gj6 Dr. Fordyce on the Cause of the additional Weight 
I reduced this metal to a calx, by dissolving it in vitriolic 
acid diluted with water, and precipitated it by kali purum dis- 
solved in water. 
In doing this, the acid should be diluted with four or five 
times its weight of water, and the zinc should be dissolved 
very slowly, avoiding heat as much as possible during the 
solution. If this precaution is not taken, a quantity of vo- 
latile vitriolic acid will be produced, and spoil the experi- 
ment. 
In the precipitation the alkali is apt to re-dissolve the calx, 
if care be not taken to use it in solution in water, and that the 
solution is diluted with a large quantity of water : the propor- 
tion in which the water is in aqua kali puri of the London Dis- 
pensatory is a convenient solution of the alkali. 
Care must likewise be taken, in the precipitation, that the solu- 
tion of the kali be poured into the solution of the zincum vitriola - 
turn in water by a little at a time, and that the whole be perfectly 
mixed together before a fresh quantity is poured in, otherwise 
part of the calx will be re-dissolved. It is farther necessary 
that the exact quantity of kali purum be used : if too little is 
used, the whole calx will not be separated ; if too much, part 
of the calx will be re-dissolved. It is also necessary that the 
alkali be perfectly pure, especially free from fixed air,* as 
that would be transferred to the calx, and as it flies off when, 
the kali is simply united with vitriolic acid, the accuracy of the 
experiment would be thus destroyed. 
The weight of the calx, by which it exceeds the weight 
* I use the name of fixed air, although certainly not proper, in order to avoid 
running into confusion by employing those which have been given to this substance,, 
until the plurality of voices shall fix an appropriated name to it. 
