REAGENTS AND PROCESSES 553 



get any of the medium on the upper portion of the tube where the cotton plug 

 would be likely to come in contact with it, else the plug would later be difficult 

 of removal. 



A solid nutrient medium which will remain solid at a higher temperature than 

 the gelatine medium may be prepared from agar-agar, a substance obtained from 

 certain gelatinous algae, as follows: Two Gm. of the agar are broken into small 

 pieces and soaked in cold water for 24 hours. Then the water is poured off and the 

 swollen agar is added to i liter of the peptonized meat infusion. The mixture is 

 boiled for several hours until the agar is completely dissolved. The solution is then 

 neutralized with a solution of carbonate of soda, filtered, distributed in flasks or 

 test-tubes, and sterilized by steaming for i hour at 2 or 3 successive intervals of 

 24 hours. 



Cooked potatoes afford a solid nutrient medium which is quickly prepared and 

 which is particularly adapted for the culture of chromogenic bacteria. Potatoes 

 free from wounds are selected and scrubbed in water until they are perfectly clean, 

 and the eyes and any unsound spots, if these could not be avoided, are cut out 

 with a knife. Then the potatoes are placed for an hour in a solution of I part of 

 mercuric chloride in 500 parts of water to disinfect the surface. They are next 

 steamed for about an hour in a steam sterilizer, and after 24 hours the steaming 

 is repeated for about half an hour. The sterilized potatoes are then placed in glass 

 Petri dishes, are cut in halves with a sterilized table-knife, and the cut surfaces are 

 inoculated. If the source of the inoculation is not a pure culture, an isolation of 

 forms may be approximated by making long scratches over the surface of the potato 

 with a sterilized platinum needle which has been in contact with the source of the 

 inoculation. It will add to the security of the process of sterilization if each potato, 

 before being placed in the bath of mercuric chloride, is wrapped in a piece of tissue 

 paper, and so protected until it is cut open for inoculation. 



Another method of preparing potatoes wkich is, on the whole, more convenient 

 and certain, is to cut out long cylindrical plugs from sound potatoes by means of a 

 cork-borer or any metal tube of the proper size, and then to cut the potato cylin- 

 ders very obliquely in two pieces, each of which is then to be placed in the bottom 

 of a test-tube so that the oblique surface stands uppermost. After plugging the 

 tubes with baked cotton, the potato cylinders are subjected to a temperature of 

 iooC. in the steam sterilizer for one hour at three successive intervals of 24 hours. 

 A sterilized paste made from potatoes or bread serves well for the culture of molds 

 as well as of bacteria. 



A decoction of horse-dung furnishes a good medium for the culture of mucor 

 and various other molds. The decoction is prepared by boiling the dung in water, 

 then filtering and sterilizing the solution. By placing the dung of different kinds 

 of animals in a moist chamber, as, for instance, in dishes floating on water and 

 covered with a bell-jar, characteristic fungi will after a time appear on it. 



Single spore cultures of mucor may be obtained in the following manner : Glass 

 slides are thoroughly cleaned and sterilized by baking. By means of sterilized 

 forceps a single sporangium of mucor is picked from a spontaneous growth of this 

 fungus on horse-dung or stale bread kept in a moist chamber. The sporangium is 

 placed in a sterilized decoction of horse-dung contained in a sterilized watch-glass, 

 which may be placed on an inverted tumbler in a plate of water and then covered 

 with a bell-jar which should dip into the water and form a germ-proof moist cham- 

 ber. After a few hours the sporangium will have burst open and the spores, which 

 are now distributed through the decoction, will have swollen to several times their 

 original diameter, and can all the more readily be discerned in subsequent manipu- 

 lations. A needle which has been disinfected by heating in a flame is now dipped 

 into the decoction and the point of it drawn along the surface of a glass slide which 



