240 PEOGRESS IN CHEMISTRY. 



ATOMS AND MOLECULES. 



One of the most noteworthy of the discoveries of the eentury was 

 made by Gay-Liissac (1778-1850) in the year 1808. In conjunction 

 with Alexander von Humboldt, Gay-Lussac had rediscovered about 

 three years before what had previously been established by Cavendish, 

 namely, that, as nearly as possible, 2 volumes of hydrogen combine 

 with 1 volume of oxygen to form water, the gases having ])een 

 measured at the same temperature and pressure. Humboldt suggested 

 to Gay-Lussac that it would be well to investigate whether similar 

 simple relations exist between the volumes of other gaseous substances 

 when they combine with each other. This turned out to be the case; 

 it appeared that almost exactly 2 volumes of carbonic oxide unite 

 with 1 volume of oxygen to form carbon dioxide; that equal volumes 

 of chlorine and hydrogen unite to form hydrochloric acid gas; that 

 2 volumes of ammonia gas consist of 3 volumes hydrogen in union 

 with 1 volume of nitrogen, and so on. From such facts, Ga3"-Lussac 

 was led to make the statement that: The weights of equal volumes 

 of both simple and compound gases, and, therefore, their densities, 

 are proportional to their empirically found combining weights, or 

 to rational multiples of the latter. Gay-Lussac recognized this dis- 

 covery of his to be a support for the atomic theory; but it did 

 not accord with many of the then received atomic weights. The 

 assumption that equal volumes of gases contain equal numbers of 

 particles, or, as they were termed by him, molecules integrantes, was 

 made in 1811 by Avogadro, professor of ph3^sics at Turin (1776-1856). 

 This theory, which has proved of the utmost importance to the 

 sciences both of physics and of chemistry, had no doubt occurred to 

 Gay-Lussac, and had been rejected by him for the following reasons: 

 A certain volume of hydrogen, sa}' 1 cubic inch, may be supposed to 

 contain an equal number of particles (atoms) as an equal volume of 

 chlorine. Now, these two gases unite in equal volumes. The deduc- 

 tion appears so far quite legitimate that 1 atom of hydrogen has 

 combined with 1 atom of chlorine. But the resulting gas occupies 

 2 cubic inches, and must, therefore, contain the same number of par- 

 ticles of hydrogen chloride, the compound of the two elements, as 1 

 cubic inch originally contained of hydrogen or of chlorine. Thus we 

 have 2 cubic inches containing, of uncombined gases, twice as many 

 particles as is contained in that volume after combinations. Avo- 

 gadro's hypothesis solved the difficult3\ By premising two different 

 orders of particles, now termed atoms and molecules, the solution was 

 plain. According to him, each particle, or molecule, of hydrogen is 

 a complex, and contains 2 atoms; the same is the case with chlorine. 

 When these gases combine, or rather react, to form hydrogen chloride, 

 the phenomenon is one of a change of partners; the molecule, the 



