192 PRACTICAL LESSONS IN SCIENCE. 



ute or so after closing; put the three tubes in a warm place and 

 note the results. Bacteria are developed promptly in the un- 

 boiled solution, and also appear in the boiled solution of the 

 unclosed tube, but not as promptly as in the first, while none ap- 

 pear in the other if the experiment was well performed. Experi- 

 ments show that every form of bacteria may be filtered out of 

 the air by a mass of cotton wool and in other ways; that they 

 may be filtered out of water by such material as porous pottery- 

 ware, and that if a body of air remains at rest for a few days 

 these germs of bacterial life will settle out of it so that it is as 

 pure as if filtered. 



It is by excluding the germs of bacteria that vegetable and 

 animal substances are preserved from decay by the process of 

 canning. The discovery of bacteria and the recognition of their 

 importance in the economy of nature has been of great value to 

 man. It has promoted a more rational treatment of disease, 

 and has greatly extended the domain of surgery. 



Another interesting one-celled plant, sometimes called a green 

 slime, is found as a green coating on the north side of old fences 

 or trees, especially after a few days of wet weather. Pieces of 

 wood or bark bearing this plant may be kept dry for a long time, 

 becoming fresh when moistened. Notice the color, and with the 

 pocket lens notice that it appears as if dusted over the surface. 

 Scrape off some of the slime and put it in a little alcohol, and after 

 an hour or so notice the color of the alcohol. The green color of 

 the plant and the color it imparts to the alcohol is due to the 

 delicate coloring matter called chlorophyll, which, in general, is 

 formed only in the sunshine. Potato stems growing in the sun- 

 light are bright green, but are nearly white when growing in the 

 dark. This slime is called Protococcus. It multiplies rapidly, 

 not by the process of budding, as did the torula, but by division, 

 one cell dividing into two and these dividing again, etc. 



A dark blue-green scum is very common on stagnant water 

 about watering troughs, along street gutters and other places. 

 It can be kept for observation in water in an open dish. If a 

 small portion of this scum be allowed to remain for a few hours 



