PROGRESS IN CHEMISTRY. 243 



compound in a jraseous state is twice its density referred to hydroo-en. 

 Thus equal volumes of ammonia and hydrogen chloride unite to form 

 ammonium chloride. It was to be expected that the density should 

 be half the molecular weight, thus: 



NH3+HC1 = NH,C1; and 53. 5 --2 = 26. 75 = density. (14+3) (l+85.,5) 



53.5. 



But the density actually found is only half that number, viz, 13.37; 

 and for long this and similar cases were supposed to be exceptions to 

 the law of Gay-Lussac, viz, that equal volumes of gases at the same 

 pressure expand equally for equal rise of temperature. In other 

 instances the gradual decrease in density with rise of temperature can 

 be followed, as with chloral hydrate, the products of which are chloral 

 and water. 



It was recognized ])y 8t. Claire Deville (1857) that the decrease in 

 density of such mixtures of gases was due, not to their being excep- 

 tions to Avogadro's- law, but to the gradual decomposition of the com- 

 pound body with rise of temperature. To this gradual decomposition 

 ho gave the name dissociation. This conception has proved of the 

 utmost importance to the science, as will be seen in the sequel. To 

 take the above instance of ammonium chloride, its abnormal density 

 is due to its dissociation into ammonia and hydrogen chloride; and the 

 gas which is obtained on raising its temperature consists, not of gas- 

 eous ammonium chloride, l)ut of a mixture of anuuonia and hydrogen 

 chloride, which, as is easily seen, occup}^, when separate, twice the 

 volume that woidd bo occupied by the gaseous compound. Of recent 

 years it has been shown by Broreton Baker that, if perfectly free from 

 moisture, ammonium chloride gasifies as such, and that its density in 

 the state of vapor is in fact 26.75. 



The molecular complexity of gases has thus gradually become com- 

 ])rehended, and the truth of Avogadro's law has gained acceptance. 

 And as a means of picturing the behavior of gaseous molecules the 

 kinetic theory of gases has been devised by Joule, Clausius, Maxwell, 

 Thomson (Lord Kelvin), and others. On the assumption that the 

 pressure of a gas on the walls of the vessel which contains it is due to 

 the continued impacts of its molecules, and that the temperature of a 

 gas is represented by the product of the mass of the molecules, or 

 the square of their velocity, it has been possible to offer a mechanical 

 explanation of Boyle's law, that at constant temperature the volume of 

 a gas diminishes in proportion as the pressure increases; of Gay- 

 Lussac's law, that all gases expand equally for equal rise of tempera- 

 ture, provided pressure is kept constant; the condition being that equal 

 volumes of gases contain equal luimbers of molecules. A striking sup- 

 port is lent to this chain of reasoning by the facts discovered by Thomas 

 Graham (1805-1869), professor at University College, London, and 



