804 
Proceedings of the Royal Society 
Added 17th November. 
The following series of observations was made on the 31st May. 
The readings for the undried state, with different spark-lengths, were 
first observed, the air was then dried once for all, and readings 
taken for the several spark-lengths. This method of comparison 
has the advantage of saving time in drying the air, but the disad- 
vantage of requiring the distances of the plates to be readjusted. 
An error in this respect may account for the different result obtained 
in the comparison at 3 mm. Four readings were taken in each case. 
Wet bulb thermometer 16 '8. Dry bulb thermometer 20 ’2. 
Spark-length 
in 
Millimetres. 
State of the 
Air. 
Mean of 
Differences 
of Potential. 
State of the 
Air. 
Mean o 
Differences 
of Potential. 
Percentage 
Ratio of In- 
crement to 
Reading for 
Undried. 
1 
Undried. 
67 
Dried. 
80 
19-4 
2 
99 
119 
” 
126 
5-9 
3 
9 9 
168 
9 9 
166 
-1*2 
4 
9 9 
222 
” 
’ 238 
7’2 
5 
99 
280 
9 9 
285 
1*8 
6 
99 
339 • 
9 
347 
2-4 
5. The Heats of Combination of the Metals with the 
Halogens. By A. P. Laurie and C. I. Burton. Com- 
municated by Professor Crum Brown. 
Professor Chrystal, in his article on “ Electricity ” in the Encydo- 
pcedia Britannica , mentions the following formula of Sir William 
Thomson’s, connecting the heats of combination in a voltaic cell 
with the EMF of the cell. 
E = JeO. 
(Where E = EMF of the cell; J = Joule’s equivalent; e = amount 
of zinc, dissolved by unit current in unit time ; 6 = heat of com- 
bination of one grm. of the metal in the cell.) 
This formula has been tested for the Daniell and other cells. 
It occurred to me that it was peculiarly applicable to the cuprous 
