Aids to Microscopic Inquiry. 81 



great ship drives a current of water behind it by means of a 

 screw. As its own mass is large and heavy, it must move a 

 proportionate quantity of water one way for the reaction to 

 carry it in an opposite way. Paddles and screws hence make 

 a noticeable turmoil, and a line of waves and foam marks out to 

 the eye the wake of an advancing ship. When, however, we 

 come to extremely minute bodies, measuring infinitesimal 

 fractions of an inch, and weighing infinitesimal fractions of a 

 grain, an amount of action upon the water, the corresponding 

 reaction of which will cause the little body to move with 

 extreme slowness through a space utterly invisible to the 

 naked eye, will not give us even a miniature resemblance to the 

 turmoil of paddle-wheels, and we must not expect to see, even 

 with the microscope, that the little ship-like diatom will leave 

 a water trail as it goes along. 



Suppose an object in water moves one-thousandth of an 

 inch in a second, and we magnify both it and the surrounding 

 space one thousand times. When a second has passed, it will 

 appear to have traversed a whole inch, and consequently its 

 rate of progress will appear tolerably fast, if the thing itself 

 even under that high power still looks small in proportion to 

 the space through which it has gone. We should be wrong, 

 however, if we expected to find the water disturbed ; a very 

 minute thing taking a whole second to get through about one- 

 tenth of the thickness of an ordinary sheet of writing paper, 

 would neither require a noticeable current to move it, nor cause 

 a noticeable current when it moved. Endosmotic, or exos- 

 motic s currents might move such bodies and be themselves 

 unseen. 



When fluids are in capillary tubes they are attracted (or re- 

 pelled) on all sides by the tube surfaces, and no force can move 

 them that does not prevail over the surface action, whatever 

 it may be. A curious illustration of this occurs when fluids 

 so situated are exposed to freezing temperatures. The water 

 particles want to arrange themselves so as to form solid ice ; 

 but the capillary tubes hold them so tight that they cannot do 

 so. " M. Despretz was able to lower the temperature of water 

 contained in fine capillary tubes to — 20° without their freez- 

 ing.-"* The application of this fact to many microscopic 

 organisms, which do not perish when the water containing 

 them is frozen, is too obvious to need pointing out. 



* Ganot's Physics, by Atkinson, p. 228. 



