llfiS Journal of Applied Microscopy 



College of Physicians and Surgeons, to whom I am much indebted for help in 

 this bacteriological work. Microscopical slides are easily prepared thus : Hold 

 upside down the test tube in which bacteria are growing, and carefully remove 

 the cotton from the mouth. Touch one of the colonies of bacteria with the point 

 of a needle, and then rub the needle point on a clean glass slide ; add a drop of 

 water to the spot touched by the needle, cover with a cover-glass. Stains (Loef- 

 fler's methylen blue and Ziehl's carbol fuchsin) bring out more clearly the struc- 

 ture of the bacteria. Each of the thirty-five pupils in a division examined the 

 stained bacteria, and watched under another microscope the motion of the living 

 forms. One pupil's written account of this study is here given : 



"Microscopic Study of Bacteria. 1. The bacteria which I saw under 

 the microscope last Wednesday were of red and blue colors. This was caused 

 by the coloring matter (stains). 



" 2. They were of three different shapes, round, pencil-shaped, and corkscrew. 



" 3. The bacteria which I saw to-day under the microscope are moving around. 



" 4. There were also under the microscope egg-shaped animals which were 

 moving around." (This slide was prepared from the hay infusion and contained 

 infusoria.) 



A little mathematical problem worked out by each student helped to make 

 real the rapidity of multiplication among these micro-organisms. The pupils 

 were told that a rod-shaped bacterium, when conditions are favorable, divides 

 in about an hour to form two bacteria. The problem was stated something like 

 this: Suppose we start with a single bacterium this morning at 10 o'clock; if 

 conditions are favorable, how many cells would be seen at 11 o'clock? The 

 answer was "two." Between 11 and 12 o'clock each of the two would divide to 

 form two ; hence at 12 o'clock it was evident that there would be four bacteria 

 in place of the single cell at 10 o'clock. The pupils, continuing the calculation, 

 found that if the process were to go on until 10 o'clock the next morning, the origi- 

 nal bacterium would give rise to 16,776,216. The completion of this calcula- 

 tion for a second day's crop of bacteria was not attempted for obvious reasons. 



Thus far the experiments and discussions had made real to the pupils the 

 existence of countless millions of micro-organisms. They had learned something 

 of the form, size, and motions of the individual bacteria ; and they had become 

 acquainted with some of the results of their activity in causing decay, in souring 

 milk, and in producing colors. 



Some of the conditions which tend to check the growth of bacteria were 

 learned from the milk experiment performed at home. A laboratory demonstra- 

 tion developed this subject still further. One of the boys described the experi- 

 ment thus : 



" Sterilization. Mr. Peabody took three test tubes and inoculated some 

 of the bacteria from the hay infusion. The first test tube contained nourishment 

 in a solid form (nutrient gelatin), and after the bacteria had been inoculated it 

 was set aside. The second test tube was prepared the same way, but Mr. Pea- 

 body poured some corrosive sublimate over the surface of the gelatin. The 

 third test tube was prepared in the same way as the first, but was put in the 

 (steam) sterilizer for five minutes, and then set aside. 



