548 ILLUSTRATIVE EXPERIMENTS 



(e) Effect Oil Cyclops quadricornis. Place one or two of these pond crus- 

 taceans in two Petri dishes containing the niinimuni amount of tap water. 

 Cover one dish with a 26-oz. glass. Note movements. Now place both 

 dishes below the lamp so that they are equally radiated. Note the time. At 

 first mobility in the uncovered dish is greatly increased. Soon, liowever, 

 movement is gradually slowed down and stops in less than 1-2 minutes. 

 Note time. The cyclops in the covered glass should now receive attention. 

 They live eight or nine times as long as the uncovered ones. Glass, therefore, 

 cuts off some of the abiotic radiations. If vita-glass or other similar glass is 

 available it is instructive to cover a third Petri dish with it. 



(/) Effect on living skin. Cut three holes of various patterns in line on a 

 piece of thick brown paper. Clean a part of the back of the arm with spirit 

 soap and dry wnth methylated spirit. Fix the brown paper over this part of 

 the arm with rubber bands. Cover hole No. 1 with a thin layer of vaseline ; 

 leave No. 2 uncovered, and cover No. 3 with either one layer of muslin or a 

 piece of vita-glass. Expose to the rays at a distance of 1 metre for two 

 minutes. Remove the paper and examine the arm immediately and one hour 

 afterwards. Compare notes next day with the rest of the class. Does 

 the colour of hair and eyes have any influence on the amount of erythema 

 produced ? 



57. Indicators of Potential Difference. 



Two of the more common indicators of the existence of a potential 

 difference between two points have been considered in detail, viz. the 

 capillary electrometer (on p. 50, Experiment 21) and the string galvanometer 

 (Chap. XXVI.). If the potential difference is sufficiently great or if it is 

 amplified (one valve resistance-capacity amplifier) it may be demonstrated 

 and measured by an ordinary mirror galvanometer. A wireless head- 

 phone or, if two or three valves (resistance-capacity coupled) are used 

 in series, a loudspeaker makes a very efficient detector of differences of 

 E.M.F. 



58. The current of injury, etc., of muscle (pp. 152 and 179) is usually 

 measured by compensation. 



A cell of constant known E.M.F. is put in the same circuit as the tissue 

 giving rise to the current, but sending its current in the opposite direction 

 (Fig. 107 and Experiment 23). By moving the jockey along the slide wire 

 (E-^-R.) till the E.M.F. from the cell exactly balances the demarcation current, 

 i.e. till the meniscus at the mercury-acid interface becomes steady, one may 

 determine what relationship the constant E.M.F. bears to the muscle E.M F. 

 Non-polarisable electrodes must be used. 



59. The Membrane Potential of the Skin of an Apple. 



Select an undamaged apple. With a cork cutter remove a small circle of 

 skin from one side. Place the apple whole side downwards in a Petri dish 

 containing some OT N . KCl solution. A non-polarisable electrode is placed 

 in this solution in contact with the apple. The other similar electrode is 

 brought into contact with the upper cut surface of the apple, taking care not 

 to touch any of the skin. Lead these electrodes to a capillary electrometer 

 or to a sensitive mirror galvanometer. The damaged portion is, of course, 

 the negative pole. 



60. Model of Mucoid Secretion. (Fischer.) 



Grind up in a mortar a small quantity of gum acacia and 1 or 2 c.c. of 

 cottonseed oil. Put a drop of this mixture on a microscope slide with cover 

 glass and examine. Place a drop of water at the edge of the cover slip and 

 note what happens when it touches the oil layer. 



