198 Kepokt of thb Chemical Department of the 



the whole piece, the excess of water dripping into the lower 

 trough. It is necessary occasionally to boil these pieces of felt 

 cloth in water slightly acidulated with some acid, like acetic or 

 hydrochloric, in order to remove mineral salts tha.t accumulate 

 and interfere with capillary action. The use of distilled or rain 

 water would obviate this difficulty. Thus far we have kept the 

 moisture as nearly as possible at 75 i^er ct. of saturation, though 

 variations of 5 per ct. below and 10 per ct. above may occur at 

 times. The natural tendency is toward a higher percentage of 

 relative moisture at lower temperatures. 



METHOD OF DETERMINING MOISTURE IN AIR. 



The relative amount of moisture in air can be determined by 

 means of an instrument known as a hygrometer, of which there 

 are several forms. One form indicates the percentage of mois- 

 ture directly by means of a needle or hand; this is the most 

 convenient kind of hygrometer, and is probably sufficiently 

 accurate for ordinary purposes. A more accurate instrument 

 consists of two sensitive, standard thermometers. The bulb of 

 one is exposed to the air directly, like any thermometer, and is 

 known as the dry-bulb or dry thermometer, while the other has 

 its bulb wrapped in a piece of muslin to hold water, and is 

 known as the wet-bulb or wet thermometer. The wet ther- 

 mometer should be fixed in a frame that enables one to whirl 

 it easily. The form of hygrometer used by us is made by Julian 

 P. Friez, Baltimore, Md. The dry thermometer indicates the 

 temperature of the air in the room. The wet thermometer, 

 properly used, indicates a lower temperature than does the dry 

 thermometer, because the water in the muslin bound about the 

 bulb evaporates and the evaporation is accompanied by a lower- 

 ing of the temperature immediately around the bulb. The less 

 moisture there is in the air, the more rapidly does evaporation 

 take place and the lower is the temperature indicated by the 

 wet thermometer. The greater the moisture in the air, the less 

 rapid is the evaporation and the smaller the difference between 



