STRUCTURE AND PROPERTIES OF PROTOPLASM 67 



What effect is produced upon protoplasmic streaming by heat 

 in its different degrees? 



For this it is needful to use some method by which the temperature 

 of the living Protoplasm can be raised and lowered at will, while the 

 rate of movement is measured at each degree. 



Experiment. Select the best of the available materials showing 

 protoplasmic streaming, preferably Nitella or Chara, and mount as 

 for the preceding studies. Then place the slide in position on a tem- 

 perature stage, and slowly raise the temperature, determining the 

 rate of streaming at regular intervals until it ceases. Then take 

 another specimen and lower the temperature, measuring as before 

 until streaming ceases. 



Again, for a more accurate determination wholly from the same 

 individual specimen, prepare good material as before, and first cool 

 the stage to zero. Then slowly raise the temperature through the 

 entire course of the movement, measuring as before. Plot the result 

 in a polygon designed to exhibit graphically the relation of rate of 

 movement to temperature. 



Precautions. Those given for the study on page 60 should be carefully 

 observed. The raising and lowering of the temperature must be slow, not 

 faster than a degree in two minutes, else the change acts as a shock to affect 

 the movement. Care must be taken to keep the specimen from drying out, 

 which can be managed by giving it abundance of water at the start; if need- 

 ful to add it later, the water must be of the same temperature as the specimen, 

 and preferably taken from a small vial kept for the purpose on the stage. 

 The flat mirror, without a condenser, should be used, to minimize the focus- 

 ing of heat with the light. It is difficult to measure the rate for every degree, 

 and practically it suffices to measure it at alternate degrees, preferably the 

 even numbers to facilitate comparison of results. 



Temperature Stages. Of these there are many forms, the chief of 

 which are mentioned below. The requisites of a good stage are these: 

 that its temperature can be readily carried under perfect control from telow 

 zero to at least 6o°, and that thermometer and object are always at the same 

 temperature. According to whether the latter is effected perfectly in prin- 

 ciple, or only approximately, they divide into grades from precision down- 

 wards. 



Of precision forms the first seems to have been Nageli's (1867, in Nageli 

 and Schwendener, " Das Mikroskop," described by Velten in work cited 

 below in Literature), in which the object and thermometer are completely 

 surrounded by flowing water of known temperature; but it is elaborate 

 and does not admit use of high powers. Then followed the Stricker electric- 

 heated stage (1871, described by Velten), which is also elaborate. Then 

 Sachs developed his microscope chamber (1873, figured in his later Text- 

 books, and by Detmer, 421), which was a double-walled box almost com- 

 pletely enclosing the microscope, with a window for light and small openings 



