226 Royal Society : — 



motes or dust that prevent or lessen adhesion between it and the 

 aqueous part of the solution, and apparently render an inactive solid 

 active. When a glass rod &c. has been kept in water or passed 

 through flame and dried, or cooled out of contact with the air, it is 

 more or less chemically clean, and remains so while sheltered. When 

 Ziz found a knitting-needle active on one solution, and by pass- 

 ing it through the cork which confined a similar solution it became 

 inactive, he simply made the wire chemically clean by the friction. 

 Air is not a nucleus, and when it appears to act as such, it is simply 

 as a carrier of some solid particle not chemically clean. Hence 

 narrow-necked flasks when opened retain their solutions liquid longer 

 than wicle-necked ones, as the former are less likely to catch motes 

 &c. from the air than the latter. Supersaturated solutions are best 

 preserved by plugging the necks of the flasks &c. with cotton-wool, 

 since in cooling down the air is filtered in passing through the plug, 

 and motes and dust are thus kept back. 



Tubes made chemically clean by the action of strong sulphuric acid 

 may be filled with a strong solution of sodic sulphate, and when cold 

 the tubes may be placed in a freezing-mixture at 10° F. without any 

 separation of the salt. Hence the author differs from M. Lowel's 

 theory, which supposes a molecular change to take place when strong 

 solutions of the salt are cooled down below 60°. Supersaturated solu- 

 tions of various salts were cooled down to various temperatures from 

 32° to 0° F. without crystallizing. Sodic acetate, for example, was 

 kept for some hours at 14°, when on touching it with a wire it became 

 solid, and the temperature rose to 104°. Sodic arseniate, sodic suc- 

 cinate, sodic borate, sodio-potassic tartrate, potash alum, and other 

 saline solutions' were treated in this way. Some of these solutions 

 become viscid at a low temperature, and do not immediately crys- 

 tallize on removing the cotton-wool plug. If they be touched, or 

 the side of the flask scratched with a chemically clean wire, there 

 is no action ; but if the wire be not chemically clean, the scratches 

 immediately become chalky white by being covered with minute crys- 

 tals of the salt, and the action then spreads until the solution becomes 

 solid. 



Some salts that are not very soluble in water, such as the plumbic 

 acetate, form highly charged supersaturated solutions, and retain 

 their liquid state below ordinary atmospheric temperatures. When 

 at a certain point they suddenly solidify. Other solutions merely 

 deposit the excess of salt above the condition of supersaturation, 

 leaving the mother-liquor saturated ; the cupric sulphate is an ex- 

 ample of this. 



The memoir contains a number of details respecting the action of 

 nuclei, whether derived from the air, from the flask, from the salt 

 itself, from the filter, or the cotton-wool used in closing the vessels. 

 If the solution touch the wool, crystallization immediately sets in ; or 

 if the upper part of a chemically clean tube be touched with a finger 

 slightly greasy before filtering into it the hot solution, the latter will 

 cool clown to the temperature of the air without crystallizing, nor will 

 there be any effect if the tube be inclined so as to touch the clean 

 portions of the inner surface ; but the moment the solution comes 



