of Edinburgh, Session 1884-85. 331 
The heat of combination can seldom be obtained with much, 
accuracy from books on thermal chemistry, the conditions under 
which the reactions have been performed in the calorimeter differing 
considerably from the conditions in the cell. 
For example, three numbers are given by Nanmann as represent- 
ing the heat of combination of zinc and iodine in presence of water — 
1st, The heat of combination of zinc and iodine in water, 60540 
calorics. 
2nd, The heat of solution of zinc iodide in water, 11310 
calorics. 
3rd, The heat of combination of zinc and iodine obtained by 
subtracting the second from the first, 49230 calorics. 
JSTow 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 repre- 
sents fairty well the heat of combination of zinc and iodine. 
The above criticism applies to Thomson’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 recom- 
mends 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 1407 volts. 
* Electrician, Feb. 18, 1882. In recent papers Wright states the electro- 
motive force of a Dan. cell differing in some details at 1*112 volts. 
VOL. XIII. 
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