23 



let the other stand for half-an-hour or more in the air, 

 so that dust may fall upon it, and then put this also 

 under a bell glass jar. In two or three days the latter 

 will be covered more or less with growths of bacteria and 

 other fungi, whilst the former will be almost or quite free 

 from them. 



To grow pure cultivations on potatoes, they should first 

 be washed, and then soaked for half-an-hour in a solution of 

 bichloride of mercury (i part to i,ooo of water), as no 

 micro-organism can grow in this, and afterwards steamed 

 until they are cooked ; each potato should then be carefully 

 cut in two with a knife which has been heated to redness, 

 and each piece, with the cut surface uppermost, put on a 

 plate under a bell glass. On the plate should be laid three 

 or four thicknesses of blotting paper, soaked in the aforesaid 

 solution of bichloride of mercury, to supply moisture which 

 is necessary for the growth of the organisms, the whole 

 being covered with a bell .glass. The potatoes are then 

 inoculated by being scratched with a platinum needle, set 

 in a glass handle, which has been previously heated to 

 redness, in a Bunsen's burner or spirit lamp, and dipped 

 in the organism to be grown. 



On the table are several specimens of such cultivations, 

 the blood-red Micrococcus Prodigiosus, the scarlet Micro- 

 coccus Indicus, the yellow Micrococcus, the black Torula, the 

 violet Bacillus, &c. 



They may also be grown in milk, in various meat 

 broths, and in solutions of certain organic materials. 



The method now most generally used is one introduced 

 by Koch, the German bacteriologist, by which they are 

 grown in solid media in test tubes, because in these they form 

 what are called colonies ; and it is an extraordinary fact that 

 nearly every bacterium has some peculiarity in its growth, by 

 which it may be distinguished from every other. The 

 microscope will shew whether an organism is a yeast or a 

 micrococcus, a bacterium or a bacillus; but one micrococcus, 

 for instance, so examined, is just like another, though when 



