198 Report or tue CuemicaL 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 that 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 per 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 
