13^ ON SUPERSATURATED SOLUTIONS. 



blotting paper makes it give the normal salt at once — but a drop on 

 some thin blotting paper which had been scraped with a knife was 

 absorbed. 



(<?.) Rapid absorption often prevents crystallisation. There 

 seems to be no time for the crystals to form. If the carbonate be 

 melted in a test-tube and allowed to cool, a very strong solution is 

 obtained. Small drops of this can be put in a circle round the 

 edge of a sheet of filter paper and will all be absorbed ; but if two 

 or three large drops are put on the paper together they will crys- 

 tallise as the normal.'^ StroUg solutions of the same which give 

 the normal generally on lumps of dry earth are frequently absorbed 

 by pounded earth, because the absorption is more rapid. A cake 

 of plaster of Paris was repeatedly active, while the same cake 

 pounded was inactive. The scraped blotting paper mentioned 

 above was probably inactive to ammonia alum because the rough 

 surface promoted rapid absorption. 



Sow absorbents act. It seems probable that these absorbents act 

 by abstracting water, and that therefore their action is analogous to 

 that of evaporation. The abstraction of water in the Carbonate 

 takes place in regular steps, depending on the amount of water 

 present, but always modified by the tendency of rapid absorption 

 to prevent crystallisation. Thus a slow rate of absorption as of 

 large drops of very strong solutions gives the normal salt, both on 

 filter paper and earth. Solutions of medium strength give salts 

 with less water on paper and earth. Weak solutions are entirely 

 absorbed and only give the modified salts by slow evaporation. 



It is worth noticing that the chemical affinity for water of the 

 Acetate, Carbonate, and Sulphate of soda, agrees exactly with their 

 behaviour to 'absorbents. The acetate has the strongest affinity 

 for water ; the drysalt does not effloresce except in very dry air, 

 and then only very slightly ; the anhydrous salt is very deliquescent. 



* This experiment, with others, was repeated at the meeting of the 

 Chemical Section mentioned, (1878.) 



