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. 
