194 



Mr. Charles Tomlinson on 



[Mar. 14. 



action derived from the atmosphere (rain having fallen during some 

 hours and having scarcely ceased) hut was due to another force. Last 

 summer, as noticed in my former paper,* drops of sodic acetate ex- 

 posed on glass always started the crystallisation from the edge. So, 

 in these wide shallow glasses in the open air, crystallisation set in from 

 the edge, or capillary curve, where the solution is most drawn out and 

 attenuated. Now, supposing evaporation to be active all over the 

 surface, this thin portion, becoming thinner by the process, would be 

 the first to detach a molecule of the salt, and this, once set free, would 

 act as a powerful nucleus to the whole. Lines, or rather thin planes 

 of crystals, diverge from the point, and this so quickly as not to afford 

 time for the molecules to arrange themselves into crystalline forms. 



On one occasion the mass of seven-watered salt rose so near the 

 surface, that the solution resting on it was scarcely more than a film. 

 This, by evaporation, also disengaged a minute portion of the normal 

 salt, which acted as a centre of crystallisation. 



Solutions containing two parts of salt to one of water behaved 

 much in the same way as the stronger ones. A weaker solution, con- 

 taining only one of salt to one of water, accommodated itself to the 

 conditions under which it was placed. It did not deposit any 

 modified salt, and on being uncovered in the open air, crystallised from 

 the edge. 



In the case before referred to, where the solution crystallised in 

 cooling, as in an open evaporating dish, namely, in the form of large 

 prisms of the normal salt, the vessel was a shallow dessert dish with a 

 scolloped edge, so that the protecting cover did not confine the air 

 over the surface of the solution, but allowed it to circulate ; hence, in 

 cooling down, the molecules were also in a condition to circulate and 

 gradually to arrange themselves into groups, ready to assume the 

 crystalline form when the temperature had sufficiently declined. 

 Whereas, in closely covered vessels, the strong adhesion to the side 

 prevents this circulation and re- arrangement, the adhesion holding the 

 molecules in a forced state, bearing some kind of analogy to the glass 

 in Prince Hup erf s drops and the Bologna phial. 



It must also be noted, that in the last case also, there was no 

 nuclear action ; for, had there been, the solution would have cooled 

 down to a certain point, and then have suddenly crystallised in closely 

 packed planes ; whereas, in the case now referred to, the crystals were 

 large and of the usual shape, while the mother liquor was no longer 

 supersaturated. 



Solutions of zincic sulphate and of magnesic sulphate (3 to 1 each) 

 in these covered shallow vessels, also threw down a good deal of the 

 modified salt, in the case of the zinc solution the monohydrated salt 



* Proc. Boy. Soc, xxvi, 528. 



