122 Review of Dr. Thomson 



oxide of zinc ; the second, of the specific gravity of oxygen gas ; 

 and the third of that of hydrogen gas . 



He sublimes ordinary zinc in an earthen retort, dissolves a 

 given weight of it in nitric acid, and then expels the acid, by 

 heating the nitrate to redness in a green glass retort. In this 

 way, he obtains at once, to the minutest fraction, every thing 

 which Dr. Prout's atomic multiples require, viz., 5.25 grains of 

 oxide, from 4.25 grains of metal. This felicity of coincidence 

 between his experiments and his theoretical aim is so usual with 

 Dr. Thomson, and with him alone, as to excite no surprise in our 

 minds. Another chemist, in quest of final precision, would have 

 procured his zinc by reviving the metal from a pure carbonate, 

 precipitated from a well crystallized sulphate ; as he would know 

 that it is impossible to obtain pure zinc by subliming the metal 

 of commerce. Proust long ago sought to determine the com- 

 position of oxide of zinc in the above way. He obtained 125 

 grains of oxide, by igniting the nitrate formed from 100 grains of 

 metal. This made its atomic weight 4 ; and it is probable that no 

 chemist could get the number 4.25 by oxidizing zinc purified by 

 sublimation alone, as it usually contains arsenic, cadmium, &c, 

 which rise along with it. 



We by no means assert that the atomic weight of zinc is not 

 4.25, for this is the mean number deducible from the experiments 

 of Sir H. Davy, and M. Gay Lussac ; and it is a number Dr. Thom- 

 son contrives to obtain in another, but very complicated, way. 

 His atomic certainty of result springs out of the following ope- 

 rations. He decomposes a given weight of sulphate of zinc by 

 carbonate of soda, he washes this carbonate on a nitre, dries it, 

 and finally exposes it to a red heat. After these manipulations, 

 Dr. T. gets his experimental weight to tally with his theoretic 

 wishes, to the third decimal figure ; a comfort rarely enjoyed by 

 other practical chemists. In the last edition of his System of 

 Chemistry, crystallized sulphate of zinc is declared to hold 5 

 atoms of water; 7 are now recognised, being 2 more than Ber- 

 zelius and some other inquirers met with. We believe, indeed, 

 that the Doctor's salts have been often in a peculiar state, as to 

 their water of crystallization. 



In the section on the specific gravity of oxygen gas, he has the 

 hardihood to refer to his experimental results, published in the 

 sixteenth volume of the Annals of Philosophy, as being perfectly 

 accurate, though it has been fully demonstrated, that they were 

 wide of the truth *. What renders this reference peculiarly ab- 

 surd, is, that in the present book he abandons the notions he 

 formerly held, on the weight of moisture existing in a gas stand- 

 ing over water, and adopts a formula for ascertaining its amount, 



* See Quarterly Journal of Science for June, 1822. 



