ON SUPERSATURATED SOLUTIONS. I37 



(a.) Absorbent substances in large quantities are active to large 

 drops, while the same absorbents in small quantities can be put 

 into drops or into the solutions, and being saturated at once, are 

 inactive. INIany lumps of earth, large and small, were put on a 

 glass plate and drops of sodium carbonate put on each ; all the 

 small lumps were inactive ; all the large but one were active. 

 Earth which had been in an open box in my laboratory for two 

 years was pounded and put into many drops of the sulphate and 

 was inactive 3 the drops put on the earth crystallised. Swedish 

 filter paper was cut into small pieces and was put into flasks, 

 test tubes, and drops of the carbonate, sulphate, and acetate, and was 

 inactive J drops of the solutions on the same filter paper crystallised 

 as the normal salt. Plaster of Paris was inactive in small quantities 

 in various solutions, and often active in large quantities, A 

 supersaturated solution of sodium sulphate in sulphuric acid can be 

 rubbed on a glass plate with bone, or hard close grained wood, 

 but the end of a match makes it crystallise even without rubbing j 

 small shavings of wood when at once saturated by being pushed 

 into drops of this solution were inactive ; when left floating or 

 when larger in size they were active. 



(h,) The same absorbents produce different results in different 

 solutions. Small bits of filter paper are almost invariably inactive 

 in the sulphate carbonate and acetate, but are active in strong 

 potash alum. In the experiments described above the sodium 

 sulphate may be either weak or strong, it will always crystallise on 

 filter paper -, the carbonate must be strong -, drops of weaker 

 solutions are absorbed entirely and evaporate as the 7-atom modified 

 salt, or if of moderate strength they often give the modified salt at 

 once. Swedish filter paper gives the normal carbonate much more 

 readily than thin blotting paper does. Filter paper acts only on 

 the strongest solutions of the acetate. The slightly absorbent 

 action of thin straw paper makes drops of the sulphate often 

 evaporate as the anhydrous instead of the 7-atom salt which it gives 

 on glass. Ammonia alum is exceedingly sensitive to absorbents. 



