92 
DR. ANDREWS ON THE HEAT DISENGAGED 
the physical and chemical properties of equivalent solutions of different salts of cop- 
per be compared, they will be found to present almost a complete identity, and the 
same remark applies to the solutions of the salts of zinc which remain after the re- 
actions are finished. We have, therefore, every condition favourable to the produc- 
tion of simple thermal results. For the present object, it is not necessary to inquire 
in what state the metallic element exists in an aqueous solution of its salts, or what 
changes actually occur between the first addition of the zinc and the final precipita- 
tion of the copper ; it is enough to know that the final result is the same, whether 
we employ a solution of an oxy-salt, or of a haloid salt. 
The general result of the whole investigation may be stated in the following terms: — 
When an equivalent of one and the same metal replaces another in a solution of any 
of its salts of the same order, the heat developed is always the same ; hut a change in 
either of the metals produces a different development of heat. 
By the expression ‘‘ solution of a salt of the same order” is understood, a solution 
in which the same precipitate is produced by the addition of an alkali, or, on one 
view of the composition of such salts, in which the metal exists in the same state of 
oxidation. 
Salts of Copper with Zmc. 
Two distinct series of experiments were made with the salts of the black oxide of 
copper and metallic zinc. In the first series, concentrated solutions were taken and 
introduced into a small glass vessel, in which was also placed a glass tube, open 
above, and containing pure zinc in a state of fine subdivision. The glass vessel, 
carefully closed, was introduced into a larger vessel of copper furnished with a lid. 
The latter was filled with water adjusted to the proper temperature and suspended 
in an outer vessel of tin plate, and the whole introduced into a cylinder closed with 
a lid and capable of being rotated*. After all parts of the apparatus had acquired 
the same temperature, a very sensible thermometer was introduced into the water 
contained in the copper vessel through a small orifice in the lid, and the position of 
the mercury in the tube observed. The thermometers having been removed and the 
orifice closed with a cork, the lid of the outer vessel was shut down, and the rotating 
wheel moved through half a revolution, by which means the metallic zinc was brought 
into contact with the copper solution. The rotation was afterwards continued for 
five minutes and a half, which was found to be sufficient not only to complete the 
precipitation of the copper, but also to diffuse the heat arising from the reaction 
uniformly through the apparatus. The temperature of the water was so adjusted as 
to render the corrections required for the heating and cooling influence of the air 
very inconsiderable ; their amount was, however, ascertained in each experiment and 
the results altered accordingly. 
* For a description and representation of a similar apparatus, see Transactions of the Royal Irish Academy, 
vol, xix. 
