310 PRINCIPLES OF CHEMISTRY 



law may now be formulated from two points of view In the first place, 

 from a physical aspect : equal volumes of gases (or vapours) at equal tem- 

 peratures and pressures contain the same number of molecules or of 

 particles of matter which are neither mechanically nor physically 

 divisible previous to chemical change. In the second place, from a 

 chemical aspect, the same law may be expressed thus : the quantities of 

 substances entering into chemical reactions occupy, in a state of vapour, 

 equal volumes. For our purpose the chemical aspect is the most im- 

 portant, and therefore, before developing the law and its consequences, 

 we will consider the chemical phenomena from which the law is deduced 

 or which.it serves to explain. 



When two isolated substances interact with each other directly and 

 easily as, for instance, an alkali and an acid then it is found that the 

 reaction is 'accomplished between quantities which in a gaseous state 

 occupy equal volumes. Thus ammonia, NH 3 , reacts directly with 

 hydrochloric acid, HC1, forming sal-ammoniac, NH 4 C1, and in this 

 case the 17 parts by weight of ammonia occupy the same volume 

 as the 36-5 parts by weight of hydrochloric acid. 7 Ethylene, 

 C 2 H 4 , combines with chlorine, C1 2 , in only one proportion, forming 

 ethylene dichloride, C 2 H 4 C1 2 , and this combination proceeds directly 

 and with great facility, the reacting quantities occupying equal 

 volumes. Chlorine reacts with hydrogen in only one proportion, 

 forming hydrochloric acid, HC1, and in this case equal volumes in- 

 teract with each other. If an equality of volumes is observed in cases 

 of combination, it should be even 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, C0 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 2 , 

 and carbonic anhydride, CO 2 , and as all the elements of phthalic acid 

 enter into the composition of these substances, it follows that, although 



This is not only seen from the above calculations, but may be proved by experiment. 

 A glass tube, divided in the middle by a stopcock, is taken and one portion filled with 

 dry hydrogen chloride (the dryness of the gases is very necessary, because ammonia 

 and hydrogen chloride are both very soluble in water, so that a small trace of water 

 may contain a large amount of these gases in solution) and the other with dry ammonia, 

 under the atmospheric pressure. One orifice (for instance, of that portion which contains 

 the ammonia) is firmly closed, and the other is immersed under mercury, and the cock is 

 then opened. Solid sal-ammoniac is formed, but if the volume of one gas be greater 

 than that of the other, some of the first gas will remain. By immersing the tube in the 

 mercury in order that the internal pressure shall equal the atmospheric pressure, it may 

 easily be shown that the volume of the remaining gas is equal to the difference between 

 the volumes of the two portions of the tube, and that this remaining gas is part of 

 that \vho:.o volume was the greater. 



