350 



NATURE 



{^August 7, 1879 



take up 209 parts of potash alum, but at 32° only 4 

 parts. A boiling solution of alum containing 209 parts 

 of the salt in 100 of water may be reduced, in a covered 

 vessel, to the freezing-point of water without depositing 

 any salt ; in such case the water at 32° holds 52^ times 

 more salt in solution than it can take up at that reduced 

 temperature. 



The phenomena of supersaturation are, perhaps, best 

 exhibited by means of Glauber' s salt, or sodic sulphate, 

 of which there are three forms, namely, the normal salt, 

 containing ten atoms or proportionals of water of crystal- 

 lisation, the seven-atom salt and the anhydrous. If two 

 or three ounces of the normal salt in one ounce of water 

 be gradually heated in a flask to 93° F., its point of maxi- 

 mum solubility, it will be completely dissolved, but if the 

 heat be applied too suddenly or too fiercely, the anhydrous 

 salt is thrown down, and occasions violent bumpings of 

 the vessel. If the solution be properly conducted, it can 

 be raised to the boiling-point, and so be left to cool (the 

 mouth of the flask being covered) to the temperature of 

 the air, without depositing any salt. It is now super- 

 saturated. If, however, the solution be cooled to about 

 40° F. and under, the modified seven-watered salt is 

 thrown down, the mother-liquor remaining supersaturated. 

 If the flask be left uncovered, or the solution be touched 

 with a proper nucleus, radiating lines, or rather plates of 

 crystals of the normal salt, diverge from a point on the 

 surface, and proceeding rapidly downwards, seem to inter- 

 penetrate the seven-watered salt, destroying its trans- 

 parency, changing its molecular structure, whereby it 

 assumes the appearance of boiled white of egg, and im- 

 parting to it three additional equivalents of water. The 

 solution has now become solid, and the temperature con- 

 siderably raised in consequence of change of state. 



The condition of supersaturation was first noticed by 

 Dr. Black towards the end of the last century, and from 

 that time to the present many scientific men, both of this 

 country and of the Continent of Europe, have studied the 

 subject, often with contradictory results. The pheno- 

 mena connected with the nuclear action of bodies in pro- 

 ducing the sudden crystallisation of these solutions seem 

 to behave differently in the hands of different inquirers ; 

 so that the facts affirmed by one writer are denied by 

 another ; and the theory which seems to have been dis- 

 proved by one is again brought forward by another. Thus, 

 to quote only a few of these contradictory statements, Ziz 

 found that air and solids act best as nuclei when dry ; if 

 wet, or boiled with the solution or thrown into it while 

 hot and allowed to cool with it, they are inactive : Lowel 

 denies that air has any nuclear action, but that solids ex- 

 posed to the air become active by a sort of catalytic 

 force, and that alcohol is active : Liversidge and others 

 deny that alcohol is active : Selmi states that dry air is 

 nuclear, and acts by getting rid of water at the surface 

 and producing crystals which continue the action : Gay- 

 Lussac had before expressed a similar opinion : Lieben 

 found that soot, platinum black, and pounded glass are 

 nuclei : Schroder remarks that it is always a matter of 

 chance whether such or such a substance produces crys- 

 tallisation ; Gernez, VioUette, Schiff, and Liversidge arc 

 satisfied that there is only one nucleus, and that is a salt 

 of the same kind as the one in solution or isomeric there- 

 with ; and that when liquid and solid bodies apparently act 

 as nuclei they are really contaminated with minute particles 

 of the salt which is supposed to be always floating in the 

 air : Jeannel opposes this theory, and Pellogio " gives 

 proofs that the phenomena of supersaturation are not so 

 simple as the French physicists would imply;" he finds 

 that porous bodies act as nuclei : VioUette and Liver- 

 sidge, on the contrary, find that porous bodies and 

 bodies greedy of water and capable of being hydrated, 

 such as absolute alcohol, fused sulphate of copper, qu'ck- 

 lime, &c., are quite inactive. 



In the midst of all this conflict of testimony, my own 



results were not likely to escape censure. Indeed, they 

 have been attacked with a degree of acrimony which 

 seems to be very much out of place in the calm inquiry 

 after scientific truth, and in the presence of so many 

 contradictory results produced by eminent conscien- 

 tious observers. I was not able for a long time to 

 account for the negative results which were opposed to 

 my positive ones, and I threw the subject aside. At 

 length, however, I was led to suspect that nuclei are 

 powerfully affected by varying atmospheric conditions, 

 whereby they sometimes determine the solidification of 

 these solutions, and at other times not. Accordingly I 

 undertook a series of daily observations which extended 

 over some months, working with a standard solution of 

 sodic sulphate and an essential oil, and the result was 

 that when the wind at Highgate was northerly or easterly, 

 the oil was nuclear, but with a westerly or southerly wind 

 it was inactive. It was obvious, then, that there was 

 some force present in the air which rendered the oil 

 active, which being absent, the same oil remained pas- 

 sive. I succeeded in identifying this force with ozone, 

 and judging that wherever ozone was present, irrespec- 

 tive of the direction of the wind, the oil would become 

 nuclear, I visited several places, notably one by the sea- 

 side, and was confirmed in the view I had taken. For 

 example, at Eastbourne a westerly wind produced in the 

 course of ten minutes a deep orange brown stain on ozone 

 test-paper, and oils exposed to its action became singu- 

 larly active after a few minutes' exposure. Freshly dis- 

 tilled oil of lemons, oil of cajuput, oil of turpentine, &c., 

 were inactive in the house on supersaturated solutions of 

 sodic sulphate and of alum, but by the sea-side far away 

 from the town, with the wind blowing in the direction of 

 the tovni, or at the end of the pier, 1,012 feet from the 

 shore, with the wind blowing from the sea, the oils were 

 allowed to fall in drops from a dropping tube into a small 

 clean beaker for about three minutes, when they became 

 powerfully active in rendering the solutions soHd. 



In my first papers on this subject {Phil. Tians., 1868- 

 1871) a nucleus is defined as a body that has a stronger 

 attraction for the salt, or for the water of a solution, 

 than subsists between the salt and the hquid. Examples 

 are given in which oils, fixed and volatile, alcohol, ether, 

 &c., spreading on the surface of the solution, produce a 

 separation of salt, and the rapid solidification of ths 

 solution ; whereas, if such liquids, instead of spreading, 

 assume a lenticular form, they are inactive, and may, 

 by shaking the flask, be dispersed through the solution 

 in numerous globules without any immediate nuclear 

 action. 



Several circumstances favour the action of these liquids 

 as nuclei : (i) Chemically clean flasks and solutions, so as 

 to maintain (2) the surface tension of the solutions as high 

 as possible, in order to spread a drop of oil, &c., into a 

 film ; (3) bright and clear weather, with strong evapo- 

 rative force. 



A newly distilled essential oil is inactive, on account of 

 its strong cohesive force, and when a drop of it is dispersed 

 in globules through the solution, each globule retains its 

 surface-tension, whereby it is prevented from coming into 

 contact with, and separating a molecule of the salt held in 

 solution. The action of ozone is an oxidising one, dimi- 

 nishing the cohesive force among the particles of the oil, 

 just as rust weakens the cohesive force of a bar of iron. 



The effect which ozone produces quickly on a newly dis- 

 tilled volatile oil, is produced slowly on the same oil by 

 long keeping. Its cohesive force is so far weakened, that a 

 drop of it on the surface of the solution no longer tends to 

 assume the lenticular form, or to disperse in globules 

 through the solution. Then, as oil can adhere much more 

 strongly to salt than to water, and the supersaturated 

 solution being a highly charged system capable of yielding 

 to a force that is exerted in the right direction, such an oil 

 adheres to and separates a molecule of the salt, and the 



