14 BULLETIN 227, U. S. DEPARTMENT OF AGRICULTURE. 



geous, 1 particularly with heavy oils. They also attempted to improve 

 upon the usual method of inoculating the surface of the culture with 

 the mycelium of the test organism by cutting a small block out of the 

 medium, placing the transferred mycelium in the aperture, and then 

 covering this with the portion of medium which was originally re- 

 moved. They claimed this would give a more accurate indication 

 of whether the fungus was really growing on the treated medium 

 or only on the fragment of medium which must necessarily accom- 

 pany the mycelium when it was transferred. This appears, how- 

 ever, from work in our laboratory, to be a refinement of doubtful 

 expediency, for it has often resulted that when fresh, actively grow- 

 ing mycelium is placed in intimate contact with the poison it will be 

 directly killed, while if it has the opportunity to recover its vigor to 

 a certain extent after the disturbance in its growth equilibrium, due 

 to cutting and removal from the original culture, it may eventually 

 withstand concentrations which would otherwise be fatal. 



The more important results of these different investigators are 

 presented in Table IV (pp. 31-34). 



TESTS CONDUCTED AT THE FOREST-PRODUCTS LABORATORY. 



SCOPE OF THE WORK. 



The experimental work in wood preservation at the Forest-Prod- 

 ucts Laboratory includes a physical, chemical, and pathological 

 examination of various substances which may have a possible value 

 in the industry (32). Therefore, since toxicity is but one factor, 

 conclusions regarding the service value of these substances should 

 not be drawn without giving due consideration to other factors. 2 The 

 pathological tests are made in Petri dishes, using agar media, or by 

 injecting the preservatives into wood and exposing the wood to the 

 action of wood-destroying organisms. Only the Petri-dish method 

 is herein described. This method has the advantage of giving 

 results from which at least tentative conclusions can be drawn in a 

 relatively short time. Conversely, it is open to certain objections 

 for which due allowance must be made in generalizations regarding 

 the possible behavior of a preservative when placed under service 

 conditions. However, in experimental work on the toxicity of dif- 

 ferent chemical substances it is often very necessary to secure indi- 

 catory results as soon as possible. In this way many substances 

 may be eliminated which arc not worthy of further trial. After a 

 preservative has been shown to possess high toxic properties under 



1 From the purely physical side of preparing the preservatives so they can bo more readily handled 

 the gum-arabic emulsions have proved satisfactory to the writers, but the gum arabic apparently reduces 

 the toxicity to such an extent as to forbid its use in comparative tests. This fact has been determined 

 since this manuscript was prepared. 



2 See U. S. Dept. of Agriculture Bulletin 145, "Tests of wood preservatives." 



