136 ON SUPERSATURATED SOLUTIONS. 



(3.) The Contact Theory. — Jeannel stated that mere contact with 

 a solid substance as a glass rod was sufficient to make these solu- 

 tions crystallise. He gave sodium acetate as an example. The 

 experiments described above with the acetate and other salts 

 render it unnecessary to refer to this theory any further. 



(4.) Catalysis. — This seems to be only a way of saying tliat 

 the subject is inexplicable, and it need not detain us. We may 

 have to fall back on it if all explanations fail, but we are not 

 reduced to that yet. 



(^.) The Absorption Theory, — Mr. Liversidge noticed that dry 

 lycopodium powder is active in supersaturated solutions, while 

 the same powder when wet is inactive. He suggested that it 

 might absorb water and thus cause crystallisation. Mr. Tomlinson's 

 nuclei would then be not little particles with a film of grease on 

 them, but little absorbent particles. He did not, however, pursue 

 the subject any further. I arrived independently at the same 

 conclusion, and am able to give clear evidence that absorbent 

 substances do cause crystallisation, provided that they are not at 

 once saturated by the liquid, in which case no separation ensues. 

 In the letter to " Nature " already referred to, I showed that I 

 could make little mud pies of earth on the top of a flowerpot with 

 the strongest solutions of sodium sulphate, and that earth from the 

 flower beds could be dropped into the solution which deposited the 

 7 -atom salt on the top of the earth ; I found at the same time that 

 drops of the solution put on the earth in the bed invariably 

 crystallised at once. I could not explain the phenomenon at the 

 time, and therefore did not mention it in my letter. I soon found 

 that absorbent substances of all kinds act in just the same way, and 

 on all kinds of solutions. I made an immense number of experi- 

 ments and arrived at some interesting results, of which the 

 following is a partial summary : — 



section of the Bristol Naturalists' Society, held Feb. 26th,.i878, many of the 

 above experiments, such as rubbing the sulphate on the palm of the hand, 

 rubbing the acetate and carbonate, &c., were successfully shown. 



