430 SCIENCE PROGRESS 



advantage being taken of natural or artificial marks to ensure 

 this condition, and undue pressure should be avoided (some 

 workers use exactly the same pressure each time by putting 

 the bottle on a compression balance and pushing the stopper 

 down so that the pointer of the balance moves to a definite 

 mark (4), but we have not found this to be necessary). For 

 lower temperatures the cup is placed on the stopper and is 

 half filled with the liquid ; this is omitted for higher tempera- 

 tures than that of the room. The cover is replaced, and the 

 bottle is fastened to a metal holder which keeps it in a vertical 

 position in the thermostat. The holder consists of two parts 

 made to slide on a metal rod about eight inches long and held 

 in position by small thumb-screws. The lower part consists 

 of a short support to which are attached three short arms 

 bent upwards and covered with rubber tubing. The upper 

 piece consists of a short support terminating in two rubber- 

 covered arms which encircle the upper part of the bottle below 

 the neck : the holder must not be fastened so tightly as to 

 deform the bottle. The time allowed in the thermostat 

 depends on the accuracy desired ; if accuracy to the fifth 

 place is required, the temperature of the liquid must not differ 

 from that of the bath by T ^ of a degree, and one hour 

 is generally sufficient before the final adjustment is made 

 (Wade and Merriman recommend two hours) ; (2) to obtain 

 results accurate to the fourth place, twenty minutes to half 

 an hour is quite enough. The cap is now removed and care- 

 fully dried both inside and out. The excess of liquid on the 

 top of the stopper is then removed so that the meniscus is 

 just visible on looking through the stopper from the side, and 

 the top is quite dry. The bottle is now taken from the bath 

 and quickly dried round the neck, and the cap replaced. The 

 outside of the bottle is wiped with a cloth moistened with 

 alcohol, dried, and the bottle placed in the balance-case, and 

 after half an hour weighed with the usual precautions (7). 

 The liquid is then poured out and the bottle cleaned, the final 

 washing being done with distilled water. It is filled with 

 water from which the dissolved air has been removed by con- 

 necting with the filter pump ; the weight of water is then 

 obtained in exactly the same method as was used in obtaining 

 the weight of liquid. If the highest accuracy is desired, it is 

 necessary to determine the water content on each occasion, 



