234 Domestic Science 



has been deposited from the air. The sodium thio- 

 sulphate in dissolving absorbs heat. Since no heat is 

 specially supplied to the apparatus, the heat absorbed 

 is taken from that originally present in the beaker 

 and its contents, and thus the temperature of ,the 

 solution and of the vessel is considerably lowered. 

 Hence the air in contact with the exterior of the beaker 

 is also cooled. Now this cooled air cannot retain as 

 much water in the form of vapour as it held before 

 cooling, and as a result the excess of water-vapour is 

 condensed, appearing as small drops on the outside of 

 the beaker. 



(Note. The absorption of heat by the solution of 

 the sodium thiosulphate, i.e. by its passage from the 

 solid to the liquid state, should be compared with the 

 heat-absorption which attends the conversion of ice 

 into water, as considered in Chapter x.) 



EXPERIMENT 96. Expose on a watch-glass a few 

 small lumps of granulated calcium chloride. Examine 

 the chloride at the end of an hour, and compare the 

 appearance of its exterior with that of some which has 

 not been exposed. Put the watch-glass in an exposed 

 position, and examine its contents once more at the 

 end of 24 hours, and again after it has been left for a week. 



Substances like calcium chloride, which freely absorb 

 moisture, are termed " hygroscopic ". The most hygro- 

 scopic substance known is phosphorus pentoxide. 

 (Refer to your observations in Experiment 82 on the 

 behaviour of this substance when left in the open air.) 

 It is hence the best absorbent of water which we possess. 

 To show its great attraction for water, try 



EXPERIMENT 97. Remove a small quantity of 

 phosphorus pentoxide from a bottle of the substance, 

 closing the bottle immediately after its removal. 



