92 M. Hess on the Scientific Labours of Richter. 



the nitrates, taking for starting-point 1000 parts of nitric acid. 

 His numbers therefore varied continually, which must have 

 kept many relations concealed from his sight. 



Nevertheless these tables constructed by Richter have an- 

 other peculiarity which merits our attention. The names of 

 metals are not found in them in writing, but the signs then used 

 are substituted for them, as 6 manganese, $ iron, 5 zinc, 

 ]) silver. But here signs fail him, for several metals had just 

 been newly discovered ; these Richter expresses by the two 

 initial letters of the name, for example, %g for chrome, Ti for 

 titanium, Te for tellurium. Here then is the first idea of the 

 notation so happily completed by M. Berzelius. 



We see Richter continually occupied with the phenomenon 

 of neutrality. What then is the neutrality of a solution ? This 

 is a thoi'ny question, and one to which, even at the present 

 time, many authors answer only in an obscure and evasive 

 manner. Neutrality, says Richter, is absolute or relative : it 

 is absolute when the solution does not exert any reaction on 

 test papers ; it is relative when the neutral salt nevertheless 

 exerts an acid or alkaline action. But in this case, he says, 

 however decided may be the reaction exerted by a metallic 

 solution (for example the nitrate of silver), you recognize, 

 nevertheless, that it is neutral, because the least addition of an 

 alkali causes a precipitate which will not dissolve again with- 

 out adding an acid. 



Although Richter had recognised the fact that different 

 metals required the same quantity of oxygen in order to be 

 dissolved in the same quantity of acid ; notwithstanding, he 

 says, when metals become charged with oxygen without the 

 intervention of an acid, that by no means prevents them 

 from taking very different quantities. Richter, as we see, was 

 not ignorant that there were different degrees of oxidation, 

 and he employed himself in determining several of them. 

 As, however, the works of Richter which relate to the oxides 

 of metals are far from being very exact, let us examine an 

 example in order to discover to what the inaccuracies met 

 with in his determinations are to be attributed. 



He knew, for example, that arsenic formed two combi- 

 nations with oxygen, arsenious acid and arsenic acid. He 

 determines by a direct experiment the quantity of oxygen 

 which arsenious acid takes to become converted into arsenic 

 acid, and find's that 100 parts of acid absorb 17*2 of oxygen, 

 which is not far distant from the real number, 16*17. He after- 

 wards seeks to determine the quantity of oxygen which me- 

 tallic arsenic absorbs to become converted into arsenious acid, 

 and he finds for 100 parts of metal 15*1 parts of oxygen, de- 



