908 



SCIENCE. 



[N. S. Vol. XXIII. No. 598. 



against the density of the acid solution. 

 Both the table and the diagram indicate 

 that the solution of sulphuric acid which 

 would be best for keeping that particular 

 tobacco in the condition in which it is put 

 on the' market, has a density lying between 

 1.283 and 1.296, rather nearer the former 

 figure than the latter. Reading from the 

 curve, the solution which causes no loss or 

 gain in the weight of the tobacco has a 

 density of 1.285. 



No precautions were taken in the above 

 experiment to control the temperature, and 

 I have assumed that the temperature was 

 20° C, which is very close to the mean 

 temperature of the laboratory at that sea- 

 son. The vapor pressures of solutions of 

 sulphuric acid at different temperatures 

 have been determined by Richards' and 

 from his tables, the solution of sulphuric 

 acid, having a density of 1.285, has a vapor 

 pressure of 10.8 mm. at 20° C. The result 

 of the experiment has been to show that 

 for that particular tobacco a solution hav- 

 ing a vapor pressure at 20° of 10.8 mm. 

 will keep that tobacco (Old Gold plug cut) 

 in the same condition of moisture as it had 

 when the box was opened. 



Now there is no automatic method of 

 regulation of the composition of this solu- 

 tion, and so some method for this regulation 

 had to be found. Here the principles of 

 physical chemistry suggested the use of a 

 saturated solution of some salt, which solu- 

 tion must have the proper vapor pressure. 

 A saturated solution automatically controls 

 the vapor content of the air space above it, 

 for if the vapor pressure is above that of 

 the solution, there is condensation tending 

 to dilute the solution; but, as there is al- 

 ready solid salt present in the solution, it 

 goes into solution until saturation is again 

 reached. Conversely, if the vapor pressure 

 of the air space is below that of the solu- 

 'Proc. Am. Acad., 33, 23 (1897-98). 



tion, there is evaporation from the surface 

 of the solution, causing some of the salt to 

 crystallize out. Thus a saturated solution 

 containing some of the crystals of the salt 

 is a means of preserving a constant vapor 

 pressure in a confined space. A dry to- 

 bacco will absorb water and a very moist 

 tobacco will lose water, the salt solution 

 acting as a control. 



There is, of course, this requirement to 

 be filled, that the solid shall not itself 

 evaporate and give any unpleasant taste or 

 odor to the tobacco, such requirement being 

 filled by almost any inorganic salt. From 

 the measurements of Lescoetir' the follow- 

 ing vapor pressures of solutions saturated 

 at 20° have been taken: 



Vapor pressure of saturated solutions of various 

 salts at 20°. 



The saturated solution of barium bro- 

 mide has a vapor pressure very nearly 

 equal to that which was found for the 

 tobacco, and Lescoeur's results also show 

 that the solid salt (BaBr2.2H20) has prac- 

 tically the same vapor pressure. Professor 

 Bancroft has kept cigars for a long time 

 in a desiccator containing barium bromide 

 with very good results. This procedure 

 has another advantage, for it not only 

 keeps cigars and tobacco in good condition, 

 but it may also be used to bring to a prop- 

 er moisture condition, tobacco and cigars 



'Ann. Chim. Phys. (6), 16, 378; 19, 35, 533; 

 21, 511; 25, 423; 28, 237; (7) 2, 78; 4, 213; 7, 

 416; 9, S.-iT. 



