470 Prof. Williamson on Chemical Nomenclature, 



than that with which'their hydrates react on basic hydrates. It 

 is well known that when two saline molecules such as SO 4 H 2 

 and BaO 2 H 2 react on one another with liberation of water and 

 formation of a salt, the force of combination, as measured by the 

 heat evolved, is less than that which the acid and base exert in 

 direct combination; for the process of double decomposition 

 separates the water from SO 3 and from BaO, and in doing so 

 absorbs just as much heat as was evolved when water combined 

 with SO 3 and with BaO; so that the force with which the two 

 hydrates react on one another is by so much less than that with 

 which SO 3 combines with BaO. 



Mr. Foster expresses an objection to applying the term " com- 

 bination '■■ to the reaction of such bodies as anhydrous acetic acid 

 (C 2 H 3 O) 2 on water, because by a process of double decompo- 

 sition the two molecules, acid and water, give rise to the forma- 

 tion of two new molecules ; but if his objection is admitted to 

 have weight, it applies equally to the reaction of free chlorine on 

 free hydrogen, where two molecules of the elements form two 

 molecules of the compound by a process of double decomposition. 

 If such reactions as that of chlorine on hydrogen, and of anhy- 

 drous acetic acid on water, are not combinations, the word 

 might perhaps be retained for such reactions as the combina- 

 tion of carbonic oxide and chlorine ; or SO 3 and water, where 

 two molecules unite to form one ; but if Mr. Foster seriously 

 proposes such a restriction of the word, it will be time enough to 

 consider it. The present usage is to describe as combinations 

 those reactions in which the resulting molecules are less various 

 than the original molecules, as in the cases of 



2 +(H 2 ) 2 =(H 2 0) 2 ; C1 2 + K 2 =(C1K) 2 ; 



N 2 5 + H 2 = (N0 3 H) 2 ; (C 2 H 3 0) 2 + H 2 = (C 2 H 4 2 ) 2 ,&c.; 



and, in like manner, to describe as decompositions those reactions 

 in which the products are more various than the materials, as 



S0 4 H 2 =S0 3 + H 2 0;Pb(N0 3 ) 2 = PbO+0 + N 2 4 ; 



(C 2 H 3 K0 2 ) 2 =C 3 H 6 O + CO 3 K 2 ; (HgO) 2 =2Hg + 2 , 



/C 2 H 5 Q J + s0 4 H * _ /C 2 H* Q ^ + H « + S0 4 R2# 



Many of these processes are known to consist of a series of 

 double decompositions, and the fact is often mentioned in allu- 

 ding to them ; but it does not seem likely that we should abandon 

 the use of the terms combination and decomposition. 



Mr. Foster has discussed in his paper what he calls the ori- 

 ginal use of the words acid and base, which is sufficiently cha- 

 racterized by his own words, " anything but particularly clear." 



