654 PLUCKER ON THE ACTION OF THE MAGNET 



keeper (B), as high as the preceding, as broad as the poles of the 

 magnet and 176 milUms. in length, rounded off circularly on one 

 side, and so narrow that the terminal surfaces formed circles 

 25 miUims, in diameter. Upon these, pieces of different shapes, 

 such as conical apices, could be screwed. These two halves of 

 the keeper may also be advantageously substituted for the two 

 perfoi'ated cylindrical appendages with the inserted pointed cy- 

 linders, in the experiments described in the two preceding me- 

 moirs. Even with the most powerful magnetic excitation and 

 the greatest approximation of the conical apices, these two halves 

 of the keeper do not fly together. Thirdly, two heavier halves of 

 the keeper (C) ; these were originally intended for optical pur- 

 poses, 40 millims. in height, 133 millims. broad and 203 millims. 

 in length, circularly rounded at one end, gradually tapering at 

 the other, and prolonged into a rectangular surface of 40 millims. 

 and 59 millims. In the middle of these two halves of the keeper, 

 throughout their whole length, a groove is cut, 20 millims. ia 

 breadth, of the same depth, and the section of which is semicir- 

 cular at the bottom. The halves of the keeper (A) and (C), 

 when the excitation is very intense and the approximation suffi- 

 cient, are attracted by each other with great force; they are 

 kept apart by pieces of brass of different thicknesses placed be- 

 tween them. 



In the experiments upon the free ascending gases, the rectan- 

 gular glass-case of the torsion-balance, which, when the latter is 

 removed, has a rectangular aperture at the top in the middle, 

 254 millims. in length in the equatorial direction (corresponding 

 to the breadth of the case) and 92 millims. in the axial direction, 

 was generally placed upon the moveable table with the two round 

 holes through which the arms of the magnet passed. 



In the experiments to be described I used from five to ten 

 Grove's cells, the larger number only being set in action when 

 the nitric acid had been frequently used previously. 



§ 1 . Ott the Diamagnetism of Gases. 



3 . Faraday devotes the paragraphs 2400 to 24 1 6 of the twenty- 

 first series of his 'Experimental Researches on Electricity' to 

 the action of magnets upon air and gases; and in the last para- 

 graph arrives at the following conclusion: — 



" Whatever the chemical or other properties of the gases, 

 however different in their specific gravity, or however varied in 



