2 BULLETIN 227, U. S. DEPARTMENT OF AGRICULTURE. 
economic considerations as far as the preservatives used are con- 
cerned. These preservatives have in a general way proved to be 
fairly good, but recently further efforts have been made to supple- 
ment them, or perhaps to substitute for them under certain condi- 
tions other substances which may be more applicable to certain 
requirements. 
In addition to the toxicity of a preservative toward wood-destroy- 
ing organisms, its value to the trade will depend largely upon its 
varied physical and chemical properties, as well as upon certain 
economic considerations involved. The present publication, how- 
ever, deals only with the toxic properties. Considerable literature 
on the toxic effect upon plant and animal life of various chemical 
substances, both inorganic and organic, has accumulated, but rela- 
tively little work has been done with wood-destroying fungi, and any 
attempt to draw analogies would be misleading. 
Even among fungi the toxic concentration of a given preservative 
will vary, depending upon the organism, the concentration of the 
preservative, and the growth conditions, such as the composition of 
media and the temperature. In order to call attention to the funda- 
mental character of certain of these variations and to illustrate the 
points involved, a brief survey of some of the work of other investi- 
gators is here presented before the results of our own work are dis- 
cussed. No attempt has been made to review all the literature on 
this extensive topic, and only a few references which serve to illus- 
trate the points the writers desire to emphasize are included. 
HISTORICAL. 
STIMULATION BY TOXIC SUBSTANCES. 
Many investigators have established the fact that certain sub- 
stances that are poisonous in higher concentration exert only a stimu- 
lating effect in extreme dilution. For many years the fluorin com- 
pounds have been known to have this stimulating effect upon fer- 
mentation. Ono (22) 1 and Raulin (24) have found similar favorable 
influences exerted by lithium nitrate and sodium fluorid upon alga? 
and by mercuric chlorid and copper sulphate upon fungi. Fred (9), 
in his studies on nitrogen fixation, denitrification, ammonification, 
and putrefactive processes in soils, due to certain bacteria and yeasts, 
concludes that the }ueld is in proportion to the toxic stimuli. Like- 
wise, Clark (3, p. 400), in his work on the toxic effect of many acids 
and salts upon fungi, found that many deleterious agents which at 
certain concentrations retard germination or early growth after- 
wards cause a great acceleration of mycelial development. 
1 Figures in parentheses refer to the bibliography at the end of this bulletin. 
