VIII. OTHEK MODES OF FOEOE. 



, or the chemical action induced by the 

 mere presence of a foreign body, embraces a class of 

 facts which must considerably modify many of our notions of 

 chemical action : thus oxygen and hydrogen, when mixed in 

 a gaseous state, will remain unaltered for an indefinite pe- 

 riod ; but the introduction to them of a slip of clean plati- 

 num will cause more or less rapid combination, without being 

 in itself in any respect altered. On the other hand, oxygen- 

 ated water, which is a compound of one equivalent of hydro- 

 gen plus two of oxygen, will, when under a certain tempera- 

 ture, remain perfectly stable ; but touch it with platinum in 

 a state of minute division, and it is instantly decomposed, 

 one equivalent of oxygen being set free. Here, again, the 

 platinum is unaltered, and thus we have synthesis and analy- 

 sis effected apparently by the mere contact of a foreign body, 

 It is not improbable that the increased electrolytic power of 

 water by the addition of some acids, such as the sulphuric 

 and phosphoric, where the acids themselves are not decom- 

 posed, depends upon a catalytic effect of these acids ; but we 

 know too little of the nature and rationale of catalysis to ex- 

 press any confident opinion on its modes of action, and pos- 

 sibly we may comprehend very different molecular actions 

 under one and the same name. In no case does catalysis 

 yield us new power or force : it only determines or facilitates 

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