306 Royal Society : — 



not crystallize, and one slowly dried up in octahedra. I have 

 repeatedly of late boiled sulphuric acid iu the laboratory, so that 

 there can be no lack of sodium sulphate in the dust. 



2. Drew a strong current of air over drops of sodium sulphate 

 in a glass tube : inactive. 



3. Drops of sodium sulphate put upon the leaves of many plants 

 in my garden. They slowly evaporated, giving the 7-atom salt. 

 The leaves were covered with dust, as the garden opens on to a 

 road, and the weather has been hot and dry ; we are not far from 

 Bristol, so we might expect to find sulphates. 



4. Carried sodium sulphate to an upper room ; drops on the 

 wash-hand stand, on the window-sill inside and out, on the iron 

 bars outside : all inactive. Washed my hands and spread a drop 

 with the finger on the window-sill, inside : inactive. Three drops 

 crystallized on the mantlepiece, and one on the window-sill. 

 Several drops on the window-frame evaporated as 7-atom salt. 



5. Potash alum on a window-sill outside gave a modified film. 



6. Sodium acetate put upon the cork of a large bottle which had 

 stood for two years untouched in my laboratory. The drops were 

 quite thick with dust, but remained liquid for more than 24 hours. 



7. Other drops of the same put on the floor of the laboratory, 

 on the dusty corners of the shelves, on paper, on every place and 

 kind of surface I could find : remained liquid in all cases. 



8. Spread a number of drops of the same on a glass plate, 

 covering nearly the whole of it. Made about half crystallize. 

 Left them exposed for three days ; they remained liquid, though 

 the normal salt effloresces slightly. 



9. Ammonia alum : many drops on a glass plate ; they formed 

 films by evaporation; made a good many crystallize, when they 

 broke open, early in the day : carried them out in a high wind 

 to the house of a neighbour, and brought them back ; then late at 

 night put a number of fresh drops on the plate, and several of 

 them remained liquid all night, 



10. Sodium carbonate is not affected by any surface in my 

 laboratory. I have spread a drop over a dirty glass plate so as to 

 cover a good many square inches, and it slowly evaporated, giving 

 crystals. Drops on the floor, shelves, bottles, &c. of the laboratory 

 invariably remain liquid. 



I could give many other instances, but these are sufficient to 

 show that the air does not ordinarily contain these salts, and that 

 it does not readily catch them up and deposit them on all kinds 

 of surfaces ; and yet these salts are remarkably sensitive to 

 crystals of the same kind. The effect of using cotton-wool is a 

 good example of this. Another is this : — Touched a crystallized 

 drop of sodium acetate with a pin; passed the pin repeatedly 

 through my coat : active at once. After touching a crystal the 

 finger needs to be washed carefully. Again, sodium sulphate 

 crystallizes almost invariably on any dirty surface in my labora- 

 tory, and ammonia alum generally. Even the sodium acetate 

 crystallizes at times when I am at work with the same salt 

 close by. 



