304 PRINCIPLES OF CHEMISTRY 



equal volumes interact with each other. If an equality of volumes is 

 observed in cases of combination, it should be all the more frequently 

 encountered in cases of decomposition taking place in substances which 

 split up into two others. Indeed, acetic acid breaks up into marsh 

 gas, CH 4 , and carbonic anhydride, CO 2 , and in the proportions in which 

 they are formed from acetic acid they occupy equal volumes. Also 

 from phthalic acid, C 8 H 6 O 4 , there may be obtained benzoic acid, 

 C 7 H 6 O 2 , and carbonic anhydride, CO 2 , and as all the elements of 

 phthalic acid enter into the composition of these substances, therefore, 

 although 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, 

 O 7 H 6 O 2 , is also 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 as these to escape his attention. Still more frequently in the 

 phenomena of substitution, when two substances act on one another, 

 and two are produced without a change of volumes that is, when the 

 first definition given (p. 4) ^corresponds with that given on p. 301 it 

 is found that the two substances acting on each other occupy equal 

 volumes as well as each of the two resultant substances. Thus, 

 in general, reactions of substitution 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 for water 

 H(OH), whose weight, corresponding 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 phenomenon, of frequent occur- 

 rence, indicating the necessity of acknowledging the law of Avogadro- 

 Gerhardt. 



<* Let us demonstrate this by figures. From 122 grams of benzoic acid there is 

 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- 

 pounds are volatile. The reactions of alkalis on acids, or anhydrides on water, Ax-., 

 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, H 2 SO 4 , breaks up into the anhydride, SO 3 ,and water, H 2 O, which exhibit an equality 



