ON SUPERSATURATED SOLUTIONS. T35 



to the rod, and this is then active. I certainly iind that when 

 drops of the sulphate or other solutions are put in the palm of the 

 hand before the solutions have been handled the drops can be 

 rubbed hard with the fingers without causing crystallisation. 

 Again, I have many times rubbed solutions of the acetate and 

 carbonate with oil on a plate without producing any effect. 



Some years ago when Mr. Tomlinson's theory was first brought 

 out, I and other observers showed that drops of oil or tallow could 

 be dropped into the solution and it prove inactive. Mr. Tomlinson 

 replied that in this case as long as the oil floated as a lens no true 

 adhesion took place between the oil and solution -, but that if the 

 flask were shaken so as to cause the oil to become a film, adhesion 

 took place and separation of salt ensued. The effect was due to 

 surface tension. He described a great number of experiments, 

 in which shaking the flask in the manner described caused crystal- 

 lisation. Without knowing the exact conditions of the experiment 

 it is difficult to say what is the explanation of his results. Other 

 observers have deposited films of all degrees of tenuity in the 

 solutions 5 M. Gernez floated oil above one solution and pushed a 

 glass rod through the oil into the solution. Mr. Tomlinson replied 

 that the conditions of the experiment altered the surface tension. 

 Hence the importance of the experiments described above in which 

 sodium sulphate, acetate, and carbonate were rubbed with oil and 

 greasy surfaces of all kinds without causing crystallisation. In 

 the face of these experiments it seems to me impossible to assert 

 that adhesion to a greasy surface is the general explanation of the 

 crystallisation of these solutions. I do not mean to assert that oil 

 has no effect on some solutions ; a large quantity of oil throws 

 down from the carbonate a modified salt which is quite inactive 

 in the solution. Ammonia alum is very sensitive to oil, giving the 

 normal salt. The fact of oil throwing down a modified carbonate is 

 curious and needs working out.'^ 



* Mr. Tomlinson has recently in a paper read before ihe Royal Society, 

 called in question my results. At a meeting of the Physical and Chemical 



