25 



however, is that the shape of the colonies is not so well 

 marked in it. 



Sterilised blood serum is another medium in which 

 some micro-organisms grow best, especially the Tubercle 

 Bacillus. This is obtained from the blood of the Ox or 

 Sheep, and is sterilised and rendered solid, without losing its 

 transparency, by an elaborate process, which want of time 

 prevents me from explaining now. 



As the atmosphere abounds in bacteria, and moulds and 

 their spores, all the apparatus employed must be sterilised 

 by heat before being used. All flasks, test tubes, beakers, and 

 the cotton wool for plugging, should be exposed for three 

 hours to a heat of I30°C. in an iron box similar to that on 

 the table. Or the glass apparatus may be heated for a 

 minute or two in the flame of a Fletcher's or Bunsen's burner, 

 but this method, though expeditious, is attended by woeful 

 destruction of the glass. The hands of the operator, after 

 being well washed in soap and water, and well cleansed under 

 the nails, should be rinsed in the aforesaid solution of 

 bichloride of mercury (i to i,ooo). 



To inoculate the jelly in the test tubes a platinum 

 needle, with a glass handle, previously sterilised by being 

 heated to redness, is dipped in the organism to be pro- 

 pagated. The plug of the tube is then carefully withdrawn, 

 whilst the tube is turned upside down to prevent dust falling 

 in, and the needle is thrust for a distance of an inch, into 

 the meat jell}^ and finally the plug is replaced. 



As before stated, the majority of micro-organisms 

 form colonies of different shapes when thus grown. On the 

 table are some thirty cultivations, and no two are alike ; so 

 that they may be distinguished from one another better by 

 them, than by their appearance under the microscope. For 

 instance, here are four specimens of micrococci, which look 

 exactly alike under the lens, but the first, a micrococcus found 

 in milk, forms merely a thin white line along the needle 

 track ; the second, found in some forms of inflammation of 

 the lungs, the m. pneumonioe, grows along the track and 



