334 HISTORICAL INTRODUCTION TO CHEMISTRY CHAP. 



" Ammonia gas is composed of three parts by volume of 

 hydrogen, and one of nitrogen, and its density compared to 

 the air is 0*596. But if we suppose the apparent contrac- 

 tion to be half of the whole volume, we find 0-594 for the 

 density. Thus it is proved, by this almost perfect 

 concordance, that the apparent contraction of its elements 

 is precisely half the total volume, or rather double the 

 volume of the nitrogen. 



" We see, then, from these various examples, that the 

 contraction experienced by two gases on combination is in 

 almost exact relation with their volume, or rather with the 

 volume of one of them " (A.C.R. IV. 15-20). 



Gay-Lussac's law of volumes. Gay-Lussac summarises 

 his conclusions in the following words : 



" I have shown in this memoir that the compounds of 

 gaseous substances with each other are always formed in very 

 simple ratios, so that representing one" of the terms by 

 unity, the other is i, or 2, or at most 3. These ratios 

 by volume are not observed with solid or liquid substances, 

 nor when we consider weights, and they form a new proof 

 that it is only in the gaseous state that substances are in the 

 same circumstances and obey regular laws. . . . The apparent 

 contraction of volume suffered by gases on combination is 

 also very simply related to the volume of one of them, and 

 this property likewise is peculiar to gaseous substances " 

 (A.C.R. IV. 24). 



In order to include all the cases discussed in the memoir, 

 Gay-Lussac's law of volumes may be stated as follows : 



When gases enter or leave chemical combination, the volumes 

 absorbed, or liberated, are in simple ratios to one another. 



Modification of Gay-Lussac's law. Later experiments 

 have shown that Gay-Lussac's law does not hold exactly. 

 Thus, Scott (Phil. Trans., 1893, 184, 566), by exploding 

 together gases of remarkable purity, found that hydrogen 

 and oxygen combine together at ordinary temperatures in 

 the ratio 2*00245 to i. This deviation from Gay-Lussac's 

 law is confirmed by the work of Morley (p. 131), who found 



