MOLECULES AND ATOMS 811 



they cannot re-form it by their direct action on each other (the reaction 

 is not reversible), still they form the direct products of its decomposition, 

 and they occupy equal volumes. But benzoic acid, C 7 H^0 2 , is itself 

 composed of benzene, C 6 H 6 , and carbonic anhydride, CO 2 , which also 

 occupy equal volumes. 8 There is an immense number of similar 

 examples among those organic substances to whose study Gerhardt 

 consecrated his whole life and work, and he did not allow such facts aa 

 these to escape his attention. Still more frequently in the phenomena 

 of substitution, when two substances react on one another, and two are 

 produced without a change of volume, it is found that the two sub- 

 stances acting on each other occupy equal volumes as well as each of> 

 the two resultant substances. Thus, in general, reactions of substi- 

 tution take place between volatile acids, HX, and volatile alcohols, 

 R(OH), with the formation of ethereal salts, RX, and water, H(OH), 

 and the volume of the vapour of the reacting quantities, HX, R(OH), 

 and RX, is the same as that of water H(OH), whose weight, corre- 

 sponding with the formula, 18, occupies 2 volumes, if 1 part by weight 

 of hydrogen occupy 1 volume and the density of aqueous vapour 

 referred to hydrogen is 9. Such general examples, of which there are 

 many, 9 show that the reaction of equal volumes forms a chemical 

 jphenomenon of -frequent occurrence, indicating the necessity for 

 acknowledging the law of Avogadro-Gerhardt. 



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

 reaction of two substances takes place in more than one proportion, acqord- 

 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 A , forms four compounds, CH 3 C1, CH 2 C1 2 , 

 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 action ol chlorine. And 

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



8 Let us demonstrate this by figures." From 122 grams of benzoic acid there are 

 obtained (a) 78 grams of benzene, whose density referred to hydrogen = 89, hence the 

 relative volume = 2 ; and (6) 44 grams of carbonic anhydride, whose density = 22, and 

 hence the volume = 2, It is the same in other cases. 



9 A large number of such generalised reactions, showing" reaction by equal 

 volumes, occur in the case of the hydrocarbon derivatives, because many of these com- 

 pounda are volatile. The reactions of alkalis on acids, or anhydrides on water, &o., 

 which are so frequent between mineral substances, present but few such examples, 

 because many of these substances are not volatile and their vapoUr densities are 

 unknown. But essentially the same is seen in these cases also ; for instance, sulphuric 

 acid, BLjSO^; breaks up into the anhydride, SO 3 , and water, H 2 O, which exhibit an equality 

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

 volumes : carbonic anhydride, C0 2> ammonia, NH 3 , and water, H 2 O {the volumes of all ar 

 equal to 2), form acid ammonium carbonate, (NH^HCOs. 



