SCIENTIFIC SUMMARY. 
91 
permanent~electric current have already been described in this Summary, 
its direction was evidently a matter of fundamental importance. Prof. 
Rowland predicted that the direction would be reversed in paramagnetic 
iron from that found in diamagnetic gold, and experiment has verified the 
prediction. Mr. Hall finds, however, that nickel and platinum — both 
magnetic substances — resemble, not iron, but gold. The fact has to be taken 
into account in endeavouring to apply the newly-discovered action to explain 
the magnetic rotation of the plane of polarization in accordance with 
Maxwell’s electromagnetic theory of light. He has, therefore, repeated 
Kerr’s experiment on rotation by reflexion from the pole of a magnet, using 
nickel for the latter instead of iron. The disc was placed between the poles 
of an electromagnet. The action on the plane of polarization, though ap- 
parently much weaker than in iron, was unmistakably in the same 
direction. Prof. Hopkinson, in a note contributed to the Philosophical Magazine , 
shows that what Clerk-Maxwell calls the ‘ Rotatory co-efficient ’ completely 
expresses the important facts discovered by Mr. Hall. Of this co-efficient 
Maxwell says, 'We have reason to believe that it does not exist in any 
known substance. It should be found, if anywhere, in magnets which have 
a polarization in one direction, probably due to a rotational phenomenon in 
the substance.’ The passage affords another remarkable instance of theoreti- 
cal prediction, afterwards confirmed by experiment. 
The Velocity of Sound in Gases has been very comprehensively compared 
by Herr Hilfer. He divides the methods into three groups : — 
I. Theoretical methods (a) from wave-theory : results for air, V. = 280 m., 
with correction, 331*88 for 0° : (6) from the kinetic theory of gases, equal to 
2 8 — 27 r 
o or of the mean value u of molecular velocity : result = 323*33 
O 7T — Z 
or 322*47 ; with correction on account of duration of collisions, 332 m. : (c) 
from a combination of the formulae of wave-motion, and of the gas-theory ; 
result = 331*94. 
II. Direct experimental methods , in some of which, with length known, 
time is measured, either in open space or in tubes ; in others, with time given, 
distance traversed : results, excluding the older experiments : — French Com- 
mission (1822) = 331*2 ; Moll and Van Beek (1823) =332*25: correction 
by Schroeder van der Kolk = 332*77 ; Regnault (1868) = 330*6 ; Le Roux, 
= 330*66 ; Szathmari, by method of coincidences = 331*57. 
IH. Indirect experimental methods, from wave-length : (a) by measure- 
ment of the length of pipes ; (6) by observation of dust-waves ; (c) by means 
of interference tubes ; ( d ) from coincident vibrations of two manometric 
flames: results, Wertheim =331*33; Zoch(1866) =332*65. — Png. Mechanic. 
The Dependence of Thermoelectric Currents on the Gases in contact with 
the Terminals having been asserted by Exner, Prof. C. A. Young has pro- 
duced a test experiment on the point. An exhausted glass tube contained an 
iron wire with platinum terminals, which were again fastened to iron wires 
leading to a galvanometer. The tube was exhausted to one-millionth of an 
atmosphere. On laying the apparatus in sunlight, and alternately shading 
the internal or external junctions, an electromotive force could be produced 
which was found to be equal in every case. He considers this to negative 
Exner’s statement. 
