906 



SCIENCE. 



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



It is a principle of physical chemistry 

 that everything, even the most infusible of 

 metals, has a vapor pressure, although this 

 is so small as to be negligible in some cases. 

 In other words, every object tends to keep 

 the air space around it saturated with the 

 vapor of that object. Thus, a body of 

 water will continually evaporate into the 

 air if the air be unsaturated with water 

 vapor at that temperature, and conversely 

 when the air becomes saturated with water 

 vapor and then is cooled for any reason, 

 precipitation ensues as cold saturated air 

 contains less water vapor than warm satu- 

 rated air. This vapor pressure of water is 

 a perfectly definite quantity at any tem- 

 perature, the pressure of the water vapor, 

 or percentage of water vapor in the air, 

 being greater at high temperatures than at 

 lower temperatures. A solution of a solid 

 in water has also a perfectly definite pres- 

 sure, the more concentrated the solution the 

 lower being the pressure. If for any rea- 

 son the quantity of water vapor is greater 

 than that with which the solution can exist 

 in equilibrium, the solution gains in weight 

 due to the condensation of the excess of 

 water vapor from the air, and thereby be- 

 comes more dilute, and will continue to be- 

 come more dilute until equilibrium is es- 

 tablished between the solution and the 

 vapor. Thus, if a beaker containing a 

 dilute solution of sugar, and a beaker con- 

 taining a more concentrated solution of 

 sugar be placed under a bell-jar, the dilute 

 solution tends to maintain a greater mois- 

 ture content in the air space than does the 

 more concentrated solution. Consequently 

 there is a slow continual evaporation from 

 the dilute solution to the more concentrated 

 one, and this process will continiie until 

 the vapor pressure of the two solutions be- 

 comes identical, that is, until the percent- 

 age composition of the solutions is the same. 

 In like manner, if the beakers contain solu- 



tions of salt and sugar, respectively, of un- 

 equal vapor pressures, there will be a dis- 

 tillation from the solution of higher vapor 

 pressure to that of lower vapor pressure, 

 the distillation continuing until the two 

 solutions have acquired equal vapor pres- 

 sures. In like manner, salts containing 

 water of crystallization have definite vapor 

 pressures depending on the temperature. 

 If the vapor content of the gaseous phase 

 falls below this vapor pressure, the hy- 

 drated salt gives up vapor sufficient to re- 

 store that vapor pressure, some of the an- 

 hydrous salt, or of a salt of a lower degree 

 of hydration, being formed. If the content 

 of the gaseous phase increases for any rea- 

 son above the vapor pressure of the hy- 

 drated salt, there is no change until the 

 vapor pressure of the saturated solution of 

 the salt is reached, at which point the salt 

 deliquesces, or becomes damp, due to the 

 formation of a film of the saturated solu- 

 tion on the surface of the crystals. The 

 saturated solution of magnesium chloride 

 (MgCL.BHoO) has a lower vapor pressure 

 than the pressure of the water vapor or- 

 dinarily present in the air about us. Con- 

 sequently, when the crystals of this salt are 

 exposed they soon become moist and if left 

 long enough they will dissolve completely 

 in the water which has been condensed 

 from the air. A hydrated salt, which is 

 not deliquescent, will prevent the vapor 

 pressure from diminishing below a certain 

 amount, but will not prevent it exceeding 

 that same amount. The vapor pressure of 

 water in capillary openings difl'ers very 

 greatly from that at a free surface, and so 

 porous substances, like paper and textiles, 

 will hold water even when the vapor pres- 

 sure of the air is below that of water at a 

 free surface. 



The moisture absorbed from a damp at- 

 mosphere by tobacco may form a solution 

 with the salts present, it may form hy- 



