4 INTRODUCTION TO GENERAL PHYSIOLOGY 



movement we are considering has been given to it on account of 

 its first description by the botanist, Robert Brown. It will be 

 seen that the smaller the particle, the more lively its movement. 

 There are difficulties in seeing the very minute particles, because 

 they are so small. Now, when such particles are brightly lit- up 

 and looked at against a dark background, they are much more 

 visible. A ray of sunlight entering a dark room through a crack 

 shows up a myriad of bright particles when we look at it from the 

 side against a dark wall, whereas the surrounding air appears 

 quite empty. This is the principle of that method known as the 

 ultra-microscope or dark-ground illumination (p., pp. 79-82). The 

 rays from a bright lamp are sent sideways through the slide on 

 the stage of a microscope, so that they do not enter the objective, 

 and, if there were nothing on the slide, one would only be aware 

 of darkness. If, however, anything solid, able to reflect light, 

 were there, it would be lit up and send rays in all directions, 

 becoming a visible object. The particles of gamboge should be 

 examined first, and afterwards the living amoeba (E., p. 166). The 

 brighter the illumination, the smaller are the particles which can 

 be seen. If it is sufficiently intense, it will be found that even in 

 the clearest protoplasm there are particles to be seen. 



The existence of these movements in the particles contained in 

 protoplasm shows that they must be free to move. In other words, 

 they are suspended in a liquid. In a solid mass, even with the 

 properties of a jelly, they would not be free to move. The 

 experiment can be made with gamboge in gelatin (E., p. 167). 



The Kinetic Theory 



The explanation of Brownian movement requires a few words 

 on the constitution of matter, as now generally accepted. 



If we imagine a crystal of common salt to be divided up into 

 smaller and smaller fragments, we should find that at a certain 

 stage, which could not be arrived at merely by mechanical crushing, 

 although possible by dissolving in water, the separate fragments 

 are such that any further division changes their chemical properties, 

 and there are now two things present of different nature. The 

 particles which are the smallest possible without alteration of 

 chemical properties are " molecules ; " the two different substances, 

 arising from further splitting, are " atoms," and, as the reader is 

 doubtless aware, are sodium and chlorine. It was at one time 

 believed that atoms were incapable of further decomposition, but 

 the study of the radio-activity of certain " elements," and that of 

 the phenomena of the electric discharge, have taught us that atoms 



