2o8 SCIENCE PROGRESS. 



alkaline solution with purified metallic zinc. The ammonium 

 chloride was sublimed now in a current of ammonia gas, 

 now in vacuo, but the results obtained showed that for the 

 complete precipitation of 100,000 parts of silver, 49,592 to 

 49,602 parts of ammonium chloride were required. In 

 other words, the extreme difference in a large number of 

 determinations carried out with very considerable modifica- 

 tions only amounted to one part in five thousand. 



Having thus proved that a compound always contains 

 the same proportion of its constituent elements it was 

 essential for his purpose as well as for the complete 

 establishment of the atomic theory to prove that the equiva- 

 lent weight of an element was not affected in the slightest 

 degree by the various elements with which it might 

 combine. To take an example, silver combines with 

 iodine to form the iodide, and with iodine and oxygen to 

 form the iodate, and these compounds are represented by 

 the formulae Agl and Ag I0 3 respectively. It was just 

 possible, one might even say probable, that the ratio of silver 

 to iodine in the one compound might not be the same as 

 that in the other, but that it would be modified by the large 

 quantity of oxygen present in that other substance. If, 

 however, the elements consist of small particles alike in all 

 respects, such a variation would be impossible, and the 

 relative masses of silver and iodine in the iodide and in the 

 iodate must be absolutely the same. To prove this may 

 seem very easy, but Stas found it by no means so, lor 

 whenever he prepared his silver iodate by precipitation from 

 the nitrate, after the reduction with sulphurous acid there 

 was always a small excess of silver over and above the 

 iodine present. This he finally traced to a minute quantity 

 of the nitrate being carried down mechanically by the 

 iodate, but so firmly held that no amount of washing would 

 remove it. By using other soluble salts of silver such as 

 the sulphate and the dithionate, however, he was able to 

 prepare silver iodate so pure that on reduction to silver 

 iodide not the slightest trace of either silver or iodine re- 

 mained in excess. In the case of that prepared from the 

 nitrate the excess of silver only amounted to one part in 



