and confirmation of these researches, most of which were putlished several years before his corresponding investigations. His work is really original only as to the magnetic properties of liquid oxygen: that which is not borrowed from my researches is a development of ideas struck out by anotheras, for instance, the experiments on electrical resistance at low temperatures, which were begun by Clausius, continued by Cailletet and Bouty, and brought ten years ago by my former fellow-worker, the late Prof. Wróblewski*, to the temperature of the freezing-point of nitrogen, then several degrees below the temperature attained in the experiments of Messrs. Dewar and Fleming, who, it is true, extended their examination to various metals, alloys, and non-metals. But the execution of these labours meets with no difficulty; for the method of getting large quantities of liquefied gases is now generally known.

In the table opposite (p. 210) I have set down the most important numbers, obtained as stated in my former experiments, concerning the liquefaction and solidification of

gases.

List of my Researches, of which I have specified the most important Results in the foregoing Summary.

All my researches were at first published in the Polish language, in the publications of the Cracow Academy of Sciences; I afterwards published an excerpt of each of them in French and German. The present list contains only French and German papers, as being accessible to students of every nationality. The numbers enclosed in brackets in the preceding summary have reference to the ordinal numbers of the following list.

1. K. OLSZEWSKI. Essais de liquéfaction de l'hydrogène.-Comptes Rendus, vol. xcviii. p. 365 (1884).

2.

3.

4.

5.

Nouveaux essais de liquéfaction de l'hydrogène, solidification et pression critique de l'azote.-C. R. vol. xcviii. p. 913.

Température et pression critique de l'azote. Température d'ébullition de l'azote et de l'éthylène sous de faibles pressions.-C. R. vol. xcix. p. 133 (1884).

Température et pression critique de l'air. Relation entre la température de l'air et la pression de l'évaporation.— C. R. vol. xcix. p. 184 (1884).

Relation entre les températures et les pressions du protoxyde de carbone liquide.-C. R. vol. xcix. p. 706 (1884).

* Comptes Rendus, ci. p. 161.

6. K. OLSZEWSKI. Température de solidification de l'azote et du protoxyde de carbone; relation entre température et la pression de l'oxygène liquide.-C. R. vol. c. p. 350 (1885).

7.

8.

9.

10.

11.

12.

13.

Liquéfaction et solidification du formène et du deutoxyde d'azote.-C. R. vol. c. p. 940 (1885).

-. Sur la production des plus basses températures.-C. R. vol. ci. p. 238 (1885).

Bestimmung der Dichte und des Ausdehnungscoëfficienten des flüssigen Sauerstoffs.-Anzeiger der Akad. der Wissensch. in Wien, 1884, nr. ix.

Bestimmung der Erstarrungstemperatur einiger Gase und Flüssigkeiten.-Monatshefte für Chemie, Akad. d. Wissensch. in Wien, vol. v. p. 127 (1884).

Erstarrung des Fluorwasserstoffs und des Phosphorwasserstoffs, Verflüssigung und Erstarrung des Antimonwasserstoffs.-Sitzungsberichte d. Akad. in Wien, vol. xciv. p. 209 (1886).

Bestimmung des Siedepunktes des Ozons und der Erstarrungstemperatur des Aethylens.-Ibidem, vol. xcv. p. 253 (1887). Also Wiedemann's Annalen, vol. xxxiii. p. 570 (1888).

Ueber das Absorbtionsspectrum des flüssigen Sauerstoffs und der verflüssigten Luft.-Ibidem, vol xcv. p. 257 (1887). Also Wiedemann's Annalen, vol. xxxiii. p. 570 (1887). 14. S. WRÓBLEWSKI and K. OLSZEWSKI. Ueber die Verflüssigung des Sauerstoffs, Stickstoffs und Kohlenoxyds.-Wiedemann's Annalen, vol. xx. p. 243 (1883).

15. K. OLSZEWSKI. Ueber die Dichte des flüssigen Methans sowie des verflüssigten Sauerstoffs und Stickstoffs.-Ibidem, vol. xxxi. p. 58 (1887).

16.

17.

18.

19.

20.

21.

Ueber das Absorbtionsspectrum und über die Farbe des flüssigen Sauerstoffs.-Ibidem, vol. xlii. p. 663 (1891). Sur l'éthane et propane liquides.-Bulletin International de l'Académie des Sciences de Cracovie, January 1889, I.

Appareil pour liquéfier et solidifier les gaz appellés permanents et pour étudier leur spectre d'absorbtion.Ibidem, January 1889.

Sur les propriétés physiques de l'acide selénhydrique soumis à une basse température et à la pression.—Ibidem, February 1890.

1890.

Transvasement de l'oxygène liquide.—Ibidem, June

Ueber den kritischen Druck des Wasserstoffs.Ibidem, May 1891.

22. OLSZEWSKI and A. WITKOWSKI. Propriétés optiques de l'oxygène liquide.-Ibidem, October 1891.

23.

Sur la dispersion de la lumière dans l'oxygène liquide.-Ibidem, July 1894.

24. K. KRZYŻANOWSKI. Sur la liquéfaction et la solidification de l'hydrogène dans les experiences de M. Pictet.-Ibidem, January 1889.

XX. Mirrors of Magnetism.

By SILVANUS P. THOMPSON, F.R.S., and MILES Walker*. UR knowledge of Electric Images is based almost entirely upon mathematical deduction. It would be difficult to satisfactorily demonstrate their properties by experiment. We are content to take the elegant geometrical proofs that Lord Kelvin has given us, and corroborate these physically at points where the conclusions are within reach of experiment. The subject, therefore, is one of purely theoretical interest, but so beautifully does it elucidate difficult problems in Electrostatics that it remains the continual delight of text-book

writers.

Magnetic Images, on the other hand (apart from one or two investigations by the same great master), are entirely neglected, and yet these are much more likely to be practically utilized in the near future than their electrical analogues. It may be in questions of dynamo and instrument shielding, or it may even be in the solution of that commercial problem, the testing of the magnetic properties of iron en masse.

The existence of magnetic images is suggested by considerations altogether apart from the mathematical.

If we take a solenoid of wire through which a current is flowing and place its end against a large plate of iron, we eliminate the effect of that end upon the distribution of the field about the coil, and so obtain a field on one side of the plate of a shape the same as if the coil were continued to double its length. If we place the coil with its end against a mirror (fig. 1), we see it reflected in the mirror so that it appears to be double its real length. Thus the iron plate gives us magnetically an image of the coil analogous to the optical image produced by the mirror, geometrically identical indeed with the optical image, virtual and erect.

If a large iron plate is placed at each end of the coil the field will be the same as if the coil were infinitely long. We know that two mirrors, facing each other, one at each end of the coil, would give it the appearance of stretching away to infinity in both directions (fig. 2).

Of course these optical effects could only be perfectly obtained if the mirror were a perfect reflector; and to include all possible points of view it would have to be infinitely large. So the magnetic effects could only be perfectly obtained if the plates were perfectly permeable and infinitely large. Observe that we are only considering the effect at a point in front of the mirror and in front of the iron plate.

* Communicated by the Physical Society: read November 23, 1894.

Behind, that is to say on the side opposite the coil, there is no effect in either case.

Fig. 1.

Fig. 2.

I

It was thought interesting to ascertain how far effects of this kind could be obtained in practice, using an ordinary iron plate somewhat less than that required for the perfect case, For this purpose two similar solenoids were wound,