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cate the examination of the organisms, whenever it is possible, in their 

 natural state, so that their appearances and characteristics may be observed 

 when they have not been subjected to the action of heat or chemical 

 reagents. It will be found, in many instances, that species, which are 

 undistinguishable one from the other microscopically, can be easily recog- 

 nised by their appearance (colour, consistence, &c.) and mode of growth in 

 cnltivating media, and, for this reason, microscopical examination should 

 always be combined with artificial cultivation. The cultivation medium which 

 is generally employed is clear sterilised meat jelly, which is made by adding 

 to a meat infusion neutralised with sodium carbonate, and sodium phosphate, 

 5 per cent, of gelatine, or 1-2 per cent, of agar, (Japanese isinglass). 

 The advantage of employing the latter is that the jelly remains solid when 

 heated to 40°, whereas jelly made with ordinary gelatine liquifies at 20-25°. 

 A very good cultivation soil is afforded by the outer surface of a cooked 

 potato. If a potato is cleansed by washing it with a solution of corrosive 

 sublimate (1-2,000), boiled, and cut in two with a heated knife, and exposed 

 on a plate beneath a bell jar, the air in which is kept moist by blotting- 

 paper steeped in water, within 1-2 days minute colonies of various coloured 

 organisms, together with moulds penicillium, aspergillus, &c, will appear on 

 the surface of the potato, and increase in size day by day. Each of these 

 coloured colonies consists of a pure cultivation of a chromogenous bacte?'ium 

 or torula, of which many varieties — white, yellow, orange, buff, red, &c. — 

 exist. Many of these are microscopically undistinguishable from each 

 other as regards their shape and size, but they are easily recognised 

 microscopically by their colour and mode of growth. 



The investigation of bacteria is required under various conditions, accord- 

 ing as they occur: — 1. In fluids, e.g., milk, water, blood, &c. ; or on solid 

 media, e.g., bread, meat, potatoes, meat jelly, &c. 2. In the organs and 

 tissues of the animal body. In the former case a minute portion of the 

 fluid, or of a colony of the bacteria, is placed on the centre of each of two 

 cover glasses, which are superimposed one over the other, and rubbed 

 together between the fingers, so as to distribute the organisms evenly over 

 their surfaces, and then separated and left to dry. They are then passed 

 several times through the flame of a spirit lamp, so as to fix the bacteria to 

 the surface of the glass. Cover glasses so prepared can be kept for an 

 indefinite time for future investigation, and if an interesting organism is 

 met with it is a good plan to preserve some in this manner. It is very easy 

 to obtain a thin, evenly-diffused specimen of bacteria on the cover glass 

 when they are present in fluids, but more difficult when they occur in the 

 form of solid colonies. To obviate this difficulty a minute portion of 

 mucilage or glycerine may be placed on the cover glasses, which will help 

 the diffusion of the bacteria when the glasses are rubbed together between 

 the fingers. It will often be found that the bacteria form very fantastic 

 patterns on the cover glass, which are artificially produced, and must not be 

 considered as typical modes of growth. 



To stain bacteria mounted on cover glasses they should be floated, with 

 the bacterial surface downwards, or a saturated watery solution of methyl 



