96 ELEMENTS OF* LABORATORY WORK 



64. The Solution of a Solid is attended with Change of Tem- 

 perature. Also the Rate of Diffusion varies with Temperature. 



When ammonium nitrate is added to water the temperature 

 is very much lowered ; a temperature as low as 27 C. may 

 even be reached. Ammonium chloride, sodium acetate, 

 silver nitrate, and potassium iodide, among others, will pro- 

 duce a fall of temperature. On the other hand, manganous 

 sulphate, magnesium chloride, and a few others, when dissolved 

 in water, cause a rise of temperature. The observations are 

 easily made by using a thermometer to indicate the tempera- 

 ture before and after the solid in question is dissolved. 



In all the above cases the solids dissolved resume their 

 original condition on raising the temperature of the solution 

 sufficiently to gasify the water, and the same observations 

 may be made with other solids and other liquids. In certain 

 cases, however, the solid cannot be restored to its original 

 condition by raising the temperature of the liquid containing 

 it. For example, when zinc and hydrogen sulphate are 

 placed in contact, and the liquid afterwards raised in tem- 

 perature, a solid quite unlike the zinc is obtained. Changes 

 of this kind will require to be afterwards considered more 

 fully by themselves. 



In either case the change coincides with a thermal change, 

 and at the same time cannot be mentally grasped, except as 

 a rearrangement of very minute particles ; those of one sub- 

 stance making their way among those of the other substances 

 imperceptibly, just as imperceptibly as electric and thermal 

 changes proceed. We adopt, then, the theory that matter is 

 discontinuous, i.e. built up of separate particles. So small, 

 however, are the particles that no changes which affect them 

 can be directly observed, and hence the difficulty in measuring 

 or understanding them. 



It is important to note, in view of a possible connection 

 between the temperature of a body and the motion of its 

 minute particles, that the rate of diffusion of two liquids 

 increases with the temperature. 



65. The Quantity of a given Solid dissolved varies with 

 the Temperature of the Solvent. The Point of Saturation is 



