246 PHILOSOPHICAL TRANSACTIONS. [aNNO 1792. 



years ago, bat various other engagements have so much interrupted me, that I 

 have had but little time to pursue any other chemical inquiry than such as were 

 necessary to form the catalogue of the ores and minerals in Dr. Hunter's 

 Museum. 



There is great difficulty in choosing the metal on which inquiry should be 

 nstituted, on account of the differences of their calces. After a number of 

 trials, I chose zinc, as that whose calces appeared to differ the least from each 

 other; in other respects there are great objections to it also, but which may be 

 got over. I took a portion of the zinc, and dissolved it in vitriolic acid, with 

 which it made a clear solution, without any of that black matter which com- 

 monly separates during its solution when we employ zinc imported from abroad. 

 After precipitating it by an alkali, and exposing the calx to the air, it remained 

 of a pure white ; so that it could contain no iron. This zinc was reduced to its 

 perfect metallic form by breaking it into small particles, and melting it with black 

 flux, taking that part of it only which was at the bottom of the crucible. I re- 

 duced this metal to a calx, by dissolving it in vitriolic acid diluted with water, and 

 precipitated it by kali purum dissolved 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 vola- 

 tile vitriolic acid will be produced, and spoil the experiment. In the precipita- 

 tion 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 be diluted with a large quantity of water: 

 the proportion in which the water is in aqua kali puri of the Lond. Dispensatory 

 is a convenient solution of the alkali. Care must also be taken, in the precipi- 

 tation, that the solution of the kali be poured into the solution of the zincum 

 vitriolatum 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 further necessary that the exact quantity of kali purum be 

 used : if too little be 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 per- 

 fectly pure, especially free from fixed air,* as that would be transferred to the 

 calx, and as it flies ofi^ 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 of the metal, shows 

 that there is a substance added to the wholQ, metal ; or, that while some substance 

 is driven ofl", another is added in greater quantity ; since it is clear, from various 

 experiments well known to this learned body, that all matter gravitates, and that 

 * 1 use the name of fixed air, although certainly not proper, in order to avoid running into con- 

 fusion by employing those which have been given to this substance, until tlie plurality of voices shall 

 fix an appropriated name to it. — Orig. 



