[Illustration: POTATO IN A ROUX TUBE PREPARED FOR CULTIVATION Glycerine is placed in the bulb of the tube] There are two common forms of test-tube culture, viz.: on the surface and in the depth of the medium. In the former the medium is sloped, and the inoculating needle is drawn along its surface; in the latter the needle is thrust vertically downwards into the depth of the solid medium. Plate cultures and anaërobic cultures will be described at a later stage. When the medium has been inoculated the culture is placed at a temperature which will be favourable. Two standards of temperature are in use in bacteriological laboratories. The one is called _room temperature_, and varies from 18° C.-20° C.; the other is _blood-heat_, and varies from 35° C.-38° C. It is true, some species will grow below 18° C., and others above 38° C. The pathogenic (disease-producing) bacteria thrive best at 37° C., and the non-pathogenic at the ordinary temperature of the room. The different degrees of temperature are regulated by means of incubators. For the low temperatures gelatine is chosen; as a medium for the higher temperatures agar. [Illustration: STAPHYLOCOCCUS PYOGENES AUREUS × 1000] [Illustration: INCUBATOR (Temperature of blood-heat, registered by thermometer, and regulated by thermo-regulator)] _Moisture_ has been shown to have a favourable effect upon the growth of microbes. Drying will of itself kill many species (_e. g._, the spirillum of cholera), and, other things being equal, the moister a medium is, the better will be the growth upon it. Thus it is that the growth in broth is always more luxuriant than that on solid media. Yet the growth of _Bacillus subtilis_ and other species is an exception to this rule, for they prefer a dry medium. [Illustration: CULTURE MEDIA READY FOR INOCULATION] _Temperature._ Most bacteria grow well at room temperature, but they will grow more luxuriantly and speedily at blood-heat. The optimum temperature is generally that of the natural habitat of the organism. In exceptional cases growth will occur as low as 5° C. or as high as 70° C. Indeed, some have been cooled to-20° C. and-30° C., and yet retained their vitality,[8] whereas some few can grow at 60-70° C. These latter are termed _thermophilic_ bacteria. The average thermal death-point is at or about 50° C. [Illustration: INOCULATING NEEDLES Plantinum wire fused into glass handles]