COAL-TAR AND WATER-GAS TAR. CREOSOTES. 67 



controlled, and of inoculating these blocks with timber-destroying 

 fungi. The test at the laboratory has not been successful in des- 

 troying the treated wood. In the light of present knowledge only 

 negative results could be expected from such tests. 



Chapman {28) brush-treated his test specimens and set them in 

 sheep manure. Experiments with brush-treated fence posts and 

 telephone poles show that such a test will not be a test of the pre- 

 servative, but that the wood will decay because checks and cracks 

 develop and permit the entrance of fungi into the untreated section 

 long before the treated portion has lost its resistance to decay. 

 The original conception of the fungus-pit test was that the life of a 

 preservative could be determined in a shorter time than by service 

 tests, but the tests so far carried out have failed to obtain this result. 

 The reason has been either that specimens which have decayed were 

 not treated in such a manner as to give the preservative life of the 

 creosote; or that the conditions in the fungus-pit were such that the 

 creosote did not lose its vitality as it does under exposure. 



PETRI-DISH TESTS. 



There are various ways of making Petri-dish tests, which vary 

 with different operators. Objections or criticisms can probably be 

 brought against any one of the systems now in use. The general 

 principles of the tests, however, are the same. A nutrient medium 

 is prepared, in which the fungi can thrive under certain known 

 conditions. To this medium, which usually consists of agar agar 

 mixed with some nutrient such as beef broth, are added known 

 amounts of the preservative, and the whole is poured into shallow 

 covered glass dishes known as Petri dishes. After the agar agar has 

 solidified to a jelly, it is inoculated with the fungus or other organ- 

 isms to be used in the test. The organisms that have been used in 

 testing wood preservatives include molds, yeast, timber-destroying 

 fungi, and bacteria. The objection to the use of molds, yeasts, and 

 bacteria for testing wood preservatives is that the killing points for 

 these organisms might be entirely different from that for timber- 

 destroying fungi, and it is already known that timber-destroying 

 fungi differ among themselves in their resistance to certain wood 

 preservatives. The chief advantage is that the test can be made 

 in a few days instead of the weeks necessary when timber-destroying 

 fungi are used. Although this difference in behavior in the various 

 kinds of low organisms tested must be recognized, the information 

 obtained from these comparative tests performed with such cultures 

 is not entirely without value; for the experiments give very quickly 

 some general idea of the relative toxicity, and approximately indi- 

 cate the order in which the toxicity may reasonably be expected to 

 fall when the preservatives are tested against timber-destroying fungi. 



I 



