.MOLECULES AND ATO.MS 305 



But the question arises : What is the relation of volumes if the 

 ivac-tion <f two substances takes place in different proportions, accord- 

 ing to the law of multiple proportions ? A definite answer can only be 

 given in cases which have been very thoroughly studied. Thus chlorine, 

 in acting on marsh gas, CH 4 , forms four compounds, CH 3 C1, CH 2 CL>, 

 CHC1 3 , and CC1 4 , and it may be established by direct experiment that 

 the substance CH 3 C1 (methylic chloride) precedes the remainder, and 

 that the latter proceed from it by the further direct action of chlorine. 

 And this substance, CH 3 C1, is formed by the reaction of equal volumes 

 of marsh gas, CH 4 , and chlorine, C1 2 , according to the equation CH 4 + 

 C1.,=CH 3 C1 + HC1, A great number of similar cases are met with 

 amongst organic that is, carbon compounds. Gerhardt was led to 

 the discovery of his law by having investigated many such reactions, 

 and from observing that in them the reaction of equal volumes precedes 

 all others. 



But if nitrogen or hydrogen give several compounds with oxygen, 

 the above proposed question cannot be answered with complete clear- 

 ness, because the consecutive formation of one degree of combination 

 after the other cannot be so strictly defined. It may be supposed, but 

 neither affirmed with assurance nor confirmed by experiment, that nitro- 

 gen and oxygen first give nitric oxide, NO, and only afterwards the brown 

 vapours N 2 O 3 and NO 2 . Such a sequence in the combination of nitro- 

 gen with oxygen can only be supposed on the basis of the fact that NO 

 forms N 2 O 3 and NO 2 directly with oxygen. If it be admitted that NO 

 (and not N 2 O or N0 2 ) be first formed, then this instance would also 

 confirm the law of Avogadro-Gerhardt, because nitric oxide contains 

 equal volumes of nitrogen and oxygen. So, also, it may be admitted 

 that, in the combination of hydrogen with oxygen, hydrogen peroxide 

 is first formed (equal volumes of hydrogen and oxygen), which is de- 

 composed by the heat evolved into water and oxygen, all the more as 

 this explains the presence of traces of hydrogen peroxide (Chap. IV.) 

 in almost all cases of the combustion or oxidation of hydrogenous sub- 

 stances ; for it cannot be supposed that water is first formed and then 

 the peroxide of hydrogen, because up to now such a reaction has not 

 been accomplished, whilst the formation of H 2 from H 2 O 2 is very 

 easily reproduced. 10 



of volumes. Let us take another example where three substances combine in equal 

 volumes : carbonic anhydride, COo, ammonia, NH 3 , and water, H 3 O (the volumes of all are 

 equal to 2), form acid ammonium carbonate, (NH 4 )HCO 3 . 



10 This opinion as to the primary origin of hydrogen peroxide and of the forma- 

 tion of water by means of its decomposition, which was always held by me (in the first 

 editions of this work), has in latter days become more generally accepted, thanks more 

 especially to the work of Traube. Probably, it explains most simply the necessity for 



VOL. I. X 



