EXPERIMENT STATION BULLETINS 235 



circumstances and conditions, and yields a growth peculiar to itself whichi 

 is studied closely in connection with the biological history of every species 

 of bacteria. They all assist greatly in the diagnosis of species. 



PLATINUM NEEDLE. 



This instrument is of very common use in the laboratory. It consists of 

 a glass rod in the end of which is fastened a platinum wire of medium size. 

 Platinum is used because it will withstand any number of sterilizations in 

 the flame without destruction. It is employed to inoculate tubes and to 

 convey bacteria from one medium to another. Sometimes it has a loop at 

 the end and sometimes is straight. It may be suited to whatever pur- 

 pose desired. 



HANGING DROP. 



It is always very desirable to study a plant or animal in its natural con- 

 dition. The form, the consistency of the protoplasm, spore formation and 

 motility in the case of bacteria are all best observed as they appear in 

 nature. The form may be studied carefully, with a view to decide the 

 exact classification of the species, the manner of grouping, whether in 

 threads or chains, or other groups, and any variation that may exist in the 

 form. The consistency of the protoplasm evokes much interest in many 

 species. Peculiarities prevail which require close and prolonged investi- 

 gation, and often lead to results of a particularly practical nature. The 

 spore-bearing bacteria may illustrate their methods of forming spores in 

 their natural condition much better than when stained. All of these fea- 

 tures are within the scope 

 Preparation, of ^ hanging drop. It is 

 made by taking a very thin, 



flat piece of glass, called a _ _ ^'s. is. Hanging Drop, 



cover glass or slip, and placing a drop of sterilized water in the center of 

 it, about the size of a pin-head. Into this drop the bacteria which are 

 under investigation are conveyed by the platinum needle. This cover 

 slip is then placed over a hollow in a glass slide, the dimensions of which 

 slide ar'e one inch by three, and the thickness that of a window pane. The 

 drop is turned downward when placed over the circular hollow, and ^till 

 adheres to the cover slip. This done, the drop is virtually suspended in a 

 glass cell, and to prevent evaporation a little vaseline is spread about the 

 border of the hollow, thus making the cell air tight. It is now ready to 

 study under the microscope. 



