74 PHYSICAL DETERMINATIONS. 



In use, it is essential that the stem remain dry. Beam's 

 directions for use are : The stem of the lactometer being dry 

 the float is passed over it and allowed to rest on the bulb. The 

 lactometer is then lifted by the point of the stem, and gradually 

 let into the milk. If there is any doubt as to the instrument 

 having found its proper level, the base of the jar may be held 

 firmly to the table by one hand and the jar gently tapped with 

 the other. 



When removing the lactometer the float should be taken out 

 first, in order to keep the tube dry and ready for another test. 



The following directions are due to Vieth : 



Use of Lactometer. In order to determine the specific 

 gravity, the milk is poured into a vessel at least J inch greater 

 in diameter than the widest part of the lactometer, and deep 

 enough to allow the instrument to float. A cylindrical glass. 

 jar (Fig. 7), with foot, is the most suitable vessel for the pur- 

 pose if Soxhlefc's lactometer or the thermo-lactometer be used ; 

 Vieth's lactometer may be used in a can or tin cup. The lacto- 

 meter is gradually lowered into the milk to the 25th degree, 

 care being taken that the instrument is entirely wetted by the 

 milk and that no air adheres to it. When released, the lacto- 

 meter will move up and down, and after a little while become 

 stationary. That degree of the scale which coincides with the 

 surface of the milk is then noted. It will be observed that, 

 where the milk touches the vessel and the stem of the lactometer, 

 the surface is not level, but, in consequence of the adhesion of the 

 milk to the glass, forms a curve (Fig. 8). There is no difficulty, 

 however, in ascertaining the extension of the curve sufficiently 

 near, and this has to be allowed for in reading off the specific 

 gravity. When using instruments of ordinary size, the curve 

 will be found to extend to about one-half degree. 



Lactometers indicate the exact specific gravity at a tempera- 

 ture of 60 F. It is, therefore, necessary, as soon as the position 

 of the lactometer has been noted, to remove the instrument 

 from the milk, immerse a thermometer, and ascertain the 

 temperature. 



If the temperature is found to be 60 F., the observed specific 

 gravity is correct, but should the temperature of the milk be 

 higher or lower than 60 F., the specific gravity must be corrected 

 by the aid of the Table XV. (page 80), which is used as follows : 

 Find the temperature of the milk in the vertical column, and 

 the observed specific gravity in the first or last horizontal line ; 

 under the latter, and in the same line with the temperature, 

 is given the correction to be added if the milk is above 60 

 and subtracted if below. For example Supposing the tem- 

 perature to be 51 and the specific gravity 34, the correction 



