MOLECULES AND ATOMS 297 



simple multiple proportion. 3 This forms thejirst law of those discovered 

 by Gii ;i- 1, tixxnc. This law may be formulated as follows : The amounts of 

 nnfistances entering into chemical reaction, occupy under similar physical 

 conditions, in a gaseous or vaporous state, equal or multiple volumes. 

 This law not only refers to elements, but also to compounds entering 

 into mutual chemical combination ; thus, for example, one volume of 

 ai 1 11 uonia gas combines with one volume of hydrogen chloride. For in 

 the formation of sal-ammoniac, NH 4 C1, there enters into reaction 17 parts 

 by weight of ammonia, NH.,, which is 8^ times denser than hydrogen, 

 and 36 -5 parts by weight of hydrogen chloride, whose vapour density is 

 18| 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 are equal to each other. By taking into considera- 

 tion 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.* 



space b is heated to a constant temperature t (by the surrounding vapours of a liquid of 

 constant boiling point), and the air (or other gas enclosed in this space) is allowed to 

 attain this temperature, and when it has done so a glass bulb containing a weighed quan- 

 tity of the liquid to be experimented with is dropped into the space. The liquid is imme- 

 diately converted into vapour," and displaces the air into the graduated cylinder e. The 

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

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

 of the apparatus is given in fig. 55. 



5 Vapours and gases, as was explained in the second chapter, are subject to the same 

 laws, which are, however, only approximate. It is evident that for the deduction of the 

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

 perfect gaseous state (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 fliat it may be clearly seen that the 

 laws of gaseous volumes, presently to be described, are in the most intimate connec- 

 tion with the laws of the variations of volumes with pressure and temperature. And 

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

 therefore, applies to the laws presently to be described. And as it is possible to find more 

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

 Van der Waals' formula, Chap. II. Note 38), 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 density remains 

 very constant even when the deviations from Mariotte's law are very considerable. How- 

 ever, in this respect the number of data is yet too small for forming an exact conclusion. 



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



