Aids to Microscopic Inquiry. 79 



If a bladder, filled with strong syrup, or other fluid denser 

 than water, and capable of mixing with it, be immersed in 

 water, and furnished with a tube that rises out of the water, 

 it will be found that the water will enter the bag through its 

 pores, and, by overfilling the bag, cause its contents to ascend 

 in the tube. 



For this sort of action to take place, the membrane or other 

 porous body must be permeable to one liquid, if not to both ; 

 the liquids must be capable of mixing and differ in density. 

 Gases can exhibit these actions ; and when a fluid is capable 

 of absorbing a gas, a membrane or other porous body may be 

 the means of bringing a gas and a fluid into sufficiently close 

 contact for chemical union to ensue. 



It is evident that these considerations enable us to appreciate 

 many particulars in the process of animal respiration, and in 

 those changes in the contents of vegetable cells that result 

 from the action and reaction that takes place between cell 

 contents and the gases or liquids with which the cell walls come 

 into contact and which they absorb. 



The small organisms which the microscope is needed to 

 reveal, are very frequently — we might say by far most fre- 

 quently — of a soft and porous structure. Often they present 

 no appearance of a distinct integument, and where an integu- 

 ment is present it is commonly readily permeable to air or 

 water. Now, an organism with a permeable integument, or 

 one composed of a permeable substance, must be subject to 

 currents of endosmose or exosmose whenever its fluid contents 

 differ in density from the fluid in which it is immersed, and the 

 two fluids are capable of mixing together. Thus it will appear 

 that animalcules or minute plants, such as desmids, confervse, 

 etc., will be continually subject to physical changes of this 

 nature, and these physical changes will very frequently 

 promote and bring about chemical changes, by which old 

 materials will be re-arranged and new materials formed. 

 We shall, in a future paper of this series, glance at such of 

 the more obvious aids to the microscope which chemical con- 

 siderations afford. At present we merely desire to call attention 

 to the fact that physical agencies, under appropriate circum- 

 stances, provide the conditions under which chemical changes 

 must take place. 



We have treated surface action and capillary action as the 

 same thing under different conditions. So soon as a set of 

 particles belonging to a fluid, whether gaseous or liquid, come 

 into contact with a surface, their liberty of action is curtailed. 

 The particles of the solid surface attract or repel them, and 

 the adjacent particles of their own nature which he behind 

 them are only capable of a partial resistance to the new force 



