MODERN SCIENCE. 185 



THE DIFFUSION of gases, " The diffusion-rates of any two- 

 gases are inversely as the square roots of their densities " \ 

 invented the diffusion tube by means of which the rates of 

 diffusion of different gases can be compared ; formulated the 

 conception of acids of different basicity ; discovered the 

 PROCESS OF DIALYSIS, in which, by reason of the law of 

 diffusion of liquids, the (difficult) separation of colloids and 

 crystalloids is easily effected ; discovered also the process of 

 atmolysis, which determines the separation of one gas from 

 another, inventing the tube atmolyser for that purpose ; 

 studied the motion of gases, and their absorption by metals; 

 devoted his energy to trace the movements of atoms and 

 molecules, showing that gases pass through walls as they 

 pass through other gases, and, by measuring their speed> 

 showed how it is possible to determine the rate of motion 

 of a molecule, and to follow the molecule into organic 

 membranes all of which discoveries place their author 

 among the greatest chemists. 



(?) 1811. Avogadro contributed to elucidate the atomic 

 theory by discovering the LAW which bears his name : 

 "Equal volumes of all gases, compound as well as simple, 

 measured at tJie same temperature and under the same pressure ', 

 contain equal numbers of molecules " one of the paramount 

 laws of chemistry; he established the distinction between 

 the ultimate particles of compounds and elements intro- 

 ducing, and explaining too, the idea of molecules. Avogadro's 

 law has, later, been deduced mathematically from the first 

 principles of the mechanical theory of gases by ProL 

 Neumann (1869). 



1816 56. Gerhardt established a THEORY OF TYPES of 

 great simplicity and convenience : the compounds of am- 

 monia, for instance, are grouped into amides, amines, alkala- 

 mides, subdivided according as they are derived from one, 

 two, three molecules of ammonia ; then the compounds con- 

 taining water are grouped and subdivided according to the 

 same principle, and so forth. In this manner we have tables 

 of multiple and mixed types, such as : Bichloride, Dihydrate, 

 Diamide ; then Trichloride, Trihydrate, Triamide ; then 

 Chlorid-hydrate, Chlorid-amide, Hydrat-amide. The deter- 



