252 
PROFESSOR TYNDALL’S FURTHER RESEARCHES 
As in the case of distilled water, vve observe a deflection in one direction when the 
current is “direct” and in the other when it is “reversed,” the action in the first 
case, in passing from position 1 to 3, amounting to five and in the latter case to six 
divisions of the scale. The polarity exhibited is that of diamagnetic bodies. 
DEPORTMENT OF MAGNETIC BODIES. 
Thus far we have confined our examination to diamagnetic substances ; turn we 
now to the deportment of magnetic bodies when submitted to the same conditions of 
experiment. Here we must select the substances suitable for examination, for all are 
not so. Cylinders of iron, for example, of the same size as our diamagnetic cylinders, 
would, through the intensity of their action, quite derange the apparatus ; so that 
we are obliged to have recourse to bodies of smaller size or of feebler magnetic capa- 
city. Besides, the remarks of writers on this subject render it of importance to 
examine whether bodies through which the magnetic constituents are very sparingly 
distributed present a veritable polarity the same as that exhibited by iron itself. 
Slate rock usually contains from eight to ten per cent, of oxide of iron, and a frag- 
ment of the substance presented to the single pole of an electro-magnet is attracted 
by the pole. A cylinder of slate from the Penrhyn quarries near Bangor was first 
examined. It was not found necessary to increase the effect by using two cylinders, 
and the single one used was suspended in the right-hand helix H'E'. The deport- 
ment of the substance was as follows : — 
xvni. 
Cylinder of Penrhyn Slate. 
length 4 inches, 
diameter 0'7- 
Current direct. 
Position 1. 620 
Position 2. 647 
Position 3. 667 
Current reversed. 
280 
240 
198 
Comparing these deflections with those obtained with diamagnetic bodies, we see 
that they are in the opposite direction. With the direct current a change from posi- 
tion 1 to 3 is followed, in the case of diamagnetic bodies, by a motion from higher to 
lower numbers ; while in the present instance the motion is from lower numbers to 
higher. In the former case the north poles of the astatic magnet are attracted, in 
the latter they are repelled. We also see that a direct current acting on diamagnetic 
bodies produces the same deflection as a reverse current on magnetic ones. Thus, as 
we protnised at a former page, the opposite polarities of diamagnetic and magnetic 
bodies are transferred from the region of deduction to that of fact. 
