130 ^1^ Address to Chemists. [FeS. 



ployed to precipitate the oxide. But if a fixed alkali has been 

 employed, the result may be too great, as well as too small ; for if 

 the precipitate has not been well washed, there will remain in it 

 nitrate of potash or of soda. Or if too much alkali has been em- 

 ployed in precipitating, a portion of that alkali for the most part 

 combines with the oxide, and cannot afterwards be removed by 

 washing. But I should never have done, were I to point out here 

 all the circumstances requisite to make an accurate analysis, and to 

 judge of its accuracy. 



It is obvious, that the theory of chemical proportions is inti- 

 mately connected with the general theory of chemistry ; but it is 

 necessary, that those who labour either to examine it, to verify it, 

 to extend, or apply it, should have their eyes constantly fixed upon 

 the whole of chemistry. Tliey must not adopt any theoretical 

 explanation, till they have seen that it is not in contradiction with 

 any other part of the theory, which we have reason to consider as 

 ■well founded. This is a circumstance which chemists often neg- 

 lect, though it is of the highest importance ; and without attending 

 to it, we cannot expect to make any valuable improvement in the 

 theory of chemistry. Long custom is necessary, and a very ex- 

 tensive acquaintance indeed with chemistry, to have, so to speak, 

 the whole science before our eyes, in order to be able to judge, 

 without long reflection, whether an ingenious explanation of some 

 phenomenon be inconsistent or not with some other part of the 

 theory. It is from not attending to this circumstance, that che- 

 mists of the greatest distinction sometimes adopt ideas respecting 

 some points, which are inconsistent with other opinions equally 

 adopted by them. 



I shall here give an example of this. Dr. Thomson, whose 

 merits as a skilful chemist do not stand in need of any panegyric 

 from me, has examined, {Annals of Philosophy, iii. 1S9,) the 

 laws relating to the combination of oxides with each other; an 

 examination for which chemists are much obliged to him. He 

 then says, " the more I have examined this law, the more correct 

 in general does it appear." In the same volume, page 106", speak- 

 in o- of iodine, this philosopher expresses himself as follows : " How 

 much these new discoveries must alter the presently received 

 chemical theory, and how they serve to confirm Davy's opinion 

 respecting muriatic acid, is too obvious to escape attention." But 

 had tlie author at this moment called to his mind the constitution 

 of the submuriates with water of combination, he would have 

 found that tlie laws concerning the combination of oxides, and the 

 theory of Davy respecting muriatic acid, are absolutely irrecon- 

 cileable ; and that ehhcr the one or the other of these must be 

 abandoned as erroneous. If, at this time, the external resem- 

 blance of crystallized iodine to the oxide of manganese crystal- 

 lized had iccalled the last of these to his mind, he would perhaps 

 have found, that it is not more difficult to conceive how tlie first 

 disengages an excess of oxygen to combine with the bases, tlian 



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