ACCOMPANYING BASIC SUBSTITUTIONS. 
27 
The sulphate was rendered anhydrous by cautious ignition. The nitrate was 
evaporated till it became a solid mass on cooling. 20T7 grains of the hydrated salt 
left 6’85 grains of the oxide on ignition. The chloride was cautiously fused and weighed 
in a covered crucible. The bromide and iodide were dried on a hot sand-bath, but 
not fused. The three latter compounds are slightly decomposed by the heat required 
to deprive them of all moisture. Hence the increments of heat obtained with them 
must be a little below the truth. All the above salts of zinc were precipitated by an 
exact equivalent of potash, and the precipitate thus formed was found to consist of 
the hydrated oxide. But when the acetate of zinc is treated in the same manner, the 
precipitate which falls is a subsalt. The supernatant liquid still contains a portion 
of the salt of zinc, and the addition of more potash produces a further precipitate. 
For this reason, on precipitating an equivalent of this salt by an equivalent of potash, 
the thermometer rose only 1°*3 1 . On attempting to effect a more complete decom- 
position by using a double equivalent of potash, the heat obtained was rather less; 
but it is doubtful whether the additional quantity of alkali effected a more perfect 
substitution, while the precipitate was, at the same time, to a great extent redissolved. 
This is an interesting example of an apparent exception to the law of equibasic heat 
arising from a corresponding anomaly in the chemical reaction. 
Salts of Mercury. 
The only salt of mercury adapted to these inquiries is the chloride. Half the usual 
equivalent of it (17*1 grains) and of the potash solution gave, in three experiments, 
0 o, 90, 0°*86 and 0 o, 89, which, all corrections made and the final results doubled, cor- 
respond to l 0- 89, 1°*8 1 and 1°*87- I have not been able to confirm this result by 
precipitating the oxide from any other salt. The bromide has too little solubility in 
cold water. The cyanuret is not decomposed by potash, and accordingly, no heat is 
produced when their solutions are mixed. That the potash has not decomposed the 
salt is further proved by the circumstance, that on neutralizing it with an acid, the 
same increment of temperature occurred as if the alkali had been in a free state. 
The sulphate and nitrate are both decomposed when their solutions are diluted. It 
has indeed been lately asserted that a solution of tbe neutral nitrate may be obtained 
by precipitating the chloride with nitrate of silver. This is a mistake, as the usual 
decomposition occurs in this case. In fact, the solution of the supposed neutral 
nitrate, instead of the acid reaction of the chloride from which it is formed, intensely 
reddens litmus paper. If a similar experiment be made with other metallic chlorides, 
capable of forming neutral nitrates, no perceptible change of reaction will be found 
to occur. These observations fully explain the anomalies which I formerly pointed 
out in the action of the dilute acids on the red oxide of mercury. 
