154 EECENT RESEARCHES 



cultivation, it is sometimes found difficult to prevent various 

 species growing together, so as to mask each other, and to render 

 it impossible to determine what effects are due to each species. 

 Lister, in order to overcome this difficulty, proposed to in- 

 troduce a drop of an infusion, which might probably contain 

 twenty species, into a large quantity of water, the dilution being 

 calculated so that a single drop of the diluted Bacterial mixture 

 would probably contain a single Bacterium ; which drop, when 

 removed to a tube of sterilised fluid nutriment, would only 

 produce a progeny of that particular Bacterium. But Koch 

 has devised a still more ingenious plan. He spreads a layer 

 of gelatine, so saturated with water as to become solid on 

 cooling, on a glass slide. It is steriHsed by boiling, and into 

 it can be introduced some nutrient matter required by the 

 Bacteria. In order to obtain pure cultivations a sterilised needle 

 is dipped into a mixture containing various species, and with it a 

 streak is made down the gelatine fluid. Bacteria are dropped at 

 intervals ; when, owing to the medium being solid, no mixture 

 occurs ; but each Bacterium produces round it a spherical nest 

 of its own kind, from which individuals can be removed (with a 

 sterilised needle) to start fresh, pure cultivations ; by which means 

 alone their specific characters and distinctive properties can be 

 studied. 



We now come to some of the special series of observations 

 which have been carried out upon the Bacteria found to be 

 associated with various specific diseases. The first epidemic 

 which was thoroughly investigated was a parasitic disease of 

 silk-worms — called Pebrine — which, in 1865, was brought under 

 the notice of Pasteur. This disease, which for fifteen years 

 had devastated the silk-producing districts of France, was 

 proved by him to be caused by a multitude of minute 

 corpuscles — the Micrococcus Bombycis — which bred in the worm, 

 and so caused its death. 



In 1850, Messrs. Rayer and Davaine found that the blood of 

 animals affected by Splenic Fever, Anthrax, or Charbon, contained 

 minute transparent rods ; and since then the history of these 

 organisms has been beautifully worked out by Koch. He 

 cultivated these Bacilli in the aqueous humour of the eye of an 



