The Babcock Test. 51 



done by weighing. Since the specific gravities of iron, 

 copper, brass, and glass are 7.2, 8.7, 8.5, and about 2.7, 

 respectively, pieces of these materials replacing 2 ec. of 

 a liquid, will weigh 14., 17.4, 17.0 and 5.4 grams, for 

 iron, copper, brass and glass in the order given. 



A measure of the right weight may be suspended by 

 a very fine copper or platinum wire (melted into the 

 glass rod if this material be chosen), and is used di- 

 rectly for calibrating test bottles • as described above. 

 Before a measure so made is used as a standard, its ac- 

 curacy should be determined by weighing the amount 

 of water of a temperature of 20° C, whii'h it replaces. 

 The specific gravity of glass especially, varies somewhat 

 according to its composition, so that a standardization 

 of a measure by weight alone cannot be depended upon 

 to always give correct results. 



In submerging the measure in the test bottle to be 

 calibrated, care must be taken that all air bubbles are 

 removed before the position of the meniscus of the 

 water is noted ; if a metal standard mea.=;ure is used, it 

 must be kept free from ru.st or tarni.sh. 



57. (C.) Calibration with mercury. 27.10 grams of 

 metallic mercury are \ieighed into the perfectly clean .and dry 

 test bottle. Since the specific gravity of mercury is 13.55, 

 double this quantity will occupy a volume of exactly 2 cubic 

 centimeters (48). The neck of the test bottle is then closed 

 with a small, smooth and soft cork, or a wad of absorbent cot- 

 ton, cut off square at one end, the stopper being pressed down 

 to the first line of the graduation. The bottle is now inverted 

 so that the mercury will run into its neck. If the total space 

 included between the and 10 marks is just filled by the two 

 cubic centimeters of mercury, the graduation is correct. 



The mercury may be conveniently transferred from one test 

 bottle to another, by means of a thin rubber tube which is 

 slipped over the end of the necks of both bottles, and one weigh- 



