494 



CHAPTER XXV 



process gives the weight of an equal volume of the material being examined, 

 when the specific purity follows by a simple division, the degree Brix being 

 obtained from reference to tables. 



In very exact work all determinations should be made at one fixed 

 temperature, now selected as 20° C. As this is inconvenient in rapid 

 technical work in the tropics, the writer worked as follows : — The mean 

 temperature of a laboratory was 27-5° C. The weight 

 of water in the pycnometer was determined for each 

 tenth degree between 25° C and 30° C. The weight of the 

 juice or other material was determined at whatever tempera- 

 ture obtained at 'the time of the determination, and was 

 compared with the weight of water at that temperature. 

 The corresponding degree Brix was then taken from a 

 table calculated for 27-5° 0/27-5° C. The error intro- 

 duced by accepting an equal expansion for water and 

 sugar solutions between these limits does not appear till 

 the third decimal place in the degree Brix. 



2. A weight is suspended by a thread of silk from 

 the end of an arm of the balance. Its weight is observed 

 in air, in water and in the material under examination. 

 If X, y, z be the weights respectively in air, water and 



Fig. 327 



Fig. 328 



material, the specific gravity of the last is given by the ratio 



x — z 

 x—y 



This 



method may be employed on any balance as in Fig. 326, or the specially 

 designed Mohr-Westphal balance, Fig. 327, whereby the specific gravity is 

 read directly from the rider weights used, may be employed. 



3. The hydrometer. Fig. 328, consists of a glass tube on which is blown 

 an elongated bulb. Beneath this bulb is a second loaded with lead shot or 

 quicksilver. The upper portion consists of a slender stem, in which is located 

 the scale. When immersed in a liquid, the instrument will sink, or float with 



