-196 CHAPTER XXV 



It further follows that the delicacy of the instrument depends on the cross 

 section of the stem, or rather on the relation between cross section of stem 

 and volume displaced by the bulb. The smaller the cross section, the 

 longer will be the division corresponding to each difference in specific 

 gravity, or degree Brix. 



In making the determination, the approximate degree Brix is found. 

 The instrument is then removed and the stem wiped dry. It is then im- 

 mersed in the liquid a very short distance below the approximate degree 

 already observed, allowed to come to rest and the reading again observed. 

 The position observed is the actual level of the liquid and not the level of the 

 meniscus which forms on the stem. A simultaneous observation of the 

 temperature is made and the appropriate correction added or subtracted. 



For certain purposes a greater degree of refinement than is obtainable as 

 described above is necessary. With this object in view, the writer has 

 devised the following arrangement, Fig. 329. 



The cylinder in which the hydrometer floats is provided with a flared-out 

 upper portion. In this is located an interior overflow controlled by a cock. 

 By filling with liquid above this overflow, and allowing the excess of liquid 

 to escape slowly, a constant level can be obtained. The scale of the instru- 

 ment is read, not at the level of the liquid, but at a known distance, con- 

 veniently i above this level. One device for obtaining this end consists 

 of a vertical piece of glass with a horizontal scratch opposite to a mirror. 

 This arrangement is carried on a horizontal holder capable of vertical ad- 

 justment, and is adjusted until the scratch is exactly i above the level of 

 the liquid.* The eye of the observer is levelled so that the scratch and its 

 image are coincident, the point where the former appears to cut the scale 

 being taken as the reading. This can be estimated to i/ioo of a degree 

 and repeated readings to this accuracy can be obtained. The holder also 

 carries three horizontal pointed screws which, when adjusted to a certain 

 point, restrain the instrument in a central position without interfering with 

 its freedom of vertical motion. A portion of the holder is hinged and swings 

 out in a horizontal plane so as to permit of removal of the instrument. The 

 appliance is provided with a charging container filling the cylinder from 

 below, with a discharge cock and with level screws, f 



Since hydrometers are seldom accurate to less than 0-05 Brix, that one 

 selected for use with this device must be accurately standardized with the 

 pycnometer. This is best done by ascertaining the degree Brix of the solu- 

 tion used with the pycnometer and adjusting the level of the scratch until it 

 indicates exactly i too little. 



The hydrometers found in use are standardized as correct at 17 5 C. or 

 2 7'5 CJ : that is to say, they indicate the degree Brix correctly at these 

 temperatures. The necessary corrections to be applied when the liquid is at 

 another temperature are given in the Appendix. When the hydrometers 

 are standardized at either 17*5 C. or 27-5 C. the writer understands that 

 the specific gravity of water at these temperatures is taken as unity. 



The general feeling of chemists is to abandon all these standards and to 

 adopt a temperature of observation of 20 C. compared with water at 4 C., 

 but the writer has seen no spindles based on this system yet in use. 



* Alternatively, specially graduated hydrometers incorrect when referred to the liquid level may be used, 

 t The complete apparatus, termed a " Brixometer," is sold by the Sugar Manufacturers' Supply Co., Ltd., 

 2 St. Dunstan's Hill, London, E.C.3. 



$ French practice still retains 15.6 C. as a basis of reference. In the British West Indies 84 F. is used. 



