THK CONTEXTS OF THK SOIL 35 



F.rp. Sta. The tt-nns (jt-nn, microbe, iHicttTium (plitrul bartenu), 

 are popularly used in the Bame sense as luifro-orgaiiisni. Thesi< 

 beings are usually unicellular (each one consisting of only a 

 single cell). They are generally classitied with plants. The rAle 

 of mifro-orgJ»nisnis in renilering soil eU'inents available to plants 

 is Ttry complex and not yet well understood. A general dis- 

 cussion of tli«-s«« organisms will be found in Lipnian's " Haeteria 

 in Ti"Iation to Country Life " The relation to germs in nitrification 

 is briefly diseuss»'«l in King's "Soil," i)p. 125-134, :i!id l<<>l>frts' 

 "Fertility," '244-248. Fig. 6 illustrates one of the q c-fV* 



common bacteria, very much magnified. This ^^ ^o<i 



species (Bacillus ubiquitus) is abundant in water, ^ ff ^ 

 air, and decaying substances. •' ^'^Jfc 



38<i. Obser\-e tin- deposits of sand in the quiet ^^ 



side (usually the concave side) of streams, and Fit:- s Mirro-or 

 also the delti, where a rapid rill flows into a slow «»">*">». Krpatly 

 one. When the rill flows into a rapiil stream, "^ 



the largi r curn-iit carries away the deposit so that it may not be 

 seen. Kecall how snml-bars form again and a;:ain in lakes, and 

 how streams must be frequently dredged to keep the channel 

 opeo. The slower the stream the more quickly does it drop its 

 sediment ; and the more winding, also, is its course, lying in the 

 bed of its own de|K>sits. (See Fig. 4.) 



38fc. Dip a glass of water from a roily stream, and ohser^-e 

 the earth wliich st-ttles to the bottom. 



39a. Glaciers are still iil>!iiidiint in alpine and arctic regions. 

 It was from the study of glaciers in the Alps that Agassiz con- 

 coived the hypothesis that large parts of the earth had once 

 been subjt-cted to glacial action. A good popular discussion of 

 glaeiers and their action may bo found in Chap. xvii. of Tarr's 

 "Elementarj' Physical Geography." Dfliijhtful readings may also 

 be made from Agasaiz's "Geological Skc-tehes." 



40a. L«'t the pupil cafch a few rain drops on a perfectly clean 

 and clear pane of glass, and oh8er>'e if any sediment is left when 

 the drops have evaporat»^l. Is there any difference in the amount 

 of dust brought down 8ftera"drj* spell" und after a periotl of 

 rainy weather, or at the begianing and end of a shower T I'he 



