106 BACTERIOLOGY. 



the contaminated tube can be rejected, and the experiments con- 

 tinued with the remaining pure cultivations. 



When, however, one tube containing a liquid medium is in- 

 oculated from another, as in the process of preparing plate-cultures, 

 or when a cultiire is made from a tube in which the growth has 

 liquefied the gelatine, it is obvious that the tubes cannot be inverted 

 or held horizontally, and they must then be held and inoculated as 

 in Fig. 38. To inoculate those tubes of nutrient media which have 

 been solidified obliquely, the point of a straight sterilised needle 

 charged with the material to be cultivated is traced over the surface 

 of the jelly from below upwards, or the inoculated material may be 

 spread out with a hooked or looped needle. 



Examination of Test-tube-cultivations. — The appearances pro- 

 duced by the growths in test-tubes can be in most cases sufficiently 

 examined with the naked eye. In some cases the jelly is partially 

 or completely liquefied, while in others it remains solid. The 

 growths may be abundant or scanty, coloured or colourless. The 

 nutrient jelly may itself be tinged or stained with products resulting 

 from the growth of the organisms. When liquefaction slowly takes 

 place in the nesdle track, or the organism grows without producing 

 this change, the appearances which result are often very delicate, 

 and in some cases very characteristic. The appearance of a simple 

 white thread, of a central thread with branching lateral filaments, 

 of a cloudiness, or of a string of beads in the track of the needle, 

 may be given as examples. 



In some cases much may be learnt by examining the growth with 

 a magnifying glass. Here, however, a difficulty may be encountered, 

 for the cylindrical form of the tube so distorts the appearance of its 

 contents, that the examination is rendered somewhat difficult. To 

 obviate this, a very simple contrivance may be employed with 

 advantage. This consists of a rectangular vessel, about four inches 

 in height and two inches in width, which may be easily constructed 

 by cementing together two slips of glass to form the back and front, 

 with three slips of stout glass with ground edges forming the sides 

 and base (Cheshire). The front may be constructed of thin glass, 

 and the base of the vessel made to slope so that the test-tube when 

 placed in the vessel has a tendency to be near the front. The 

 vessel is filled with a mixture of the same refractive index as the 

 nutrient gelatine. The latter has a refractive index rather higher 

 than water, which is about 1-333 ; alcohol has a refractive index of 

 1-374. The vessel is filled with water, and alcohol is then added 

 until the proper density is reached. The test-tube is placed in the 



