General Principles. 33 



different from solution. A stone or an eartli 

 may be reduced to powder and mixed with water 

 for a certain time, but the mixture will be 

 muddy, and in a little time the solid particles 

 will sink to the bottom ; whereas if salt or sugar 

 be dissolved in water, the compound will be al- 

 ways clear, and cannot be separated otherwise 

 than by a chemical process. 



These processes are generally produced by 

 means of heat. They are chiefly evaporation, or 

 sublimation, which will be explained in the next 

 lecture. 



Fluid matters in which others are dissolved are 

 called menstruums ; and there is a certain limited 

 quantity of the solid which they will dissolve or 

 take up in solution by the power of attraction. 

 Thus the quantity of salt which water will actually 

 dissolve is limited, and whatever is beyond that 

 will sink undissolved to the bottom. This point 

 or limit is called the point of saturation. 



Chemical attraction differs from the attraction 

 of cohesion or aggregation in this, that it exists 

 between the particles of bodies differing from 

 each other, as between the salt and water; whereas 

 the attraction of aggregation' operates only be- 

 tween particles of the same kind. The particles 

 of certain bodies are chemically attracted by cer- 

 tain menstruums more strongly than by others. 

 Thus sulphuric acid will dissolve a certain por- 

 tion of copper, but if iron is added it will let fall 

 the copper and take up the iron, and if an alkali 



c 5 



