3O2 W. C. ALLEE AND J. F. SCHUETT. 



100 cc. of water and then 4 gm. of pure caustic soda. Dissolve 3 

 gm. of silver nitrate in 20 cc. of water and add to the dextrine-soda 

 solution. The precipitate of silver oxide that is formed is gradu- 

 ally reduced by the dextrine, the color changing to reddish brown. 

 After 20-30 minutes, add 100 cc. of 90 per cent, alcohol and stir. 

 Allow the mixture to settle for 15-20 minutes and then pour 

 off the liquid from the particles of silver at the bottom. Add 

 about 200 cc. of water and the silver particles will generally dis- 

 perse; however, if this is not the case shake or stir until an even 

 suspension of silver results." 



With dilutions from such stock suspensions, it was easy to 

 demonstrate, as Drzewina and Bohn state, that within limits, the 

 greater the mass of animals, the better the protection. Thus with 

 Planaria dorotocephala, 12 sets of two each exposed at room tem- 

 perature to 10 cc. of water containing 10 drops of a rather weak 

 suspension of colloidal silver, showed the beginning of head dis- 

 integration in from 4.5 to 10 hours ; while 10 similar lots each con- 

 taining from 10 to 75 worms, all lasted over 36 hours in the same 

 volume and concentration. A species of Cladocera and the isopod, 

 Asellus communis, showed similar, but less marked group protec- 

 tion. 



Similar tests in which the animals were exposed to the action of 

 dilute suspensions of colloidal silver for considerable time and 

 were then washed and transferred to water similar to that to which 

 they were accustomed, yielded such results as are listed in Table 

 I. With the worms, the results are summarized in terms of sur- 

 vival after exposure, with added information concerning the num- 

 ber of worms that were able to crawl either with or without stimu- 

 lation with a camel's hair brush. Other data at hand show that 

 the protective effect of the group was greater than is indicated by 

 this summary, but the summarized account is sufficiently conclu- 

 sive. With the brittle starfish, Ophioderma, tests were made 

 concerning the righting ability of the animals after exposure 

 to colloidal silver. In these tests, after exposure, the washed 

 animals were transferred to separate finger bowls each containing 

 250 cc. of well-aerated sea water. It will be noted that the mean 

 righting time of the isolated animals is given in minutes while 

 that of the animals tested in groups of five is given in seconds. 



