60 



PRINCIPLES OF CHEMISTRY 



contact. In dissolving, a solid substance acquires a mobility of parts, 

 and a gas loses its elasticity, and therefore reactions often take place 

 in solutions which do not proceed in the undissolved substances. Fur- 

 ther, a substance, distributed in water, evidently breaks up (or ' disin- 

 tegrates ') that is, becomes more like a gas and acquires a greater 

 mobility of parts. All these considerations require that in describing 



Flo. 14. Method of transferring a gas into a cylinder filled with mercurv and wlm.-e open end is im- 

 mersed under the mercury in a bath having two glass sides. The apparatus containing the gas is 

 represented on the right. Its upper extremity is furnished with a tube extending under the 

 cylinder. The lower part of the vessel communicates with a vertical tube. If mercury be poured 

 into this tube, the pressure of the gas in the apparatus is increased, and it passes tlm>ui:li the gag- 

 conducting tube into the cylinder, where it displaces the mercury, and can be measured or subjected 

 to the action of absorbing agents, such as water. 



the properties of substances, particular attention should be paid to their 

 relation to water as a solvent. 



Everybody knows that water dissolves many substances. Salt, 

 sugar, alcohol, and a number of other substances, by dissolving in water 

 form with it homogeneous liquids. To clearly show the solubility 

 of gases in water a gas should be taken which has a high co-efficient 

 of solubility for instance, ammonia. This is introduced into a bell 

 (or cylinder, as in fig. 14), which is previously filled with mercury 

 and stands in a mercury bath. If water be then introduced into the 

 cylinder, the mercury will rise, owing to the water dissolving the 

 ammonia gas. If the column of mercury be less than the barometric 



