292 Mr. A. P. Laurie on the Electromotive Forces 



3rd. The heat of combination of zinc and iodine obtained 

 by subtracting the second from the first, 49230 calories. 



Now the first and second numbers are obviously average 

 results, the zinc iodide formed dissolving in pure water at the 

 beginning of the reaction, but in a strong zinc-iodide solution 

 towards the end. 



If care has been taken to use the same quantities of water 

 and zinc iodide in both experiments, no doubt the third number 

 represents fairly well the heat of combination of zinc and 

 iodine. 



The above criticism applies to Thomsen's book, though 

 more details are usually given by him. It is consequently 

 impossible to compare with exactness these numbers with 

 those obtained by measuring the heats of combination in the 

 iodine cell. 



In the following experiments on the zinc-iodine and zinc- 

 cuprous-iodide cell, I have used Professor Tait's Thomson 

 electrometer to measure the electromotive forces obtained. 

 Alder Wright recommends a high-resistance galvanometer 

 for the purpose, which is certainly more delicate, but not 

 more reliable, than this particular instrument. 



About 50 divisions of the scale correspond to one volt, and 

 it is possible to read to half a division. 



As a standard of electromotive force a Daniell cell was 

 used, made up according to the directions given by Alder 

 Wright*, but specially constructed to make diffusion of the 

 two liquids practically impossible. This cell has, according 

 to Wright, an electromotive force of 1*107 volt. 



To check this cell I used two Latimer-Clark cells, one 

 made up to two years ago, the other made up quite recently. 

 Taking Lord Eayleigh's recent determination of the electro- 

 motive force of this cell (1*454 volt), the three cells agreed 

 with each, other within the errors of experiment. 



The electrometer was connected to a rocker so constructed 

 that the zero of the instrument, the deflection due to the cell 

 to be measured, and the deflection due to the Daniell cell 

 could be noted one after the other. This was always done, 

 and the numbers given at the end are always the mean of 

 several readings. The Daniell did not alter at all from week 

 to week. 



When very small variations in electromotive force had to 

 be measured, two or three cells were placed in series, and in 

 some cases cells were connected to the electrometer through 



* • Electrician,' Feb. 18, 1882. In recent papers Wright states the 

 electromotive force of a Daniell cell, differing in some details, at 1*112 

 volt. 



