C. GENERAL PHYSICS. 195 



saturated solution of a mixture of nitrate of potassium and 

 sulphate of sodium solidifies at 5. A mixture of nitrate 

 of sodium and sulphate of potassium also solidifies at this 

 temperature ; but the temperatures never fall as low as the 

 point which could be reached by employing whichever of 

 the salts a x, a y, b x, or b y forms a cryohydrate with the 

 lowest temperature. Thus, in the above case, the solidifying 

 point of nitrate of sodium is 17. Nature, XL, 440. 



OX UNILATERAL ELECTRIC CONDUCTIVITY. 



Dr. Arthur Schuster states that in the course of many ex- 

 periments he has had frequent occasion to remark that elec- 

 tric currents seem to traverse copper wires more easily in 

 one direction than in the other; so that the galvanometer 

 indicates different intensities when we reverse the direction 

 of the current which traverses it. He gives to this phe- 

 nomenon the name of unilateral conductivity. He first ob- 

 served it in using the galvanic battery, but was able to 

 make more accurate observations by means of magneto-elec- 

 tric machines. The phenomenon observed with this appa- 

 ratus led him to the hypothesis that the current induced by 

 one pole of the magnet traverses a circuit more easily than 

 that induced by the opposite pole. In his second memoir 

 Schuster records another phenomenon which is not without 

 analogy with the preceding. He joined the electrodes of a 

 galvanic battery to the apparatus which he had used in his 

 first experiment, and found that, whatever the intensity of 

 the continuous current might be, or the relative positions of 

 the electro-magnet to the battery, it always happened that 

 the initial deviation of the galvanometer needle augmented 

 during the rotation of the magnet. On the other hand, as 

 long as the magnet was immovable, it exerted no influence 

 upon the deviation produced by the permanent currents. 

 Noting then, at first, the initial deviation of the needle while 

 the magnet was stationary, he interrupted the current be- 

 fore turning the magnet, and observed again the deviation 

 produced at the first passage of the current during the rota- 

 tion. The difference between these two deviations was sen- 

 sibly proportional to the intensity of the permanent current, 

 but decreased rapidly with the increase in the electric vibra- 

 tions produced by the magnet. The cause of this singular 



