PRACTICAL EXERCISES 61 



If the tubes are of the same dimensions, and the liquids with which 

 they are filled are approximately at the same initial temperature, 

 it is not necessary to immerse them in water at constant temperature. 

 It is sufficient to place them side by side in the air. Perform the 

 following experiments in this way : 



(a) Label the tubes A and B. Fill them both to the mark with 

 o'9 per cent! NaCl solution. Connect as in the figure, and move the 

 slider along the wire till the sound is a minimum. Probably the two 

 tubes are not exactly of the same dimensions, and therefore the slider 

 will not be exactly in the middle of the wire. Suppose it is at 

 49' o, the total length of the wire being 100. Then resistance of 



A : resistance of B : : 49^0 : 51*0, i.e., resistance of A= resistance 

 of B. 5 1 



(b) Fill A with defibrinated blood, keeping B filled with NaCl solu- 

 tion, and repeat the measurement. The slider must now be moved 

 much farther away from the zero of the scale. Suppose the mini- 

 mum sound is obtained with the slider at 70' o. Then resistance of 



blood = -x resistance of the NaCl solution. 

 3 49 



(c) Compare in the same way the resistance of serum with that 

 of the NaCl solution. It will be found much less than that of the 

 blood. 



(d) Centrifugalize some of the blood for as long as is convenient, 

 and compare the resistance of the blood from the top of the tubes 

 and from the bottom of the tubes with that of the NaCl solution. 

 The resistance of the blood from the bottom of the tubes will be 

 found much greater than that of the blood from the top. 



10. Opacity of Blood. Smear a little fresh blood on a glass slide, 

 and hold the slide above some printed matter. It will not be possible 

 to read it, because the light is reflected from the corpuscles in all 

 directions, and little of it passes through. 



11. Laking of Blood by Chemical and Physical Agents. (i) Put a 

 little fresh blood into three test-tubes, A, B and C. Dilute A with an 

 equal volume, B with two volumes, and C with three volumes, of 

 distilled water, and repeat experiment 9. The print can now be read 

 probably through a layer of A, but certainly through B and C, since 

 the haemoglobin is dissolved out of the corpuscles by the water and 

 goes into solution, the blood becoming transparent or laked. That 

 the difference is not due merely to dilution can be shown by putting 

 an equal quantity of blood in two test-tubes, and gradually diluting 

 one with distilled water and the other with a 0*9 per cent, solution 

 of sodium chloride, which does not dissolve out the haemoglobin. 

 Print can be read through the first with a smaller degree of dilution 

 than through the second. Examine the laked blood with the 

 microscope for the ' ghosts,' or shadows of the red corpuscles. The 

 addition of a drop or two of methylene blue will render them some- 

 what more distinct. 



(2) Heat a little dog's or ox blood in a test-tube immersed in a 

 water-bath. Put a thermometer in the test-tube, taking care that 

 there is enough blood to cover the bulb. Keep the temperature 

 about 60 C. In a few minutes the blood becomes dark and laking 

 occurs. 



(3) (a) Put a little blood into each of four test-tubes. To one add 

 a little ether, to another a little chloroform, to the third dilute 

 acetic acid in o'9 per cent. NaCl, and to the fourth a dilute solution 



