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have been about 1,000,000 years ago, and that it may have lasted about 2,000,000 years; afterwards he approximately fixed the incoming of that Period at 240,000 years ago, and its termination about 160,000 years afterwards (80,000 years ago). Prof. Prestwich considers that Palæolithic Man, that is, the man of the Valleydrifts, appeared in Western Europe probably not longer ago than about 20,000 or 30,000 years, and disappeared about 10,000 or more years ago. He was probably preceded by the Plateau or Eolithic Man (referred to in the footnote at page 640), before the time of extreme glaciation; and he was certainly succeeded, with some intervening lapse of time (page 619), by the Neolithic Man.

Before taking up the subject of glacial phenomena in Asia, Australia, and North and South America, in Chapters XL. to XLII., Prof. Geikie gives two interesting chapters on Cave-deposits, Valley-drists, and the Loess of Central Europe. The last of these has been already referred to. The first involves a consideration of the two Stone Periods (old and new), and evidences of human work and occupation in England and France. For the Valley-drifts M. Ladrière's results are given in detail, in preference to Prof. Prestwich's work; and, though the latter is not lost sight of, the treatment is one-sided (pp. 629-637), brief references being made to Prestwich's elaborate memoirs explanatory of river-gravels, and a full account given of M. Ladrière's paper illustrative of views which appear somewhat artificial. So also we should like to have seen further references to Prestwich's work and views on the Pliocene Crags and the associated Pleistocene Beds of the Eastern Counties, besides the useful notes derived from Mr. Clement Reid's excellent memoir (pp. 329–336); and, with reference in the footnote at p. 603) to the “ rubble-drift,” we may observe that the Author should have remembered that Prestwich limited its range to 1000 feet in England only, and gave several cases where it exceeds 1500 and 2000 feet on the Continent. His name also might with advantage have been given (at p. 392) at the same time as Mr. A. Collinette's. Prof. Bonney's researches on ice-work and its results seem to have altogether escaped notice. It may be that, with the multiplicity of facts and inferences

athered for assimilation whilst preparing a voluminous work, an Author can scarcely avoid handling some subjects and some writers less carefully and judiciously than others. Prof. Geikie seems to be aware of this ; and apologises in his Preface for any such inadvertencies and shortcomings.

Professor Chamberlin's two Chapters on the Glacial phenomena of North America (pages 724-775) give a concise and clear account of the enormously extensive glacial drifts covering nearly one half of the country. The great ice-field of Greenland, as now, lay formerly to the north-east; and on the mainland glaciers were . formed on the high grounds of Canada and Labrador, spreading and coalescing into the great so-called Laurentide ice-sheet. On

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the west side of the Rocky Mountains and Cordilleras confluent glaciers were another centre for the formation and dispersion of “ drift." The glacial deposits of North America, numerous, extensive, and thick, overlapping one another, reach southwards to the coast of New York and New Jersey, and over-ride the States that lie north of the Ohio and Missouri.

The till, drumlins, escars, kames, sand-plains, glacio-fluvial moraine-aprons with their valley-drifts and sheets of silt or loess, partly due to ice and water and partly to wind, are definitely handled.

One of the classifications of the great North-American driftsories suggested by Prof. Chamberlin is as follows:-

(1. Concealed deposits (theoretical).
| 2. Kansan stage of glaciation: First Glacial Epoch.

3. First Interglacial Epoch.
Glacial 1. East-Iowan Stage : Second Glacial Epoch.
Period. 5. Second Interglacial Epoch.

6. East-Wisconsin Stage: Third Glacial Epoch, perhaps com

prising a Fourth Epoch.

7. Later oscillations, Professor Geikie thinks that this arrangement is closely parallel with the glacial history which he has described for this part of our hemisphere. He regards the Kansan stage as equivalent to his own “Second Glacial Epoch,” with the maximum glaciation, and specially applicable to the Lower Diluvium of Central Europe. The East-Iowan matches his Third epoch; and he finds corresponding features in Europe for the East-Wisconsin, and even for the possible fourth epoch entertained by Prof. Chamberlin (p. 775).

In the Appendix, pages 817-826 give a well digested account of the published evidences of the results of glacial action at various geological periods, in many parts of the world. Indeed it is pointed out that proofs, more or less distinct, occur in all, or nearly all, the geological formations, of contemporary glacial boulders or glaciated surfaces. The Appendix contains also an account of Loch Lomond and its map (Pl. XVI.); and remarks on the maps, Pls. I., IX., X., XI., XIII., XIV., and XV., showing the directions of glaciation in the British Isles, Europe, Asia, and North America. Notes on the inap, Pl. XII., showing the distribution of land and sea in Northern Europe after the last great Baltic glacier come also in this Appendix, p. 832. Eight other maps

and charts are given in the volume; and 78 woodcuts of views, sections, rocks, and illustrative diagrams are scattered through the text.

A General Index and one of Authors are useful adjuncts to this elaborate history of the “Great Ice Age.”

XXXIX. Intelligence and Miscellaneous Articles.

SILVERING GLASS IN THE COLD.

BY MM. AUGUSTE AND LOUIS LUMIÈRE. FOR silvering glass in the cold, a number of methods have been

described which appear open to the objection of being very complicated, and of requiring very minute precautions.

In investigating the properties of formic aldehyde, we bave observed that this substance gives with ammoniacal solutions of silver nitrate, adherent mirrors which can be readily polished. By working under suitable conditions, we have observed that the greater part of the silver contained in the solutions is deposited on the glass, thus avoiding residues and diminishing the cost.

After numerous trials we have arrived at the following method.

100 cubic centiin. are taken of a 10-per cent. solution of silver nitrate to which ainmonia is added drop by drop, so as just to redissolve the precipitate formed at first. Care must be taken to avoid an excess of ammonia, which would hinder the formation of the deposit. The volume of the solution is made up to a litre by distilled water, and we thus obtain what we shall call A.

On the other hand, commercial formaldehyde of 40 per cent. is diluted with distilled water so as to form a 1-per cent solution. Owing to its dilution this solution, B, may be kept for some time.

The surface of the glass is carefully polished by rubbing it with chamois covered with rouge, and two volumes of A are rapidly mixed with one volume of B, and the mixture is rapidly poured over the glass to be coated.

In five or six minutes, at the temperature of 15° to 19°, all the silver in the solution is deposited in a brilliant layer which is washed with water. It is then dried, and varnished if the surface in contact with the glass is to be the reflecting surface, or polished with the ordinary precautions where the layer itself is to be used as in astronomical instruments.-Journal de Physique, January 1895.

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The results of this research are summed up by the author as follows:

1. The potential gradient v in the positive nustratified glowlight with constant current diminishes as the current i increases, and the value of this decrease is given with sufficient accuracy by the equation

v=v.-(i-1). The value 6, that is the decrease of the gradients for unit increase of the current strength, decreases as the width of the tube in the clear increases, and for the same width has almost the same value for nitrogen and for hydrogen.

2. The potential gradient in question decreases as the clear width of the tube increases; and

3. Increases as the pressure increases, but more slowly than in proportion to the pressure.

4. For nitrogen quite free from oxygen it is, for pressures between 4 and 8 mm., 1.4 times as much as hydrogen.

5. A small admixture of oxygen with the nitrogen increases it; a small admixture of aqueous vapour with nitrogen produces no change.-Wiedemann's Annalen, No. 2, 1895.

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PRIZE-QUESTION PROPOSED BY THE SCHNYDER VON WARTENSEE'S

FOUNDATION FOR ARTS AND SCIENCES, ZÜRICH, The Schnyder von Wartensee's Foundation again proposes for the year 1897 the following prize-question on problems in Physics.

“As the numbers which represent the atomic beats of the elements still show very considerable divergences, the researches conducted by Professor H. F. Weber on boron, silicon, and carbon, on the increase of the specific heat with the temperature, are to be extended to several other elements prepared as pure as possible, and also to combinations or alloys of them. The densities and the coefficients of thermal expansion of the substances investigated are further to be determined as accurately as possible."

The conditions are as follows:

Art. 1. The treatises handed in by competitors for the prizequestion may be either in German, French, or English, and must be sent in by September 30th, 1897, at the latest, to the address given in Art. 6.

Art. 2. The examination of the treatises will be entrusted to a jury composed of the following gentlemen : Professor Pernet, Zürich.

A. Hantzsch, Würzburg.
E. Dorn, Halle-on-the-Saale.
J. Wislicenue, Leipsic.
G. Lunge, Zürich, as member of the committee

proposing the prize-question. Art. 3. The Prize Committee bas at its disposal a sum of four thousand five hundred francs, of which a first prize, of no less than three thousand francs, will be awarded and minor prizes for the remaining sum.

Art. 4. The work to which the first prize is awarded remains the property of Schnyder von Wartensee's Foundation, which has to arrange with the author regarding its publication.

Art. 5. Every treatise sent in must bear a motto on the titlepage and be accompanied with a sealed envelope containing the author's name and bearing the same motto outside.

Art. 6. The treatises are to be sent to the following address, within the period named in Art. 1 :

“ An das Präsidium des Conventes der Stadtbibliothek in Zürich (concerning prize-question of Schnyder von Wartensee's Foundation for the year 1897)."

THE

PHILOSOPHICAL MAGAZINE

AND

JOURNAL OF SCIENCE.

[FIFTH SERIES.]

MAY 1895.

XL. Tests of Glow-Lamps, and Description of the Measuring

Instruments Employed. By Prof. W. E. AYRTON, F.R.S., and E. A. MEDLEY*. THE HE conditions under which the glow-lamp can be most

economically used have attracted considerable attention since a paper entitled “The Most Economical Potential Difference to employ with Incandescent Lamps

was read before the Physical Society in February 1885 † by Professor Perry and one of the authors of the present communication. In this paper they stated that it was well known, from experiments made by their students in 1880, and from results published in 1881 by Lord Kelvin, then Sir William Thomson, as well as from subsequent experiments, that the light obtained from an incandescent lamp increased much more rapidly than the power expended in it; or, that the number of candles produced per watt of power expended in the lamp increased as the filament became botter. But it was pointed out in the paper in question that such experiments by themselves gave no idea of the commercial value of any particular glow-lamp, because they afforded no indication of its life when run at different efficiencies, and it was known that the length of time a filament would last was the less the higher its temperature.

They then proceeded to show that the cost of lighting per hour per candle could be divided into two parts, viz., the cost

* Communicated by the Physical Society: read December 14, 1894, and January 25, 1895.

† Phil. Mag. April 1885, p. 305. Phil. Mag. S. 5. Vol. 39. No. 240. May 1895. 2 D

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