528 Prof, Magnus on the Changes in the Induced Current. 



not yet proved that opposite currents are produced by suffi- 

 ciently weak resistance, still at the very least it is highly proba- 

 ble that such is the case. 



Moreover, both Dr. Feddersen* and Dr. Paalzow have found 

 that when the Leyden battery is discharged, opposite currents 

 make their appearance if the resistance be trifling. 



We may therefore regard as proved that induced currents are 

 only single with a certain amount of resistance. Let the resist- 

 ance exceed a fixed limit, and they are opposite; let it likewise 

 sink below another certain limit, and they arc also opposite. 

 These boundaries vary according to the intensity of the current. 



On the Changes of Colour of Electric Light. 



In the test-tubes that I have employed, the negative light, 

 which in rarefied atmospheric air is generally of an intense blue, 

 is almost white; and in like manner the light extending from 

 the positive pole to the dark intervening space was white, though 

 it is usually red. I have endeavoured to find out the cause of 

 these variations of colour. 



When a newly-made tube of the prescribed kind is used, the 

 negative light is at first blue, and the space between the wires is 

 filled with red light ; but soon afterwards both become brighter. 

 The space between the two wires becomes brown, and finally 

 white, and in the same way the negative light becomes entirely 

 whitish. When this change has once been effected, the colour 

 in the hermetically-sealed tube remains unchanged. If, however, 

 we use a tube which can be opened, and consequently can have 

 its air renewed, the negative light is at first blue and the inter- 

 vening space red, but immediately afterwards both become white 

 again. 



This change cannot depend on the union of oxygen with alu- 

 minium ; for in nitrogen gas, which in this case would be 

 left, the colour of the electric light is very similar to what it 

 is in atmospheric air. The appearance is most like that of the 

 electric light in carbonic acid or hydrogen ; but as neither of 

 these gases was present, it occurred to me that perhaps the alu- 

 minium, during its preparation, might have come in contact with 

 some foreign substance (some greasy matter for instance), and 

 that by this means the phenomenon was brought about. Two 

 aluminium wires, which were cut out of rolled plate, were conse- 

 quently purified by scraping as much as possible, and without 

 being touched by the fingers were fused into the tube. Under 

 these circumstances the wire retained the light unchanged from 

 what it was at the first moment, that is, continually blue at the 

 negative wire, and red in the intervening space. 

 * Popgcndorff's Annalcn, vol. cxii. p. 452. 



