Dec. 15, 1914 Expansion of Milk and Cream 253 



METHOD OF PROCEDURE 



The sample of milk to be investigated is placed in the containing tube, 

 the sinker immersed, and the tube placed in position in the temperature- 

 control bath. The temperature is then brought to the point at which 

 the density is first to be measured and is allowed to remain constant 

 until the apparatus reaches a condition of temperature equilibrium. 

 Observations are then begun. First, a weighing is made with the sinker 

 immersed in the sample and suspended from the arm of the balance. 

 Then the temperature is read on each of the two thennometers; next, the 

 sinker is detached and a weighing made with the sinker off, but with the 

 suspension wire still passing through the surface of the liquid. The 

 difference between these weighings is the apparent weight of the sinker 

 in the liquid at the temperature of obser\-ation. The object of making 

 the second weighing with the suspension wire still passing through the 

 surface is to eliminate the surface-tension effect on the suspension wire. 

 In order that the suspension wire may be kept straight and in position 

 whether the sinker is attached or not, a small secondary sinker is kept 

 suspended at all times, and the large sinker of known mass and volume 

 is attached to that. When not attached, the large sinker rests on the 

 bottom of the tube and remains standing in an upright position. Imme- 

 diately after the weighing with the sinker detached, a second weighing is 

 made with it again attached, after which the temperature is again 

 observed on the two thermometers. 



The observations at each point therefore consist of two weighings with 

 the sinker attached, one weighing with it detached, and two readings 

 on each of two thermometers. The reason for making two weighings 

 with the sinker attached and only one with it detached is because in the 

 former case a slight change of the temperature of the liquid will make 

 an appreciable change in the weighing, on account of the large volume 

 of the immersed sinker, while in the latter case the change is not appre- 

 ciable. 



After completing the observations at one point, the temperature is 

 changed to the next one in the series and the process is repeated in the 

 same order. 



TEMPERATURE RANGE OF DENSITY DETERMINATIONS 



At the beginning of the work it was intended to cover the tempera- 

 ture range from 0° to 50° C, but the rate of expansion at low tempera- 

 tures was found to differ so much from the rate at higher temperatures 

 that the expansion over the entire range could not be expressed by a 

 simple equation. This is especially true of samples having a low fat 

 content. In certain samples a point of maximum density was found 

 at a temperature in the region of 5° C. This is what might be expected — 

 from the similar behavior of water. In the samples containing higher 

 percentages of fat the point of maximum density was not found, but 

 the rate of expansion was noticeably less at the low than at the high 

 temperatures. 



The rate of expansion was especially desired at the higher tempera- 

 tures, and since it was found that the results at temperatures between 

 20° and 50° C. could be well expressed by a simple equation, it was de- 

 cided to cover only this temperature range, and when approximate 

 results are desired beyond this range, to extrapolate from the results 



