452 Intelligence and Miscellaneous Articles. 



is a sensitive and accurate criterion of saturation if care be taken 

 that 



First, the air contains approximately a normal quantity of 

 solid particles of dust ; it is sufficient for this that it is not filtered, 

 and is renewed from time to time. 



Secondly, the air must be free from any bodies which act chemi- 

 cally on aqueous vapour ; solids and vapours of acids are most to 

 be feared in this way. 



If these conditions are fulfilled, adiabatic cooling furnishes a 

 convenient way of determining the degree of saturation of air ; for 

 instance, of calculating the vapour-pressure of solutions. 



This calculation gives at lower temperatures, that is below 

 30° C, values for the depression of tension, which are freer from 

 errors of observation than direct measurements by means of mer- 

 curial columns. 



In order, however, to anticipate a similar degree of accuracy at 

 higher temperatures, the ratio of the specific heats of aqueous 

 vapour should be determined with greater accuracy than it has 

 hitherto been. — Wiedemann's Annalen, No. 4, 1886. 



ON THE WEIGHT AND THE CAUSE OF THE LAYER OF WATER ON 

 GLASS AND OTHER BODIES. BY E. WARBURG AND T. IHMORI y 

 FROM JAPAN. 



The results of this investigation are thus given : — 



1. Above the dew-point no weighable deposit of water could 

 be observed on bodies with smooth surfaces insoluble in water 

 (platinum, glass with a coating of silicic acid, glass free from 

 alkali) ; from the sensitiveness of the balance used, the thickness of 

 any such layer could not exceed one or two millionths of a 

 millimetre. 



2. The film of water which forms above the dew-point on alkaline 

 glass arises from a small quantity of free or loosely combined 

 alkali on the surface of the glass. This must each time absorb 

 water until the pressure of the vapour above the solution of alkali 

 found is equal to the pressure in the place of experiment. 



3. This deposit on alkaline glass, which can be weighed, is the 

 cause of the electrical surface-conductivity which such glass shows 

 in moist air when tried by the electroscope ; any glass, which, with 

 the balance in question, showed no weighable deposit of water, 

 was found to be a good insulator when tested by the electroscope. 



4. Eock-salt showed a deposit of moisture some millionths of a 

 millimetre in thickness at temperatures for which the pressure of 

 vapour was greater over a saturated solution of chloride of sodium 

 than the pressure in the place of experiment. There was, however, 

 in the rock-salt used a small quantity of magnesium chloride, which 

 must attract moisture, until the pressure of vapour over the solu- 

 tion of magnesium chloride formed was equal to the pressure of 

 vapour in the place of experiment. — Wiedemann's Annalen, No. 4, 

 1886. 



