46 PHYSJCS OF STREAMING 



end of a smaller magnet brought near to the object formed the other pole. 

 In my own experiments the electro-magnet used consisted of two parallel 

 arms placed horizontally, and with pointed pole-pieces, which could be 

 adjusted across the stage of a microscope so as to be any distance apart, 

 from a millimetre upwards. Each iron core was 4 cm. diameter, its height 

 was 30 cm., and the diameter across the coils was 15 cm. A current of 

 12 volts was used to charge the coils, which gave 22 amperes against 

 a resistance of about ^ an ohm. 



Observations upon the directive action of a magnetic field were carried 

 out by suspending plant-cells and other objects in still moist air between 

 the poles of the electro-magnet by means of fibres of unspun silk. With 

 large objects, a strand of six to ten untwisted fibres may be used. The 

 manipulation is naturally somewhat difficult, since the object must lie 

 horizontally when suspended, but the copper loops and paper stirrups used 

 by Faraday are unsuitable, in the first case owing to the disturbing effect 

 of the currents induced at each movement, and in the second one owing to 

 the fact that paper is paramagnetic. 



Experimenting in this manner with suspended living cells of Chara 

 and Nitella, it was at once seen that they swing so as to set their long axes 

 parallel to the lines of force in the field. The same was shown when 

 suspended in water, although the movement was slower, owing to the 

 greater inertia of the water. Either end was presented indifferently to 

 either pole, and hence the movement is not the result of the existence of 

 galvanic currents in the cell, but is due to the average magnetic permea- 

 bility of the different constituents of the plant-cell being greater than that 

 of air and of water. Similar results were given by strips of leaf-cells of 

 Elodea and Vallisneria, by leaves of the onion, and more strongly by 

 peduncles of Primula, Narcissus, and Tulipa, as well as by the petioles of 

 various leaves and the stems and leaves of grasses. All these objects were 

 as strongly paramagnetic when dead, and when dry, as when living and 

 moist. The phenomenon is, therefore, a purely physical one, but the 

 results obtained are somewhat surprising, since Faraday 1 explicitly states 

 that sugar, starch, gum-arabic, wood, dried and fresh mutton and beef, 

 apples, bread, fresh and dry blood, leaves, &c., are all diamagnetic, while 

 Reinke obtained negative results with plant-cells. A possible explanation 

 of the latter's results lies in the fact that no directive action is exercised 

 upon a small object placed in a large uniform field, since in all positions it 

 is traversed by equal numbers of tubes of force. If, however, pointed pole- 

 pieces are used, or if the object is brought towards the periphery of the 

 field, it will set itself at right angles to, or parallel to, the lines of force, 

 according to whether it is diamagnetic or paramagnetic. 



1 Experimental Researches on Electricity, 1855, Vol. Ill, p. 35. 



