Minor Constituents of Air 235 



Drop the pentoxide quickly into a beaker containing 

 a little water. The combination of the pentoxide with 

 the water is attended with so much heat that a little 

 water is converted into steam, and a sharp hissing noise 

 is heard. 



The next experiment affords another striking 

 example of hygroscopy. 



EXPERIMENT 98. Make a weak solution of cobalt 

 chloride. Write your name on a clean sheet of paper, 

 using a clean nib and the cobalt chloride solution as 

 ink. Allow the writing to dry, and it will be found 

 that the characters will at best be but very faintly 

 visible. Now warm the paper carefully over a small 

 Bunsen flame. The writing will gradualty appear in 

 bright blue characters. Dry cobalt chloride has a 

 bright blue colour ; when this substance is moistened, 

 it has only a very faint pinkish tinge. Allow the blue 

 writing to remain in the open air, and examine it from 

 time to time. Account for any change which occurs. 



Cobalt chloride solution was one of the earliest of 

 the so-called " invisible inks " to be discovered. 



161. The foregoing experiments have shown clearly 

 that carbon dioxide and water-vapour exist in the 

 atmosphere, and also that they are continually being 

 produced by the process of combustion and animal 

 respiration. Water-vapour is further being continually 

 given off from the surfaces of all exposed sheets of 

 water under ordinary circumstances, and it is from this 

 source that the major portion of the aqueous vapour 

 present in the air is obtained. 



EXPERIMENT 99. Nearly fill a small beaker with 

 water. Weigh the beaker and its contents carefully 

 and mark the level of the water by gumming a strip 

 of paper vertically on the outside of the beaker, and 



