Application of the results of M. Magnus. 91 



that derived by plotting the 5th series falls considerably below 

 those obtained by plotting the 6th or 7th. A mere inspection 

 of the table exhibits the same in particular cases. For example, 

 a difference of temperature of 268°*64 between iron and platinum, 

 corresponds in the third series to a difference of 13 0, 71 between 

 bismuth and antimony ; whereas in the 6th series, a difference 

 of 268 0, 66 between the former corresponds to a difference of 

 17°- 77 between the latter; and in the 7th series, a difference of 

 268°-56 is equivalent to one of 18°-60. It hence appears that the 

 thermo-electric force of iron and platinum is relatively greater in 

 the 6th and 7th series than in the 5th. We shall now endeavour 

 to account for this hitherto inexplicable result. Turning to the 

 table at page 85 of this Report, we observe that the current formed 

 at the junction of hard and soft in an iron wire passes from hard 

 to soft, which proves that the iron is rendered more negative when 

 it is softened by heat. Let us now devote a moment's attention to 

 the result with platinum wire at page 87. In the case of two ho- 

 mogeneous wires, the current passes from warm to cold, causing 

 a deflection of 24° when both wires are hard. "When a hard and 

 soft wire are taken, and the former is heated, the current passes 

 as before from warm to cold, causing, however, a deflection of 

 only 13°. It thus appears that the soft wire is less negative, or 

 what is the same, more positive than the hard wire. Consist- 

 ently with this, if the heated wire be the soft one, the fact of its 

 being hot and soft at the same time ought to make the current 

 developed a maximum — this is the case. The deflection observed 

 under these circumstances is 36°. 



The general facts being thus established, that iron, when soft- 

 ened by heat, becomes more negative, and that platinum, when 

 softened by heat, becomes more positive, let us apply them to 

 the case before us. M. llegnault commenced his 5th series with 

 a fresh couple of iron and platinum, increasing the difference of 

 temperatures between the hot and cold junctions gradually until 

 it reached 273 0, 46. The absolute temperature of the hot junc- 

 tion at this point was in all probability 300°. After the couple 

 had been thus heated, it was allowed to cool, and the 6th series 

 was commenced : here the anomaly before alluded to at once 

 presented itself; a certain difference of temperature produced a 

 stronger current than in the 5th series, a result which might be 

 inferred a priori from the foregoing considerations. For the 

 iron by being once heated to 300° has become more negative, as 

 before proved, while the platinum has become more positive ; the 

 thermo-electric force of the couple has, in short, been increased, 

 and a more powerful current is the necessary consequence. This 

 is still more strikingly exhibited in the 7th scries, where M. lleg- 

 nault commences with a difference of 103 o, 80, and goes on in- 



