296 Brown. — S Hi dies in the Physiology of Parasitism. IX. 
may be seen from several of the tables quoted above. It is shown equally 
well by blotting-paper and by the ordinary filter-paper. The method 
adopted to get the effect was simply to place discs of blotting- or filter- 
paper in the usual way in the lids of Petri dishes, to wet the papers with 
distilled or tap water, and to allow them to remain wet for a week. Control 
experiifients showed that the effect began to appear in three or four days. 
On comparing a series of dishes in which the paper had remained wet for . 
seven days with another series in which the paper was left dry for these 
seven days and was only wetted at the time the germination test was begun, 
it was found that the latter gave germination equal to that given in the 
absence of the paper, whereas the former showed reduced germination. 
A certain latent period is thus required for production of the effect. The 
effect is also shown as markedly w r hen the wet paper is placed in the 
bottom instead of in the lid of the Petri dish, and is thus not due to 
interference by the paper with the conditions of aeration in the dish. 
The method of producing the effect being thus known, experiments 
were carried out to determine in what ways it could be removed. The 
earlier work gave a certain amount of indication that the effect was 
produced more strongly in light than in darkness, and thus a photochemical 
action of light on the paper was suggested, resulting in the formation of 
some volatile substance, such as perhaps hydrogen peroxide or formaldehyde. 
Later experiments, however, have shown that the effect is obtained in much 
the same degree with wet blotting-paper in the dark, and all the results 
obtained indicate that the action is due to some organism or organisms 
growing in the moist paper. Thus after any treatment which would 
destroy organisms present in the paper the retarding effect does not 
appear. The following treatments have been shown to be effective in 
preventing the appearance of a retarding action : Sterilizing the paper in 
a moist condition (dry air sterilization of the paper is hardly allowable 1 
as this process much reduces its hygroscopic capacity) ; placing a few drops 
of chloroform in the Petri dishes (all traces of chloroform were found to 
have disappeared at the end of the incubation period) ; wetting the paper 
with dilute caustic soda, dilute copper sulphate, or potassium permanganate 
solution. In paper which is showing the retarding effect, this can be 
removed by washing with alcohol, followed by thorough rinsing with 
distilled water, or simply by the addition to the paper of a few drops 
of dilute permanganate solution. The effect thus appears to be due to 
biotic causes. 
No attempt has been made to isolate the organism or organisms 
concerned, and thereby to produce the effects with pure cultures on the 
paper; nor "has the volatile substance been determined further than the 
experimental proof that it is not hydrogen peroxide. This was carried out 
as follows: Various dilutions of a 20-volume solution of hydrogen peroxide 
