Penetration may be tested by the use of a standard filter; a suitable filter for this purpose is one which does not remove the smoke to such an extent that measurement of its concentration becomes difficult, and one which does not become clogged quickly by the smoke. A filter consisting of two pads of felt, placed side by side and arranged so that the smoke first comes in contact with the thinner and less dense pad has been found very satisfactory. In testing penetration, smoke is produced by dispersing one gram of the toxic substance in a sheet iron box of 1000 liters capacity. After 5 minutes a steady concentration is usually attained and the smoke is then forced through a Tyndall meter, (see page 299) after dilution with air, where the initial concentration is determined. It then passes through the standard filter, and through a second Tyndall meter, where the final concentration is measured. The difference of the two readings gives the amount of smoke retained by the filter. The penetration is ordinarily represented by a series of figures, which decrease from a maximum value at the beginning of the test to a minimum at a point where the filter permits the passage of so little smoke that it cannot be measured. This decrease is due to decrease in penetrating power and concentration of the smoke, and to increase in filtering power of the filter as a result of plugging. Usually five degrees of penetration are recognized, excellent, good, fair, poor and very poor. [Illustration: FIG. 100.—Penetration Apparatus Used to Test Toxic Smokes.] A portable penetration apparatus is shown in Fig. 100. In using the apparatus, the smoke producing material is so placed with reference to the apparatus that the sample is taken about 20 feet down the wind, so that the smoke is appreciably diluted. One man is stationed at each Tyndall meter and takes readings as fast as his recorder can write them, so that the smoke density, before and after the filter, can be followed very closely. PHYSIOLOGICAL ACTION In addition to a high penetrating power a smoke should also possess great toxic, irritant, sternutatory, or lachrymatory power. These properties are tested by exposing mice to the smoke in the chamber. They are placed in the chamber at the beginning of the run, and exposed for 10 minutes to the smoke from 1 gram of the material. While these tests are only qualitative in character, they give a fairly good notion of the relative value of different materials. QUANTITATIVE RELATIONSHIPS