804 PRINCIPLES OF CHEMISTRY 



simple multiple proportion. 3 This forms the first law of those discovered 

 by Gay-Lussac. It may be formulated as follows : The amounts of sub- 

 stances entering into cJiemical reaction occupy under similar physical 

 conditions, in a gaseous or vaporous state, equal or simple multiple 

 volumes. This law refers not only to elements, but also to compounds 

 entering into mutual chemical combination ; thus, for example, one 

 volume of ammonia gas combines with one volume of hydrogen chloride. 

 For in the formation of sal-ammoniac, NH 4 C1, there enter into reaction 

 17 parts by weight of ammonia, NH 3 , which is 8-5 times denser than 

 hydrogen, and 36'5 parts by weight of hydrogen chloride, whose vapour 

 density is l8'25 times that of hydrogen, as has been proved by direct 

 experiment. By dividing the weights by the respective densities we 

 find that the volume of ammonia, NH 3 , is equal to two, and so also the 

 volume of hydrogen chloride. Hence the volumes of the compounds 

 which here combine together arer equal to each other. Taking into 

 consideration that the law of Gay-Lussac holds good, not only for 

 elements, but also for compounds, it should be expressed as follows : 

 Substances interact with one another in commensurable volumes of their 

 vapours.* 



tity of the substance to be experimented with is dropped into the space. The substance 

 is immediately converted into vapour, and displaces the air into the graduated cylinder e, 

 The amount of this air is calculated from its volume, and hence the volume at t t and 

 therefore also the volume occupied by the vapou*, is found. The general arrangement 

 of the apparatus is given in fig. 55. 



3 Vapours and gases, as already explained in the second chapter, are subject to the 

 same laws, which are, however, only approximate. It is evident that for the deductioa 

 of the laws which will presently be enunciated it is only possible to take into consideration 

 a perfect gaseous state (far removed from the liquid state) and chemical invariability in 

 which the vapour density is constant that is, the volume of a given gas or vapour 

 varies like a volume of hydrogen, air, or other gas, with the pressure and temperature. 



It is necessary to make this statement in order that it may be clearly seen that the 

 laws of gaseous volumes, which we shall describe presently, are in the mo&t intimate 

 connection with the laws of jthe variations of volumes with pressure and temperature. 

 And as these latter laws (Chapter II.) are not infallible, but only approximately exact, the 

 same, therefore, applies to the laws about to be described. And as it is possible to find 

 more exact laws (a second approximation) f or the variation of v with p and t (for example, 

 van der Waals' formula, Chapter II., Note 88), so also a- more exact expression of the 

 relation between the composition and the density of vapours and gases is also possible. 

 But to prevent any doubt arising at the very beginning as to the breadth and general 

 application of the laws of volumes, it will be sufficient to mention that the density of 

 such gases as oxygen, nitrogen, and carbonic anhydride is already known to remain 

 constant (within the limits of experimental error) between the ordinary temperature 

 and a white heat ; whilst, judging from what is said in my work on the ' Tension of Gases ' 

 (vol. i. p..9), it may be said that, as regards pressure, the relative density remains very 

 constant, even when the deviations from Mariotte's law are very considerable. However, 

 in this respect the number of data is as yet too small to arrive at an exact conclusion. 



4 We must recollect that this law is only approximate, like Boyle and Mariotte's law, 

 and that, therefore, like the latter, a more exact expression may be found for the 

 exceptions. 



