1871.] Mr. C. Tomlinson on Supersaturated Saline Solutions. 43 



both the solutions crystallized. Heated the flasks over a spirit-lamp, and 

 passed the solutions through a filter, when in each case a minute black 

 speck, which had acted as a nucleus, was left on the filter. 



Two flasks that had been standing some time on the window-ledge, each 

 containing a deposit of the seven-watered salt, were put out during the 

 heavy rain of a thunder-storm and uncovered. The drops repeatedly entered 

 the flasks, splashing up the solution ; but they did not act as nuclei. The 

 flasks were thus exposed during six hours, when one of them was brought 

 in and placed on the table ; it immediately became solid. 



After the storm, and while the sun was shining, snipped off some 

 pieces from some young leaves of the currant- and gooseberry-bush ; 

 they did not act as nuclei, even when shaken up in the solution. The 

 upper surfaces of the leaves had been washed in the rain, but the 

 under surfaces were dry. The scissors used were washed in spirit, and 

 several cuttings made in order still further to clean the blades, before 

 any of the pieces were allowed to fall into the flask. 



The conclusions which I think myself justified in drawing from these 

 and similar experiments are the following : — 



1. That a highly supersaturated solution of sodic sulphate may be 

 exposed to the open air of the country in an uncovered flask and in 

 cloudy weather for from twelve to twenty hours without any formation- 

 of the ordinary ten-watered crystals. 



2. That if the temperature fall to about 40° Fahr. and under, the 

 modified seven-watered salt is formed at the bottom of the solution just as 

 in covered vessels. 



3. That if the exposed solution suddenly crystallize into a closely 

 packed mass of needles, a nucleus may always be found in the form of 

 an insect, a speck of soot, a black point of carbon, &c. 



4. That if during the exposure rain comes on, the solution generally 

 crystallizes suddenly, in consequence of an active nucleus being brought 

 down ; but if the flask be put out during heavy rain, when we may sup- 

 pose all the solid nuclei to have been brought down, the rain- drops, now 

 quite clean, fall into the solution without any nuclear action. 



5. That the young and newly sprouted leaves of trees, such as those of 

 gooseberry- and currant-bushes, have no nuclear action. 



6. That in clear cloudless weather, when the force of evaporation is 

 strong, the solutions, after some hours' exposure, produce fine groups of 

 crystals of the ten-atom salt, just as a saturated solution would do if left to 

 evaporate slowly in an open dish. 



7. That if the solution, after being exposed to the open air, be brought 

 into a room, it crystallizes immediately under the action of aerial nuclei. 



Supersaturated solutions of Epsom salts and of alum were similarly ex- 

 posed, and the results were in harmony with the above conclusions. The 

 following are a few cases. 



