MOLECULES AND ATOMS 303 



fruitless, the hypothesis must be considered as verified, 6 and the law of 

 Avogadro-Gcrhardt must be spoken of as fundamental, and as of great 

 importance for the comprehension of the phenomena of nature. The 

 law may now bo formulated in two aspects. In the first place, from a 

 physical aspect : equal volumes of gases (or vapours) at equal tempera- 

 tures and pressures contain an equal number of molecules or of such 

 quantities 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 

 ,sv fixfances 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 stay to consider the chemical phenomena from which the law is 

 deduced or by which it may be explained. 



When two isolated substances interact with each other directly and 

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

 reaction is accomplished between quantities which in a gaseous state 

 (at equal temperatures and pressures) occupy equal volumes. Thus, 

 ammonia, NH 3 , reacts directly with hydrochloric acid, HC1, forming 

 sal-ammoniac, NH 4 C1, and in this case the NH 3 , or 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 



8 It must not be forgotten that Newton's law of gravity, or of the unity of the forces 

 causing a body to fall to the earth, and the planets to be attracted to and revolve round the 

 sun, was first a hypothesis, but it became a trustworthy, perfect theory, and acquired 

 the qualities of a fundamental law owing to the concord between its consequences 

 and reality. All laws, all theories of natural phenomena, are first hypotheses. Some 

 are rapidly established by their exact consequences, which agree with facts ; others only 

 take root by slow degrees. 



7 This is not only seen from the above calculations, but maybe proved by experiment. 

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

 (//// hydrogen chloride (the dryness of the gases is strictly necessary, because ammonia 

 and hydrogen chloride are both very soluble in water, and hence 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 the remaining gas is that whose 

 volume was the greater. 



