19 



In the first place the initial proportionality between water content 

 and time observed with sand is not seen with soil, the curve being 

 more exponential in character. This indicates that the relationship 

 of water to soil is quite different from its relationship to sand, a 

 circumstance which has been traced as already stated to the colloids. 

 This relationship has at present only been expressed empirically but 

 it is probably connected with the relation between vapour pressure 

 and moisture content. But there is clearly something else at work 

 for the curve is not of a simple exponential type. It is necessary to 

 allow for another factor : the effect on the rate of evaporation of the 

 decreasing water surface in the soil, the surface obviously diminish- 

 ing in area as evaporation continues. 



The equation finally developed is : — 



A S7 = ^(ioo + [2 ' 303 logl ° {w+K) ~ l0ge ffll 



dw 

 where ~J7 — rate or evaporation. 



w — percentage of water present by weight. 

 s = specific gravity of the soil. 

 A and K — constants. 



XVIII. "Some Notes on Soil Protozoa." C. H. Martin 

 and K. R. Lewin. Philosophical Transactions, 

 1914. 205 B, 77—94. 



The authors show that the view current among zoologists 

 limiting the range of free living protozoa to ponds, streams, the sea, 

 etc., is not justified by the observations of the older investigators. 

 A method is described by which it is easy to demonstrate in field 

 and garden soils the presence of free living protozoa in a trophic 

 state : it is as follows : — A small quantity of the soil is added to an 

 equal quantity of a saturated aqueous solution of picric acid ; the 

 mixture is then stirred very thoroughly, so that the protozoa which 

 are situated on the liquid films between the soil particles are freed. 

 The mixture is then allowed to stand for 12 — 24 hours, and a scum 

 gradually rises to the surface which contains a proportion of the 

 bacteria and protozoa of the soil. Cover slip preparations are made 

 by floating cover slips on the surface of the mixture, then transferred 

 to corrosive sublimate solution or 70 per cent, alcohol, stained and 

 mounted in the ordinary way. 



At least eight organisms, some of them new, were discovered 

 in a trophic state in a "sick" cucumber border. The thecamoebae 

 were probably present in considerable numbers, as also were the 

 amoebae, which contained large numbers of ingested bacteria. 

 Neither flagellates nor ciliates were observed in any quantity in the 

 trophic state. An organism, Vahlkampfia soli, appeared to be the 

 dominant form during August and is described in some detail as it 

 differs in certain points from Amoeba Umax. It is very active and 

 has a flagellate stage : in nuclear division it shows the phenomena 

 of promitosis. A second organism, Amoeba cucumis, is also 

 described ; this is very sluggish and has characteristic pseudopodia. 



An outdoor seedling bed was found to contain a smaller number 

 of protozoa but in larger variety : a relationship very similar to that 



