164 Dr. J. Kerr's Electro-optic 



acid. With the jar and two turns of the plate, the restoration 

 was good, though still faint ; with ten or fifteen turns it Mas 

 brilliant. The effect was always strengthened by tension 

 parallel to lines of force, and always extinguished perfectly by 

 compression. 



Isobutyric acid, examined immediately afterwards in the 

 same way, acted very similarly, but was apparently stronger. 

 Propionic acid (n = o) showed an additional fall of power — 

 gave no clear effect without the jar and five or more turns of 

 the plate. The effect of discharge at every tenth turn was 

 moderately strong, and was found to be purely positive by 

 both tests. 



Temperature was observed to have a great influence on the 

 action of this acid. In one experiment, the initial temperature 

 happened to be about 0° C, and there was no effect obtained 

 with less than twenty turns of the plate; but when the cell 

 was now gently heated by exposure for a minute in front of 

 the fire, the effect of discharge was certain at every third turn, 

 and good at every fifth. 



Glacial Acetic acid (n = 2) showed an additional fall of 

 power. "With the jar and ten turns of the plate, it gave an 

 effect so faint as to be often uncertain. With fifteen turns, 

 the restoration was clear and regular, though not vet strong. 

 Tested by the hand-compensator at this or higher power, the 

 effect was strengthened regularly by tension parallel to lines 

 of force, but hardly or not at all affected (and certainly not 

 strengthened) by compression. Acetic acid is seen thus to be 

 purely positive, and a very weak insulator. The second test 

 by the hand-compensator (neutralization by the proper strain), 

 fails here, as it failed earlier (with Leyden discharges) in the 

 series of alcohols. 



Cry stallizable Formic acid (n=l) showed a large additional 

 fall of power, with undoubted reversal of sign. Tried with 

 plate machine and strongest charges of the jar, it gave an effect 

 which was clear and perfectly regular, though very faint and 

 abrupt. The light was now restored steadily and very faintly 

 from extinction, by permanent strain of the hand-compensator ; 

 and the observations were repeated. In the case of compres- 

 sion parallel to lines of force, the effect was regular and very 

 distinct ; the light brightened sensibly for a moment at the 

 instant of discharge. In the case of tension parallel to lines 

 of force, the effect was not so regular; at the instant of discharge 

 the light was sensibly and very abruptly darkened in a few 

 instances, never certainly strengthened, and generally affected 

 merely as by an instantaneous shock, without seizable change of 

 intensity. Like results, though hardly so good, were obtained 



