166 DR. FARADAY’S EXPERIMENTAL RESEARCHES IN ELECTRICITY. (SERIES XXX.) 
As the difference of force remains constant when the surrounding' medium is varied 
(338L), it follows that the possibility exists of finding' a magnecrystal and a medium 
so related, that the attraction and repulsion of the crystal, as a whole, should be con- 
vertible terms depending upon the position of the crystal in regard to the lines of 
force. I was desirous of verifying this result experimentally, and especially in rela- 
tion to the case of mere space or a vacuum about the crystal, and therefore seleeted 
certain magnecrystals which promised favourable results, and yielded the following 
illustrations. 
3387- The tourmaline crystal already employed (3377-) was found paramagnetic, not 
merely in water and air, but also in a saturated solution of protosulphate of iron ; and 
though the difference in degree of attraction according to its position was very striking 
in all the media, the substance was for the present dismissed. 
3388. Red ferroprussiate of potassa, being attracted in water or camphine, was 
repelled in the solution of iron, and therefore promised the desired result if it could 
be protected from the action of aqueous solutions*. Some good crystals were selected, 
and shortened by grinding until the length was little more than the breadth ; then 
the angles were removed until each crystal became a rounded mass, after which they 
were made fast to suspending copper wires (3370.), 6 inches in length, so that the 
axis of the crystal prism should be in the horizontal plane, and, when in place in the 
magnetic field, either axial or equatorial at pleasure. Some wax was melted and 
kept at a temperature above the fusing-point, and the crystals being introduced and 
retained in the wax until they were above its fusing temperature, were then removed 
and carefully hung up. Afterwards a wax bath was prepared, of which part was 
fluid and part solid, and the cold crystals being suddenly dipped in and removed, 
brought away a congealed coat of wax which in the course of a few minutes became 
a compact envelope. Being left for a few hours, 1 found that they might afterwards 
be immersed and left in water, or in a solution of iron, for two or three days without 
any action between the crystal and the medium around it. No varnishing could 
thus protect them. 
3389. A small torsion balance with a single cocoon thread was constructed. The 
end of the arm intended to sustain the crystal, was bent at an angle of 90°, so that 
the crystal could be suspended from it in either of two positions at right angles with 
each other. A counterpoise to the crystal was placed on the other arm, and the 
balance was covered with ajar to screen it from air currents. The crystal, when in 
place, hung down below the edge of the jar, descending into a vessel arranged at one 
pole of a great electro-magnet (2247.), so that it could be surrounded by any medium 
in which its actions were to be observed. The pole-piece terminated either in a cone, 
or an upright edge, oi' a face inch square ; the cone was the best, and the crystal 
* Vai'nished crystals are not protected ; when put into water the salt dissolves through every part of the coat ; 
for, being soluble in alcohol, the eoating matter is a mixture of resin and the salt. In solution of iron this 
substance dissolves in a very interesting manner whether unprotected or imperfectly coated. 
