126 THE AMERICAN MONTHLY ' [July, 



The motion has been confounded by some writers with the circulation of 

 protoplasm in tlie cells of oi'ganic matter. One authority speaks of having 

 seen it in the threads of sarcode projecting from the apertvires in foraminiferal 

 shells. Another considers it to be identical with the movement of the granules 

 in saliva corpuscles. These are merely examples of what foolish statements 

 find their way into standard text and reference books. Some claim that the 

 motion ceases after a short time, while one man reports a mount made six 

 years ago in which the particles are as lively as ever. I have not examined 

 it a sufficient length of time to speak authoritatively on this point. 



It is claimed by some that those substances that can be very finely subdi- 

 vided ai^e the ones that show the movement most readily. This is evidently 

 only a fancy as some of the most palpable powders are the least active. Some 

 one states that the nearer the specific gravity of the liquid agrees with that of 

 the powder, the more active will be the motion. While I am not prepared 

 to disprove this statement, I have been totally unable to verify it by experi- 

 mentmg with pure water, pure (95"o) glycerine, mixtures of glycerine and 

 water, chloroform, alcohol, and ether as vehicles. Water gives the best re- 

 sults of any liquid tried. 



This motion should be observed by every microscopist who has sufficient!}^ 

 high magnifying powers to see it. Now that we find everything swarming 

 with bacteria there is a liability of pedesis being mistaken for some new mi- 

 cro-organism. 



The polariscope is a valuable accessory when observing the movement in 

 crystals, but it is of no avail with other powders. The parabolic illuminator 

 does not seem to be serviceable in this work. The best results come from 

 oblique reflected light passing through a small diaphragm. 



Brevoort has studied the subject as much as any recent investigator. He 

 finds that the fat globules in freshly drawn human milk, secreted at the time 

 of childbirth, are very active, but diminish in 'vitality' as the child grows 

 older. 



Permanent mounts to illustrate the phenomenon of pedesis are not difficult 

 to make, provided, however, that the motion does notecase after a few days, 

 as claimed by some authorities. I have no reason for doubting the statement 

 of one writer who says he has a mount six years old that shows the move- 

 ment nicely and as well as it ever did. I place a well-cleaned slide on the 

 turn-table and run a ring of cement on it about 0.5 mm. (-^L inch) high. 

 This warm weather, or in a warm room during winter, the cement will be- 

 come sufficiently dry in a half hour to permit of finishing the mount. I ac- 

 complish this by placing a large drop of the liquid, prepared as directed 

 above, in the cell, and placing in position a well cleaned cover glass. When 

 the cover is pressed down, the superfluous liquid will be pressed out and 

 the fresh cement will hold the cover firmly to the cell. The pressure reduces 

 the depth of the cell to about 0.25 mm. (yJny inch) . The slide should be washed 

 to remove any particles of the powder that may have run out with the liquid 

 and been deposited on the cover glass. When dried it is ready for use, and 

 such a mount, at least as far as the mechanical part is concerned, will last a 

 lifetime. Either white zinc cement or Brunswick black can be used. 



REFERENCES. 



p. Harting : — The Microscope, vol. ii, page 49. 

 L. S. Beale : — How to Work with the Microscope, p. 172. 

 W. B. Carpenter : — The Microscope and its Revelations, p. 199. 

 H. Frey : — The Microscope and Practical Technology, pp. lor and 425. 

 H. L. Brevoort : — Jouriial of the New York Microscopical Society, vol. ii, p. toi ; 

 vol. iii, p. I. 

 J. H. Wythe : — The Microscopist, pp. 53 and 120. 

 S. H. Gage : — Notes on Microscopical Methods, p. 13. 



