330 Mr. C. Tomlinson on the Action of Nuclei. [Nov. 20, 



form soluble soaps with the oils, which mingle with the solution often 

 without nuclear action. Lime and magnesia are more manageable. 

 On mixing the oil with one of these, and adding a drop or two to the 

 solution, then inclining the flask nearly horizontally, and rotating it 

 slowly on its axis, these insoluble soaps adhere to the side and form 

 smears, upon which masses of large crystals of the normal salt start 

 into existence in a striking manner, and the solution immediately 

 becomes solid, so that on restoring the flask to its erect position, a 

 vertical wall appears, with a solid mass on one side and an apparently 

 empty space on the other. 



Some of the solid and semi-solid fats in their natural state, or as 

 they leave the hands of the cook, are admirable nuclei. Such are suet, 

 dripping, fat of bacon and ham, lard, butter, and some others. But in 

 all cases a freshly cut surface acts most effectually, and a fat that 

 sometimes does not act for hours if cut one way, will act instantly 

 if cut at right angles to the former direction. Lard contains a 

 good deal of entangled water; this should be driven off. The lard 

 when cold sometimes acts better when smeared on a clean glass rod 

 than in a small lump. 



The action of solid porous nuclei, such as plaster of Paris, pumice, 

 &c, in determining the sudden solidification of these solutions admits 

 of easy explanation. When these bodies are moderately absorptive 

 they act by separating water from the solution ; but when, by exposure 

 to heat, their absorptive power is considerably increased, they become 

 immediately saturated not with water, but with a portion of the solu- 

 tion itself, and hence there is no separation of salt. When such an 

 apparently inactive body is exposed to the open air, its absorptive power 

 becomes somewhat diminished by access of moisture, and it is brought 

 into a condition to absorb more water from the solution, instead of the 

 solution itself. In the experiments of Mr. Liversidge and others the 

 porous bodies were thoroughly heated and dried before they were 

 brought into contact with the solution, and this will sufficiently explain 

 the negative results obtained by them in their carefully conducted 

 experiments. 



The action of nuclei on the supersaturated solutions of other salts 

 that have engaged my attention, such as those of alum, zincic and 

 magnesic sulphates, and one or two others, admits of similar explana- 

 tions ; but there are peculiarities belonging to each which need not at 

 present occupy the time of the Society. 



The curious condition of supersaturation was first observed by Dr. 

 Black, towards the end of the last century, and from that time to the 

 present innumerable notes and memoirs have been written on its varied 

 phenomena. Some observers who have treated the subject most elabo- 

 rately have adopted methods which, by the action of heat and the ex- 

 clusion of air, have tended to promote the cohesive or adhesive force of 



