564 - Bn Thomson on the [Nov. 



iidtis? vire^of very irffle'^Wii*, as I decd'mpOsed sulphate of zinc by 

 tirnplj/ pouriNij; cdr/nma'fe dJTspda into a cold solution of it. 



This dssertion excited iii'me a good deal of surprise. Ido 

 not say in my work how I performed the analysis, but merely 

 fliit Wfe^rilts ^vetfe ^o'ktld so. Now if a practical 6h6i^ist, 

 ^^Mst'kn6Wth^"tikilal mode of throwing dowri okide^'bf 

 Mifc'f)^ tffli:aline' carbo'iiiates, was induced from itif^ilenfe"to 

 ijbficlurfe' that I had experimented with so little regatd to'jit^- 

 dsiort, ^^ to be satisfied with a mode which would hav0 l^ft 

 ihore than one-fifth of the oxide of zinc still in solut?6'ii, ' I 

 IriL^e'reason to be apprehensive that those gentlemen Wh6 ''aife 

 6tily ifelnmencing the study of practical chemistry may be still 

 lijibrfe' injuriously misled, that they may attempt the analyisis '6f 

 iM Sulphate of zinc without being aware of the requisite 'Jii^- 

 cautions, and that the quantities which they will thus procure 

 will i)'e s6 different from those which I have stated, that they 

 will be either inclined to consider my statements as erroneous, 

 or, what would be still more unfortunate, be discouraged from 

 prosecuting their researches till they have satisfied themselves 

 respecting the truth of the atomic weights which I have given. 

 Fibnd^ive, therefore, that it will be highly proper to state with 

 some minuteness the different modes of analysis which J found 

 to arisv;er best. And the safest method of proceeding seems 

 to be to givie the steps of a few of my actual analyses. 



1. The sulphate of zinc of commerce is usually a compound 

 pf, one atoni acid and 6ne atom oxide of zinc. Bat I have never 

 Met with it absolutely free from iron, and seldom without some 

 tfabes of cadmium. When the salt is made by dissolving the 

 zinc of commerce in sulphuric acid, and crystallizing the solu- 

 tion, you often obtain a salt containing about one-third too 

 much acid, and this excess is not all got rid of even when the 

 kalt is diisdlVed and crystallized several times successively. 

 Tli^ tiest way is to put a plate of zinc into the acidulous salt, 

 aiiii'bd leave it in contact with it till all excess of acid is neutra- 

 lized, a j)r0jCess vv'hich, even when heat is applied, takes a con- 

 srd^^T^Ie'niiife'; the' bfest way to free the salt from iron is 

 tb' 3r\f t*h'e crystals,' expose them to a red heat, redissolve in 

 distilled water, filter and crystallize. It is only when the sul- 



tihate of zinc is^ pure that its coristituents are as I have stated 



tti^^^ oj iiiiijol 'liw/ »iij^ iyj ,»ji.iiqlu^ io Jii^iyv/ li^i/J§ h aaoq 



2. The water of crystaflfkteti W kl§mM^(^^n4^'Am^l'19e 

 obtained «<fite(Jtly by experiment. I usually reduced the crystals 

 to powder in a'porcelain mortar, wrapt up the powder in several 

 folds of blotting paper, and kept it for some time under a pretty 

 ifflMj^ pressure. 181*25 2,Tains of the salt thus treaft^d'4ere 

 pnt^nfito a balanced platinurti crucible, and exposed' '6h tftfe 

 satfdibtfth<4tl k'teGiperMir^ 2iH tiearly as possible of 320®, 'till 

 they ceased to give out moisture. The loss of weight in several 



