42 THE AMEEICAN MONTHLY [March, 



being only about one thousandth of an inch long, and of a very simple form. 

 It is only an oval body with very few parts. And yet, small as it is, it can 

 do wonderful things which no unanimated object can imitate. 



To see what yeast is like you must first procure a cake of Gaft\ Fleisch- 

 mann & Co.'s compressed yeast, and be sure it is very fresh, or get a penny's 

 worth of the fluid yeast from a baker. The two are much alike, the latter 

 being merely diluted with water. If you have the compressed article, take, 

 of the grayish damp yeast within the wrapper, a very small amount, a mass 

 say the size of a buckshot, put it in a watch-glass and add ten or twenty times 

 its volume of water. Then clean a slide and cover-glass very carefully ; place 

 in the centre of the slide one drop of the thinned yeast, carefully drop the 

 cover-glass over it, and, if the fluid does not extend to the edge of the cover, 

 add at the edge a little more water from a suppl}' you should have on your 

 table. ■ Your yeast is now prepared for a first look. Place it on the stage ot 

 your microscope, which must be horizontal, or only slightly inclined, else 

 the yeast will be likelv to gravitate out of the field, and examine the fluid 

 with a low power (about 50 diameters) to learn its composition. You at 

 once see that it is a transparent medium, the water, in which a myriad of 

 minute specks are scattered. Closer study shows that the specks are of two 

 sizes, one sort quite large and not very numerous, also bright and shiny in 

 most lights (these are not the yeast but starch grains) , and a second kind, 

 which are far more abundant than the starch grains, and very decidedly 

 smaller, indeed so small that you may at first fail to see them ; these are the 

 yeast plants, the active portion of yeast. 



If there is any doubt about which are the starch grains and which the 

 yeast plants, one may easilv settle the question by using the iodine* solution, 

 which can be got from a dealer in optical goods. To use this, place a drop 

 of the iodine on the slide at the edge of the cover, so that the solution mixes 

 with the yeast fluid. Place a bit of clean blotting paper ^ inch square on the 

 opposite side, and allow it to soak up the fluid from under the cover-glass 

 and to draw the iodine under in its place. This process is a convenient one 

 to remember by the name of ' irrigating.' After you have irrigated the 

 iodine through, then irrigate some w^ater through after it. A good many of 

 the starch grains and of the yeasts will have been washed out, but enough are 

 left for observation. On looking again with a low power there will be seen 

 uncolored grains of small size, and very dark blue or black grains of larger 

 size. The blue ones are the starch and the uncolored ones the yeast. 



The yeast grains are not reallv uncolored with the low power, though they 

 seem to be so. Examine them with a power of 450 diameters, or thereabouts, 

 and they will be clearly seen. They seem to have very little ' to them.' 

 Close examination will show that they are made up of a substance which 

 ' stains ' brown with the iodine and is known as protopIas7u^ which is en- 

 veloped by an outer protecting coat called the ceU-~Jcall^ that the protoplasm 

 does not wholly fill the. cell but leaves a clear space of variable size and posi- 

 tion, the vacuole^ and that the protoplasm which is not clear but faintly gran- 

 ular contains numerous bright or dark little specks which are fat droplets. 

 The yeast plant thus appears to have a few different parts. We shall next 

 inquire how to see each of these more clearly. And since we think of living 

 things as doing something, at least growing, it will be interesting to see what 

 the yeast plant can do. 



(To be continued.^ 



* The iodine which the biologist used is made by saturating a small amount ot water with potassic iodide, then 

 adding a few crystals of iodine. In use the fluid should be diluted to a light cherry brown color. The strong 

 solution is not as effective as a weaker ona, which may need to be passed through several times. 



