xiv THE ATOMIC THEORY 317 



definitely in 1811-1812 by Berzelius, who showed that 10 

 grams lead sulphide could be oxidised to lead sulphate with- 

 out leaving any excess of litharge or of sulphuric acid thus 

 proving " that lead sulphide contains its two constituents in 

 the exact ratio required for the formation of lead sulphate " 

 (Fixed Proportions, 1811-1812 ; Ostwald's Klassiker, 

 XXXV. n). 



A still more stringent test was carried out by Stas, in 

 1865 (Works, I. 481-536), in response to the criticisms of 

 Marignac. Stas removed the oxygen from silver iodate and 

 examined the silver iodide produced, in order to see if it 

 contained any excess of silver or of iodine. The excess was 

 always less than i part in a million, and was often quite im- 

 perceptible, showing that the proportion by iveight of silver 

 to iodine, in the iodate and in the iodide, is invariably the same 

 ( Works, I. 500). Similar experiments were made with silver 

 bromate, using about 20 grams for each experiment. In the 

 case of silver chlorate, " 259-4535 grams of this salt were 

 transformed into chloride without liberating a trace of silver 

 or of chlorine," thus proving that the proportion by weight of 

 silver and of chlorine is absolutely the same in th,e two salts 

 ( Works, I. 535). Although fifty years have elapsed since this 

 test was carried out by Stas, it would scarcely be possible to 

 devise a more stringent proof of the accuracy of the law of 

 reciprocal proportions. 



SUMMARY AND SUPPLEMENT. 



John Dalton, about the year 1802, whilst studying the nature 

 of the atmosphere, revived the atomic theory of the Greek 

 philosophers and suggested that the "simple atoms" of an 

 element (i) are all alike in size and weight, and (2) cannot be 

 created or destroyed ; but (3) may unite with other atoms in 

 simple ratios to form " compound atoms," or molecules, as they 

 are now called. These atoms were represented by symbols, e.g. 

 hydrogen 0, oxygen Q> water OO? which Berzelius, in 1819, 

 replaced by letters e.g.H, O, H + O. 



