530 EXCRETION 



tube, and to hang it up carefully on a stand by means of a fine flexible 

 copper wire passing through the eye. The rubber cork can be taken 

 out with the thermometer, and the platinum wire also, the bent free 

 end of the latter supporting it in the cork, or it may be fastened tempor- 

 arily to the thermometer stem by a small rubber band, which is slid 

 up over the cork when the thermometer is reinserted. Tube A can be 

 set temporarily in a specially heavy test-tube rack. Remove the lid 

 of C, and with it tube B. Now put ice and salt alternately into C until 

 it is nearly full, mixing them up well. Add some cold water from the 

 tap till the stirrer G can move freely up and down in the mixture. For 

 very exact work the temperature of the freezing mixture must not be 

 more than a few degrees below the freezing-point of the liquid which is 

 being examined. Put on the lid, and immerse tube B. Into A, which 

 must be perfectly clean, put enough pure distilled water to fully cover 

 the bulb of the thermometer, and introduce the latter. For ordinary 

 purposes distilled water previously boiled to expel the carbon dioxide, 

 and then cooled in a stoppered flask, is sufficiently pure. Immerse A 

 directly in the freezing mixture through the hole by which G comes out, 

 or through a separate hole (not shown in the figure) till some ice has 

 formed in the water. Take A out of the mixture, wipe it with a cloth, 

 and hold the lower part of it in the hand till nearly the whole of the 

 ice has melted. If there is a cake of ice at the bottom, see that it is 

 displaced by the platinum stirrer. A trace of ice being still left floating 

 in the water, place A in B, and allow the temperature to fall to a few 

 tenths of a degree below the freezing-point you expect to get, as deter- 

 mined by a previous rough experiment. The freezing mixture is 

 stirred up occasionally. The meniscus of the thermometer is to be 

 carefully followed, as it goes on falling, by means of a weak hand lens. 

 Now stir the water in A briskly. Suddenly it will be seen that the 

 mercury begins to rise. Keep stirring with the platinum wire, and 

 read off the maximum height of the mercury, at which it is stationary 

 for some time. The temperature can be estimated between the gradu- 

 ations to thousandths of a degree. Take out A, and observe the fine 

 ice crystals in the water. Heat A in the hand as before till nearly all 

 the ice has disappeared ; then replace A in B, and make another freezing- 

 point determination. A third one may also be made, and the mean 

 of the three readings taken. 



Take out the thermometer, and dry it and the platinum wire with 

 clean filter-paper, or dip them in some of the urine, which is then thrown 

 away. Dry A or rinse it with urine. Then make a determination of 

 the freezing-point of the urine in the same way as was done with the 

 water. The freezing-point of the urine will lie much lower on the scale. 



Instead of freezing the liquid first and then leaving a little ice in it 

 when A is placed in B, A may be put into B before any ice has formed. 

 Cooling is then allowed to go on with gentle stirring to a few tenths of 

 a degree below the anticipated freezing-point. A small crystal of 

 clean dry ice is then introduced through the side-piece on a clean 

 splinter of wood or the loop of a cooled platinum wire, the end of which 

 passes through a piece of cork, by which it is held to prevent conduction 

 of heat. The platinum stirrer can be raised to receive the crystal. The 

 liquid is now vigorously stirred ; freezing occurs, and the observation is 

 made as before. 



Instead of the above method, the liquid may first be cooled directly 

 in the freezing mixture, but not so much that ice forms. A is then put 

 in B, and cooling allowed to go on while it is being stirred. When it 

 has been undercooled to a certain extent i.e., cooled below its freezing- 

 point the vigour of the stirring is increased. Ice forms suddenly, as 

 before, and the temperature rises to the freezing-point. With urine 



