June 23, 1922] 



SCIENCE 



677 



bombardment of the water molecules in which the 

 particle is immersed shatters the particle beyond 

 the ability of the molecules in the solid to hold 

 together as a solid mass. The atoms of calcium, 

 magnesium, potassium and sodium in the molecule 

 of the silicate would go for the most part into 

 true solution, while the atoms of siHeon, aluminum, 

 and iron would go chieflj' into colloidal solution 

 forming the basis of the colloidal matter or the 

 ultra clay of the soil. It should be possible for 

 the mathematical physical chemist, from physical 

 constants now known, to determine empirically the 

 relative size of the particle of matter which could 

 withstand such bombardment without complete 

 disintegration. This is a problem which has not 

 yet been worked out. 



This is one way of looking at tlieir origin, 

 but the results of our experimental work on soil 

 colloids force us to adopt quite a different view. 

 One that is not based on bombardment of water 

 molecules, but one based largely on chemical 

 reactions. 



Many soil particles are hydrated silicates 

 which contain varying amounts of aluminium, 

 iron, silcon, sodimn, potassium, calcium, mag- 

 nesium and other elements in smaller quanti- 

 ties. Soil chemists claim that these particles 

 are surrounded with a water-film, and that this 

 film is held tenaciously. In the light of this 

 the salts in the outer layer of these soil parti- 

 cles are subjected to constant hydrolysis. The 

 hydrolytie products of the soluble compounds 

 of sodium, potassium, etc., are partly taken up 

 by this water film by way of solution, and part 

 of them are adsorbed by the hydrolytie insoluble 

 products of the iron and alumina salts which 

 form a gel easing for the soil particle, that is, 

 there is an equilibrimn of the soluble salt be- 

 tween the water film and the insoluble gel which 

 now surrounds the soil particle. 



When the soil becomes flooded as after a 

 rain, and the water moves down through the 

 soil, the soluble salt of the water film is partly 

 removed by diffusing into the moving water. 

 This destroys the salt equilibrium between the 

 water film and the incasing gel, and, hence, 

 some of the soluble adsorbed salt is released to 

 the water film. This continues until most of 

 the soluble material is leached from the outer 

 layer of the soil particle. This leaching may 

 be continued until the incasing hydrolytie gel 



products of alumina and silica, and ferric 

 oxide may pass into colloidal solution. Not 

 only will the freedom of electrolytes tend to 

 bring the incasing gel into colloidal solution 

 but some of the soluble salts themselves or some 

 salts that are moving through the soil under 

 the proper hydrogen ion concentration will 

 very much hasten their pepitization. 



The pepitization of the hydrolytie insoluble 

 compounds removes the encasing gel and the 

 soil particle is again exposed to hydrolytie 

 action, and in this way the weathering of the 

 silicate particles proceeds. The pepitized gel 

 or hydrosol moves through the soil, provided 

 the pepitization is great enough, until it en- 

 counters a coagulating electrolyte or different 

 hydrogen ion concentration, when it comes back 

 as the gel and may be deposited on a soil par- 

 ticle, or come down as a precipitate where it 

 remains as an adsorbent and reservoir for plant 

 food until the conditions are sufBciently 

 changed for it to pass back into the hydrosol; 

 that is, the process is reversible 



hydrogel ^ hydrosol 

 and whether it is a hydrosol or a hydi'ogel de- 

 pends on the soil environment. 



Certain soil salts in our work have brought 

 about a very beautiful pepitization, while other 

 salts have brought about an equally definite 

 coagulation. Then there are salts that lie in 

 between these extremes. Again the same salts 

 and same concentration have brought about 

 both coagulation and pepitization by changing 

 the hydrogen ion concentration. 



Neil E. Gordon 

 Chemistry Department, 



TjNnfERSITT OF MARYLAND 



A CRAYFISH TRAP 

 In ponds and streams where crayfish are 

 abundant they can be readily taken by means 

 of a trap constructed as follows : A rectangular 

 box of any convenient size, sixteen by twenty- 

 four inches for instance, is built of one-fourth 

 inch mesh galvanized screen wire. Into one 

 end of this box a removable funnel of like 

 material is fitted. This funnel should project 

 about eight inches into the box and have a 

 flattened opening about four inches wide and 

 an inch and a half deep. In setting the trap 



