No. 320. Genetic Studies of Rabbits and Rats; by W. E. CASTLE. Pp. 55, 2 pls., 7 figs. ($1.00).

9. American Association for the Advancement of Science.— The seventy-fifth meeting of the Association was held in Salt Lake City on June 22 to 24, under the auspices of the Pacific Division, of which Dr. Barton W. Evermann is president.

10. Observatory Publications.-The University of Cincinnati, JERMAIN G. PORTER Director, has recently issued No. 19 of its Publications. This gives a Catalogue of 4683 Stars for the Epoch 1900, observed by ELIOTT SMITH and prepared for publication by the director. It presents the positions of all stars determined with the meridian circle from January, 1907, to December, 1921. The proper motions of the greater part of the stars were computed in No. 18; other stars are those of the Boss preliminary general catalogue. Computations have been made by the Director assisted by Dr. YoWELL. It is announced that the next meridian work published by the Observatory will be reduced to 1925.

The observatory of Krakow has issued an eight-page publication giving the minimos of Algol and RW Tauri in 1922, reduced to the meridian of Greenwich.

The Annual Report of the U. S. Naval Observatory at Washington has also been received.

The National University of La Plata has issued part 2 of volume VI, by NUMA TAPICA, giving micrometric measurements of double and vicinal stars. From the same source comes a determination of the orbit of the planetoid (796) Saria by HUGO A. MARTINEZ.

The Hector Observatory at Wellington, New Zealand, published some time since a reprint (Bulletin 33) on observations of Southern variable stars by C. E. ADAMS; also a brief method of calculating occultations of stars by the moon by C. J. WESTLAND.

The observatory at Lyons, France, publishes a Bulletin of which No. 4 for April, 1922 (4th year) has been received (H. Georg, Editeur). The annual subscription is only 12 francs.

OBITUARY.

PROFESSOR GEORGE SIMONDS, the English botanist, died on May 4 at the age of sixty-nine years.

ARTHUR BACOT, entomologist to the Lister Institute, died at Cairo on April 12 as the result of infection contracted when prosecuting investigations on the etiology of typhus.

LOUIS ANTOINE RANVIER, the veteran French histologist, died recently at the age of eighty-seven years.

DR. HENRY MARION HOWE, the distinguished metallurgist, since 1897 professor in Columbia University, died recently at the age of seventy-four years.

THE

AMERICAN JOURNAL OF SCIENCE

[FIFTH SERIES.]

ART. VIII.-Colloids in Geologic Problems; by GEORGE D. HUBBARD.1

Introduction. It has long been recognized that solid matter occurs in more than two states. The terms "crystalline" and "amorphous" exclude quite a large body of materials, some of which are organic and some inorganic. The other condition has been called the colloidal state. Many substances which occur crystalline can be prepared in the colloidal state without any compositional difference. Amorphous substances were in many instances once in colloidal condition. Perhaps all substances can ultimately be prepared in this state whether they are normally crystalline or amorphous.

In the case of some substances, there seems to be no line fixed between the crystalline and the colloidal state. In the same way, there seems to be no line which can be definitely drawn between a true solution and a colloidal solution. In practice we say a substance is in the colloidal state when it will not dialyze or diffuse through certain membranes, as egg skin, bladder, goldbeater's skin, or parchment paper. Molecular dispersions are in true solution; dispersions of much larger aggregates (a hundred, more or less, of molecules in one particle) are in colloidal solution. Such particles are too large to pass through the membrane..

Bancroft2 says, "Colloid chemistry is the chemistry of grains, drops, bubbles, filaments, and films." Grains of solid particles, drops of fluids, bubbles of gas, filaments very small in two directions, films small only in one direc

1 The author is deeply indebted at many points in the paper to Dr. Harry N. Holmes of Oberlin College for suggestions and criticism. His General Chemistry and Colloid Manual have been of much help also. This paper was presented to the Geologic Section of the Ohio Academy of Science, April 15,

1922.

2

1 Bancroft, W. D., Applied Colloid Chemistry, p. 2 (McGraw-Hill Co.).

AM. JOUR. SCI.-FIFTH SERIES, VOL. IV, No. 20.—August, 1922.

tion, these constitute the field of the student of colloids. The smallest particle visible with the aid of the best compound microscope is about 100 millimicrons in diameter, while the largest molecules approach a diameter of 1 millimicron. Materials in the colloidal state have particles ranging between these limits.3

The term "colloid" or "colloidal state" has been expanded to include not only solid gels, but suspensions and emulsions. The former may be solids in liquids, as the very fine sediments in roily water, liquids in liquids, or even solids in solids, as the blue rock salt which is a suspension of finely divided metallic sodium in crystalline halite. In suspensoids the solid particles do not powerfully absorb or hold large quantities of water. In emulsoids, on the other hand, the particles very powerfully hold or absorb large quantities of water, forming gels. Such gels have been made solid, containing as high as 99.87% water.

Gels are substances in the colloidal state which have set or become apparently solid, even though they contain a large percentage of water. The fruit juice (acid), water, and sugar in the jelly cup have set because of a small percentage of pectin. All are essential to the phenomenon. Certain minerals, for example most orthosilicates and some zeolites, on some evaporation after solution in hydrochloric or nitric acid, will set in beautiful gels because of the silicic acid liberated. Sodium silicate or water glass sets in the same way when treated with acetic or some other acid, which makes a salt with the sodium and leaves the silica to gel.

As early as 1861 Thomas Graham1 knew that dissolved crystalline substances diffused in and out of silica gels, and that reactions occurred between salts in solution in the gels and solutions from outside which diffused in. The famous Liesegang rings were discovered in 1896. Many of these were rings of metallic crystals or crystalline salts, formed in silica gels by reactions between two solutions, one of which was in the gel and the other was gradually diffusing through the gel. The well-developed crystal had time to form because the solutions came in

8 Holmes, H. N., Colloid Chemistry, 1922 (Wiley and Sons).

Hatschek, E., and Simon, A. L. Trans. Inst. Mining and Metallurgy, London, 21, 451-480, 1911-12.

contact so slowly. Bechhold in 1905, Ziegler in 1906, and Hatschek in 1911 continued experiments with rings in gels.

Mineral Genesis through Colloidal State.-Liesegang suggested after his studies on the rings in gels that the banding of agates might be due, in many cases, to the slow diffusion of iron salts through silicic acid gels which had previously been laid in cavities or crevices. He also suggested that this process might explain the formation of large crystals in or on quartz in places where igneous activity had not operated, and where they therefore could not be due to its heat and gases.

In the discussion a the Academy meeting, two points were made. Prof. G. . Lamb reported orally the finding of good agates in recent clays; the data seem to be wanting as to whether they developed in preexisting cavities or made their own spaces. Agates in the major cavities of buried bones testify to the recency also of agatemaking, and as well to the rate of making. Long ages are not necessary. No evidence was presented as to whether or not the agates came through the gel stage, or how the banding originated.

As early as 1884, Clarke tells us, Bourgeois made opal synthetically from gels of SiO2. Clarke states also that the gelatinous silica formed by the solution of silicates in mineral acids becomes, upon drying, an amorphous mass essentially identical with opal. He also reports that Schafhäutl heated a solution of colloidal silica in a Papin digester and obtained a crystalline deposit of quartz; while de Senarmont heated gelatinous silica with water and carbonic acid gas to temperatures between 200° and 300°C. and obtained a crystalline quartz. Chrustschoff, 1873, obtained quartz from an aqueous solution of colloidal silica by heating to 250°C. for several months. The heat apparently hastened the process, which would have reached the same end in a longer time without so much heat. Silica is dissolved when silicates decompose, and is redeposited by evaporation as opal if still in an acid solution, but when alkalies are present it crystallizes. When the Simplon tunnel in central Switzerland was put through the Alps, the engineers found a vein of silica

Bourgeois, L., Production of Minerals Artificially, p. 93. French.

Clarke, F. W., Data of Geochemistry, U. S. G. S. Bull. 616, p. 357.

in the gel condition or colloidal state, thus showing that the silica gel is not purely an artificial product. If the ancient scientists who gave the name "quartz" to rock crystal had found this vein and watched it through its transformation to crystalline quartz, they might have had some ground for their theory that it was made of water which had become so thoroughly frozen in a very cold winter that its solid state had become permanent.

The experiments of Liesegang, and of many other colloid chemists who have followed him, suggest a method for the formation of the quartz-gold vein. They have shown that if a gold solution, e.g. AuCl,, is mingled with a silica gel when the latter sets, and then is treated with a reducing agent diffusing through the gel, crystals of gold will form in the gel. Oxalic acid placed on top may serve as this reducing agent. H2S in water, SO2, or CO will serve the same purpose. Sodium sulphite, and especially FeSO4, do occur in solutions in nature and would be suitable reducing agents there.

Holmes' has shown that a preparation of silicic acid gel made 0.1N with respect to potassium iodide, and then covered with 0.5N mercuric chloride, gives in the gel after a few days bands of red crystalline mercuric iodide. In some cases as many as 40 rather sharply marked bands occurred in a distance of 8 cm. In a similar manner, bands of copper chromate, cuprous oxide, basic lead iodide, basic mercuric chloride, and other substances were formed. Bands of colloidal gold with scattered crystals of gold developed in a preparation of gold chloride and H2SO4 with oxalic acid above the gel.

If the silica gel were in one crevice and an intersecting crevice should be conveying one of these reducing solutions, the conditions of the laboratory would be very nearly duplicated. The only difference would be that the solution need not become continually more dilute, but might remain essentially the same in composition for years. In such case there might be no occasion for the banding of the salts, or if bands developed, they would no doubt be evenly spaced. Banding is explained as a result of a reduction in concentration of the solution in the gel just in front of the ring, so that the diffusing

7 Holmes, H. N., Jour. Amer. Chem. Soc., 40, 1187-1195, 1918. 'Holmes, ibid.