THE STRUCTURE AND PROPERTIES OF PROTOPLASM. 55 



Carefully select an active, clean, tip-cell of Nitella, and mount in 

 water so that the cover-glass will not press upon it. Place on the 

 temperature-stage and warm the latter slowly and evenly by use of 

 a spirit-lamp. Determine the rate of movement at regular and fre- 

 quent intervals and note with care the exact point at which the 

 movement is most active (the optimum) and that at which it ceases 

 (the maximum). Later, with the same cell or one as nearly like it 

 as possible, reduce the temperature by filling the water-box gradually 

 with a mixture of ice and salt, observing and recording as before ; 

 note the point at which movement ceases (the minimum). Do not 

 be content with a single observation, but try again and average. 



(Express results in mm. per minute for each two degrees, even num- 

 bers, of temperature, to allow coniparisoii of results. The heating 

 and cooling must be done slowly, else the sudden change acts as a shock, 

 affecting the result. Use only the plane mirror, as the. concave focuses 

 some /i eat.} 



The results of these observations are to be expressed not 

 only in tables of figures, but also in curves (polygons) the points 

 of which are established by the intersection of ordinates ex- 

 pressing rate of movement with abscissae expressing degrees 

 of temperature. (On this, see page 18.) On the same sheet 

 with your individual record should be plotted the class average 

 polygon. Comparison of this with your own will give some 

 idea of the probable error in your results. 



An efficient stage, represented in Fig. 8, may be made at small cost 

 (about $1.50) by any tinsmith. It is of sheet copper, one-sixteenth of an 

 inch thick, of the breadth of a microscope-stage, rolled over as shown by 

 the figure to make a chamber for a thermometer and another for a three- 

 inch slide, with holes between objective and mirror. Forward it dips 

 down to enter a shallow (one inch deep) tin box which hangs from it by 

 one cross-wire and two stubs, as shown in the figure, this arrangement 

 allowing its removal. Both stage and box taper forward to one inch and 

 one and one-half inches respectively in breadth, a feature not necessary 

 but useful as diminishing the leverage of the box when full. A battery- 

 clamp, properly filed, holds the apparatus to the stage of the microscope, 

 while a mat of felt between prevents conduction of heat to or from the 

 instrument. A bent thermometer, as shown in the figure, may be used, 

 but the straight ones needed in the laboratory for other purposes are 

 about as good. To raise the temperature, the box is nearly filled with 

 water and heated by a spirit-lamp ; to lower it, the box is gradually filled 

 with ice and salt.* 



* Very careful tests of the accuracy of this stage have been made in comparison 



