438 H. A. Hazen — Condensing Hygrometer 



alty complained that his fluid congealed at low temperatures; in 

 this case it was not the ether at all, but the moisture taken up 

 by it, that produced the effect. In using rhigolene it is only 

 necessary to empty the box after evaporating a half pint or so; 

 no objectionable deposit is left in the box. The thermometer 

 is held in position by a cork at the top ; the cork should be so 

 bored that the thermometer may be brought very near the 

 plate. This plate on which the dew is deposited is brass, plated 

 with gold, and has many advantages over the earlier cylinder 

 of silver. Dew is much more easily detected, the plate is easier 

 to clean, and fumes of the evaporating liquid do not affect the 

 appearance of dew. The writer found the latter an important 

 matter; it was impossible to keep the sdver in a satisfactory 

 condition for a large number of observations. To guard against 

 fumes from the evaporated liquid, if in a room, the outlet 

 pipe should be carried to the outside air, and in the open 

 air, the same tube should be placed to the leeward of the 

 instrument. 



Before entering upon the use of this instrument it is essential 

 to carefully investigate the errors which are likely to occur. 

 The effect of accidental errors may be practically eliminated by 

 multiplying the observations, but there is a class of constant 

 errors which cannot be thus treated. These may be divided 

 into two classes : 1st, those that may best be investigated by 

 experiments upon a single instrument; and 2d, those which can 

 be completely determined only by the use of two or more 

 instruments. Under the first class the following questions need 

 an answer. 



Question 1. What is the effect of using different depths of 

 liquid? 



Answer. It is only necessary that the instrument be com- 

 pletely immersed. Experiments in which the instrument was 

 first pushed to the bottom of the box and then pulled to the 

 top of the liquid showed no appreciable difference. 



Q. 2. What should be the position of the inlet tube as re- 

 gards the thermometer and the plate when dew is observed? 



A. The inlet tube should be carefully arranged at the back 

 of the box, leaving as much space as possible between it and 

 the thermometer. 



Q. 3. What should be the position of the thermometer? 



A. It should be as near the plate as possible. 



Q. 4. Does the rapidity of flow of air into the liquid need 

 attention? 



To answer this question, which has also been regarded as an 

 important one by others, a test tube was arranged with two 

 thermometers placed on opposite sides and as far apart as possi- 

 ble, the inlet tube being passed down very near one of these. 



