124 Dr. Thomson's Experiments to determine the [Aug. 



exactly 9*25. This experiment being twice repeated, the result 

 was the same. It enables us to conclude that an atom of tin 

 weighs 7'25 ; and that peroxide of tin is a compound of one 

 atom of tin + two atoms of oxygen ; so that we have the weight 

 of tin and its oxides as follows : 



Tin 7-25 



Protoxide of tin 8-25 



Peroxide of tin 9-25 



III. Antimony. 



We are indebted to Proust for the first accurate ideas respect- 

 ing the oxides of antimony. Berzelius, in his elaborate experi- 

 ments on antimony, published in vol. xxxiv. of Nicholson's 

 Journal, has made a very near approximation to the true compo- 

 sition of these oxides. I have repeated both the experiments of 

 Proust and Berzelius, and have been guided by them to the 

 experiments which I am going to relate", and which I consider 

 as leaving little doubt respecting the true atomic weight of anti- 

 mony and its oxides. That I might operate upon antimony free 

 from every sensible quantity of impurity, I dissolved the antimony 

 of commerce in nitromuriatic acid, and precipitated the per- 

 oxide by means of water. This oxide was well washed, dried, 

 mixed with black flux, and exposed to a red heat in a covered 

 crucible. By this process, I obtained a button of antimony in 

 which I could not detect any sensible quantity of foreign matter. 

 This button was softer than common antimony, and its specific 

 gravity was only 6-424 at the temperature of b*0°. 



Of this antimony I weighed 5-5 grains (which, as I concluded 

 from preceding trials, represented the weight of an atom of 

 antimony, at least very nearly), put it into a platinum crucible, 

 and dissolved it by the assistance of heat in pure nitric acid. 

 The solution was evaporated to dryness, and exposed for some 

 hours to a heat of 500°. A yellow powder remained, exhibiting 

 the well-known properties of peroxide of antimony, and weighing 

 7'5 grains. This experiment was repeated four times with 

 exactly the same result. It is obvious from it, that peroxide of 

 antimony is a compound of 5*5 antimony + 2 oxygen ; or (as 

 an atom of oxygen is represented by 1) of 1 atom antimony + 2 

 atoms oxygen. Therefore, an atom of antimony weighs exactly 

 -5-5. 



If we take 7-5 grains of peroxide of antimony and expose them 

 to a red heat, the colour changes from yellow to white, and the 

 weight diminishes to 7. I repeated this experiment four times 

 with precisely the same result. We see from it (as had been 

 already established by Berzelius) that when peroxide of antimony 

 is changed into white oxide, it loses the fourth part of its 

 oxygen. If we make the experiment in a small green glass 

 retort (employing 100 grains of peroxide), and collect the gaseous 



