i8o PATHOGENIC BACTERIA. the fifth-, 30; on the sixth, 150; on the seventh, 1000, etc.; indicating that the serum exerted a destructive action upon some but not all of the bacteria, and that this power disappeared after the lapse of a certain time, allowing the bacteria to develop ad libitum. When the germicide to be studied is a gas, as in the case of sulphurous acid or formaldehyd, a different method must, of course, be adopted. It may be sufficient simply to place a few test-tube cul- tures of various bacteria, some with plugs in, some with plugs out, in a closed room in which the gas is afterward evolved. The germicidal action is shown by the failure of the cultures to grow upon transplantation to fresh cul- ture-media. This crude method may be supplemented by an examination of the dust of the room. Pledgets of sterile cotton are rubbed upon the floor, washboard, or any dust-collecting surface present, and subsequently dropped into culture-media. Failure of growth under such circumstances is very certain evidence of good dis- infection. These tests are, however, very severe, for in the cultures there are immense numbers of bacteria in the deeper portions of the bacterial mass upon which the gas has no opportunity to act, and in the dust there are many sporogenous organisms of extreme resisting power. Failure to kill all the germs exposed in such manner is no indication that the vapor cannot destroy all the ordi- nary pathogenic organisms. More refined is the method of saturating sterile sand or fragments of blotting-paper or absorbent cotton with cultures and exposing them, moist or dry, to the actior of the gas. Such materials are best made ready in Petr: dishes, which are opened immediately before and closec immediately after the experiment. A piece of cotton 01 blotting-paper or a little sand transferred to fresh culture media will not give any growth where the disinfection ha: been thorough. By transplanting from different depths the sand may be used incidently to show to what deptl the gas is capable of penetrating.