MOISTURE IN THE ATMOSPHERE 



15 



into the water reservoir. The reservoir may be made of any wide- 

 mouthed bottle. 



The difference between the readings of the wet and the dry 

 thermometer should not be more than 10 degrees or less than 7 

 degrees. The temperature of the room 

 should be between 65 and 68 F. If we 

 do not feel comfortable, more vapor is 

 needed in the atmosphere. 



Hygrometer Paper. Some hygrometers owe 

 their hygroscopic properties to chemicals which 

 change their color with the percentage of humidity 

 of the air. The following chemicals may be used 

 to color flowers, light muslin, or paper. 



1. Cobalt chlorine, 1 part; gelatine, 10 parts; 

 water, 100 parts. The normal coloring is pink; 

 this color changes into violet in medium humid 

 weather, and into blue in very dry weather. 



2. Cupric chloride, 1 part; gelatine, 10 parts; 

 water, 100 parts. The color is yellow in dry 

 weather. 



3. Cobalt chloride, 1 part; gelatine, 20 parts; 

 nickel oxide, 75 parts; cupric chloride, 25 parts; 

 water, 200 parts. The color is green in dry 

 weather. 



Hygroscopes. Wood, quill, hair, whalebone, 

 animal membranes, whip-cord, cat gut, wild oats, 

 common feathers, and grass are known as hy- 

 groscopes, since they are very sensitive to changes in moisture. 



Attempts have been made to produce a hygrometer to take the place of 

 the wet and dry bulb instruments; but they have failed because of the great 

 inaccuracy caused by temperature and dust. A good hygrometer has been 

 constructed to read within five per cent error. Such hygrometers are often 

 used in cigar cases. The "old man and old woman weather prophets" in a little 

 house is an example of a hair hygrometer. 



The following table will assist the class in determining the rel- 

 ative humidity of the room from the hygrometer. 



FIG. 9. Wet and Dry Bulb. 



