116 SCIENTIFIC AMUSEMENTS. 



The two last experiments illustrate a general rule always 

 observed in connection with diffusion, viz., the lighter the gas, the 

 more rapidly does it diffuse;* hydrogen being considerably 

 lighter than air, diffuses much more quickly ; whilst carbon dioxide, 

 being more dense than air, diffuses less rapidly. Other experi- 

 ments in connection with certain of the chemical properties of 

 carbon dioxide and hydrogen are described subsequently (Expts. 

 115, 143, 144, 154, 155, 168, 207, 208, 215, 216, &c.). 



3. Solution and Separation from Solution by actions 

 involving Chemical Changes. 



CHAPTER X. 



SOLUTION OF SOLIDS IN LIQUIDS AND SEPARATION OF SOLIDS 

 FROM LIQUIDS. 



It has been already explained (Chapter I.) that whilst, in the 

 narrowest sense of the word, the term " solution " is confined to 

 cases where the dissolved body and the solvent react upon one 

 another so as to produce a solution from which the two substances 

 are again separable by simple means (such as cooling, evaporation, 

 diluting with some other fluid so as to precipitate from solution, 

 &c.), the term is also used in a wider sense to indicate all cases 

 where a solid disappears on treatment with a liquid, no matter 

 whether the solid is chemically changed by the process or not ; 

 when such chemical change takes place, in general the original 

 solid can no longer be recovered from the solution formed by any 

 simple means, such as evaporation, &c.; what substance is thus 

 obtained is no longer the original solid used, but is something 

 different therefrom, being altered by the chemical action taking 

 place. Sometimes the chemical change is not accompanied by any 

 visible alteration, except the disappearance of the dissolved solid in 

 the solvent fluid ; sometimes a coloured solid dissolves to a colour- 



* The exact stating of the law is that the rate of diffusion is proportionate to the 

 reciprocal of the square root of the density. Thus the densities of the gases 

 hydrogen, oxygen, and chlorine are nearly in the proportions of 1, 16, and 

 36 ; the square roots of which number are respectively 1, 4, and 6. Hence 

 the rates of diffusion are in the ratio 1, |, and J ; or hydrogen diffuses four 

 times as quickly as oxygen, and six times as rapidly as chlorine. 



