10' and no=2'4 x 10' are computed for these extremes. Recalling that 4 × 10 is the number of particles per cubic centim, inferred for complete saturation, and that no is the number at the initial section of the condenser contiguous to the hard-rubber plug, where many ions must already have vanished by absorption, I hold the value of no stated to be in reasonable accord with the theory sketched in §§ 6-8, and throughout the course of the present papers.

Brown University, Providence, U.S.A.

XLVIII. A Study of Growing Crystals by Instantaneous Photomicrography. (Contributions from the Chemical Laboratory of Harvard College.) By Professor THEODORE WILLIAM RICHARDS, of Harvard University, and EBENEZER HENRY ARCHIBALD, A.M., 1851 Exhibition Scholar of Dalhousie University*.

COUN

[Plates VII.-IX.]

MOUNTLESS observers have watched the growth of crystals under the microscope. As long ago as 1839 attempts were made to study also the birth of crystals, in order to determine in what manner the new phase makes its entrance into the system. With a microscope magnifying 600 diameters, Link† thought he could detect the formation of minute globules at the moment of precipitation-globules which soon joined and assumed crystalline form. Schmidt ‡, Frankenheim §, and especially Vogelsang, made similar observations some years later, and several more recent accounts of this phenomenon have appeared. Modern investigators have been more concerned with the speed of separation from supersaturated or supercooled liquids than with the form of the first separation ¶.

* Reprinted from the Proceedings of the American Academy of Arts and Sciences, xxxvi. p. 341 (1901). From a separate impression communicated by the Authors.

+ Link, Pogg. Ann. xlvi. p. 258 (1839).

Schmidt, Lieb. Ann. liii. p. 171 (1845).

§ Frankenheim, Pogg. Ann. cxi. p. 1 (1860).

Vogelsang, Die Krystalliten (Bonn, 1875). See Lehmann, Molecularphysik, i. p. 730 (1888).

¶ Gernez, Compt. Rend. xcv. p. 1278 (1882); Moore, Zeits. phys. Chem. xii. p. 545 (1893); Friedländer & Tammann, ibid. xxiv. p. 152 (1897); Tammann, ibid. xxv. p. 441; xxvi. pp. 307, 367; xxviii. p. 96; Küster, ibid. xxv. p. 480; xxvii. p. 222; Bogajavlensky, ibid. xxvii. p. 585.

Ostwald, in 1891, accepted the interpretation of these data, which assumes that crystallization is always preceded by the separation of an initially liquid phase, consisting of a supersaturated solution of the former solvent in its former solutet.

This explanation is indeed a plausible one, and undoubtedly holds true in cases like those studied by Schmidt and Vogelsang, where a substance separates at a temperature not far below its melting-point, and often where a substance soluble in one liquid is precipitated by the addition of a consolute liquid in which the substance is insoluble. For examples, phenol always separates from aqueous solution in the form of a liquid, and manganous sulphate forms at first two liquid phases when alcohol is added to its aqueous solution. On the other hand, the separation of a high-melting salt like baric chloride from its solution in pure water is much less likely to take place in this way. The admixture of water necessary to lower a melting-point from 900° to 25° would be so large as to make the new phase, a solution of water in baric chloride, supersaturated to an improbable extent. Moreover, we have no evidence of the existence of vitreous baric chloride at low temperatures.

It has long been known that an exceedingly small particle of solid is capable of starting crystallization ‡—a fact which may not be wholly foreign to the present discussion.

In any case, the matter seemed worthy of further experimenting. Ostwald says:-" Die erste Bildung der Krystalle lässt sich bei Salzlösungen und dergleichen microscopisch nicht verfolgen, weil gewöhnlich im Gesichtsfeide an einer bislang gleichförmigen Stelle plötzlich ein Krystallchen erscheint." While this is true as far as the human eye is concerned, instantaneous photography, an art unknown in Link's time, seemed peculiarly fitted for the unprejudiced recording of the circumstances attending the genesis of crystals. An attempt in this direction is described below.

The problem resolved itself into the taking of a number of successive instantaneous microphotographs of a suitable mixture at the point of crystallization. This problem presented some difficulties, however. In order to secure a sufficiently brief exposure, very great illumination is needed. greater the magnifying power of the lenses of the microscopecamera, the more intense must be the source of light. The

The

This is obviously not the case in crystallization of water or of a metal, in freezing.-EDs. Phil. Mag.

Lehrbuch, i. p. 1039 (1891).

Ostwald, Zeits, phys. Chem. xxii. p. 289 (1897).

difficulty is increased by the fact that most crystals are so transparent as to absorb but little light, and reflexion is possible only in certain directions. Hence it is hard to obtain a distinct image even in a strong light. Moreover, the machinery necessary for shifting the plates must be so frictionless in construction, and so firmly fixed, as to impart no vibration to the camera or the mobile subject of study.

These difficulties were at least partially overcome by two different arrangements; the first of which caused the successive impression of a bright image in a dark field, and the second registered dark images in a succession of bright fields Obviously the former was the more economical as regards expenditure of sensitized film, and the more simple in execution; for when the field is dark, successive images can be obtained by a very slight motion of either object or film, while, when the field is light, the whole previously exposed surface must be replaced by a fresh surface before each

exposure.

The apparatus consisted of a good compound microscope fitted above with a vertical folding camera, which was supported by two massive steel pillars on the heavy stand. It was, in short, the regular photomicrographic outfit made by Bausch and Lomb. Between the microscope and camera, in a suitable light-tight box, was placed a revolving shutter, which allowed an exposure equal to one fifth of the time of its revolution. Thus, when the shutter made two revolutions in a second, the exposure was one tenth of a second. A Henrici hot-air motor, combined with speed-reducing double pulleys, enabled the experimenter to use any rate of revolution desired. The rate was reasonably constant, but no attempt was made to make it absolutely so. The sensitive plate or gelatine film was held above in a suitable holder, which was put in the place of the ground-glass plate used for focussing just before each series of exposures.

In carrying out the first of the two methods, it was found more convenient to move the crystallizing solution than to move the photographic plate. For this purpose, the slide bearing the drop of liquid was attached by a wire to a point just below the centre of a segment provided above with sawteeth. The segment was moved gradually by the oscillating motion of a connecting-rod, fastened by a crank to the revolving shutter at one end and playing into the saw-teeth on the other. In order to make the motion certain, the stroke of the connecting-rod slightly exceeded the distance between the saw-teeth. The segment was suspended in such a way

that its centre of gravity coincided with its point of support, and the friction of its bearings was so adjusted that it would move easily, and yet remain stationary during the return. stroke. The distance through which the observed object was moved was easily varied by altering the relative lengths of the lever-arms; distances varying from one tenth to one fiftieth of a millimetre were usually used. The shutter was so arranged that during the exposure the segment and slide were at rest, the shift in position being effected during the four fifths of the revolution through which the shutter was closed. The accompanying diagram will make the arrangement clearer (see p. 492).

As a source of light any ordinary combination of incandescent electric lights proved to be inadequate. A good Auer von Welsbach light with a powerful reflector was more satisfactory, but the best results were obtained with the help of sunlight directed by a suitably arranged mirror and condensed by reflectors and lenses. The chief, though not serious, difficulty of this arrangement was the great heat caused by the converging rays; a difficulty which was obviated partially by an absorbent screen in later experiments*.

The first photographs were taken by reflected light, the drop of solution being placed upon a ruby-coloured slide. As soon as the crystallization had begun upon one edge of this drop, the very sensitive plate was uncovered and the shutter and segment were set in motion. The exposure was stopped after fifteen or twenty revolutions, so as to avoid confusing superpositions. Even with the strongest light the images were very faint and unsatisfactory; it is not worth the space to reproduce them here.

Another mode of obtaining light images on a dark ground, applicable to all except the isometric system of crystals, is the use of polarized light †. A Nicol prism was placed in the barrel of the microscope, and another just below the stage. The main body of the light was thus intercepted by the crossed prisms, and only that which had been deffected by the crystalline structure was allowed to emerge. It is true that this method could not in all probability decide the chief point at issue; for the prenatal globular condition of crystals would probably have no effect on polarized light. Definite optical structure is of course necessary to produce the required deflexion of the plane of polarization, and such definite

* Hutchins has shown that pure water is as good as a solution of alum for this purpose (Am. Journ. Sci. cxliii. p. 526, 1892).

†This suggestion was kindly made by Professor É. C. Pickering.

A, sensitive plate or film-holder.

B, box containing shutter.

C, pulley attached to axle of shutter to communicate power from motor. D, light rod moved by crank attached to same axle ; D is guided by a stout support in which it moves loosely.

E, segment provided with ratchet-teeth; moved gradually by rod D.

F, microscope.

G, slide for object, moved by wire running to H.

H, holes to regulate amplitude of object's motion.

I, weight, balancing segment.

J, horizontal projection of revolving shutter in detail.

The diagram represents the apparatus an instant before an exposure begins.