1912-13.] Precipitation of Salts by the Corresponding Acids. 67 
We may now examine Engel’s solubility data in the light of what has 
been said. Table II. shows values of lc 0 , of B, and of their product Jc 0 B. 
Table II. Chlorides at 0°. 
Metal. 
Li. 
iMg. 
iCa. 
nh 4 . 
*Sr. 
Na* 
K* 
Pa. 
•96 
•98 
•99 
1T1 
•90 
•84 
•94 
•89 
B . 
12 
10 
9-3 
4*6 
5-2 
5-47 
3 45 
2*9 
^0® = -^-calc. 
11*5 
9-8 
9-2 
51 
4*6 
4-6 
32 
2-6 
* In connection with this table, it is necessary to observe that for the chlorides of sodium 
and potassium the solubilities found by Engel are in excess of the accepted values, which 
are 5 - 39N for sodium chloride, and 3*33N for potassium chloride ; this is quite sufficient to 
make a considerable error in 1c 0 , hence the figures tabulated in these two instances are certainly 
incorrect. The other values are correct. 
The special point to which attention is drawn is that k 0 is approxi- 
mately the same for all the chlorides, and is nearly unity. This was 
practically the claim put forward by Engel. 
It should further be mentioned that change of temperature seems to 
have little effect on the value of Jc 0 . 
As a result, it follows that the critical concentration of acid in precipita- 
tion-tests with a given chloride ( = & 0 B) is roughly equal to the concen- 
tration of the chloride in saturated aqueous solution ( = B). 
It is noteworthy that chlorides which are deposited in the hydrated 
condition are, from the standpoint of the course of their curves as expressed 
by the equation, in nowise different from those which are anhydrous — a 
fact which is in harmony with the generally accepted notion of the hydra- 
tion of all salts in aqueous solution. 
Furthermore, the only chlorides whose solubility-acidity curves have not 
an initial slope roughly = l, and with which this equation does not seem to 
agree, are those such as thallous chloride, which are sparingly soluble and 
for which simple ionic notions suffice, and those such as ferric chloride, 
which are known to form complexes with the hydrochloric acid. 
It therefore appears to be likely that, since the same equation (with 
practically the same initial constant), expresses the course of the solubility- 
acidity curves of ordinary chlorides, whether hydrated or not, therefore the 
processes controlling the solubility changes are the same in all these cases. 
Reference may now be made to the precipitation experiments of Gibson 
and Denison ( loc . cit . ; Trans., 1911, 48, 117). These authors, investigating 
