31 



A is a large water-bottle, filled with water, and so graduated that a 

 measured quantity may be drawn from it when the pinch-cock P is re- 

 leased. A thermometer T is inserted in the bottle. A rubber tube R joins 

 A with a series of U tubes. Ui contains calcium chloride freshly fused, 

 so that all the water of crystallization is driven off; U2 contains copper 

 sulphate similarly treated ; U3 contains pumice stone and concentrated 

 sulphuric acid. S is a short pliable rubber tube which may be inserted 

 into an incubator or under a hen as desired. For convenience Ui, U2 

 and U3 are mounted on one small base so that they may be weighed. 

 When a determination of the moisture in the air is desired, the U tubes 

 are detached from the bottle, weighed, and then attached again. A mea- 

 sured quantity of water is let run from A, but as it does so the same 

 volume of air must enter A, having first passed through U3, U2 and Ui 

 in succession. The U's are then detached and weighed again. The gain 

 in weight gives the amount of water contained in the air drawn through. 

 The weight of .water in unit volume of air is sometimes called the "abso- 

 lute humidity," and this method of determining the moisture content, an 

 "absolute" method. 



Investigations with apparatus such as illustrated in Fig. 1 soon showed 

 a great variation in the moisture content of the air, even at a uniform 

 temperature, also that for each temperature there was a limit — the air 

 could contain a certain amount and no more. When it contained all it 

 was capable of holding it was said to be "saturated," or to contain its 

 "saturation amount" of moisture. It was also learned that the saturation 

 amount varied with the temperature, the higher the temperature the 

 greater the amount of moisture required to saturate the air. Saturation 

 occurs during rain, mist, or fog; also near the ground when dew is falling. 

 At 32 a room 10 feet long, 10 feet wide and 10 feet high is capable of 

 holding in the air when saturated 5 ounces of water. At 70 it would 

 hold 1 lb. 2 oz. when saturated, or nearly four times as much as at 32 . 

 At ioo° it would hold 2 lbs. 11 oz. , which is more than twice as much as 

 at 70 , and more than eight times as much as at 32 . 



Relative Humidity, or "Humidity," as it is Usually Called. 



The air, however, is seldom saturated, only at times of rain, mist, 

 fog, dew, snow, or some kindred phenomenon. At all other times it has 

 less than, its "saturation amount," and if we wish to convey an idea of 

 the amount of moisture in the air at any time, we use the saturation 

 amount as the standard of comparison, e.g., at 70 the saturation amount 

 for a room 10 x 10 x 10 feet is 1 lb. 2 oz., or 18 oz. , but if by use of the 

 apparatus shown in Fig. 1 we were to find that the room at 70 contained 

 only 9 oz. , we would say that the air contained only half as much moisture 

 as it was capable of holding, or that its relative humidity was 50 per cent. 

 Thus at any particular time we may state the humidity of a room in two 

 ways : (1) by giving the actual amount of moisture per unit volume, e.g., 



