March 24, 1936] 



SCIENCE 



435 



fell to a minimum. The increase in ammonia 

 was accompanied by a decrease in nitrates, 

 whicli were practically non-existent in the 

 highly heated soils. 



The ammonia produced on heating soil has 

 been suggested by Russell as causing the in- 

 jurious action, although no evidence on this 

 point could be obtained. Pickering suggested 

 that the injurious factor was volatile in na- 

 ture, on account of its gradual disappearance 

 from the soil, but Russell disagrees on this 

 point. Russell, however, worked with low 

 temperatures, usually not exceeding 100° C, 

 and with volatile antiseptics. Under such 

 treatment, only relatively small amounts of 

 ammonia are produced directly, and seed 

 germination and plant growth are not so stri- 

 kingly affected as in soils heated to higher 

 temperatures. 



The percentage of seed germination has been 

 found to be closely correlated with the amount 

 of ammonia present in the heated soils studied. 

 The amount of ammonia required to injure 

 germination, however, appears to vary with 

 the type of soil when comparisons of different 

 heated soils are made. It appears that the 

 absorptive power of the soil is a very impor- 

 tant limiting factor in determining the extent 

 of the injurious action. 



The presenec of dihydioxystearic acid as de- 

 scribed by Schreiner could not be demonstrated 

 in the most toxic of the heated soils. That 

 the toxic substance is of a volatile nature is 

 evident by the fact that it is readily removed 

 from the soil by aeration. If collected in water 

 upon removal, its toxicity can be readily 

 demonstrated. By collecting in a hydrochloric 

 acid solution the chemical composition of the 

 resultant salt has been shown to be ammonium 

 chloride, containing ammonia in sufficient 

 quantity to account for the toxic action of 

 heated soils. 



It is improbable that all the ammonia pro- 

 duced in heated soils exists as free ammonia. 

 Large amoimts of carbon dioxide are also pro- 

 duced when soils are heated, which possibly 

 accounts for the increased acidity of heated 

 soils. The evidence at hand points toward the 

 formation and injurious action of ammonium 



carbonates particularly. These salts being un- 

 stable in the soil except when kept in a dry 

 and unaerated condition, accounts for the 

 gradual disappearance of the injurious action 

 of heated soils. It also appears that other com- 

 poimds of ammonia are formed which are 

 more stable in character. 



The beneficial action of heated soils on plant 

 growth, especially of those heated between 

 150° C, and 2.50° C, is believed to be due in 

 a large part to the direct assimilation of 

 ammonia or ammonium compounds by the 

 plants after the manner described by various 

 workers. The increased growth follows in 

 practically all cases after a period of injuri- 

 ous action to plant growth, and is no doubt 

 dependent upon the reduction of the toxic 

 substance to a point where it is stimulatory 

 or acts as a plant food. The relative impor- 

 tance of increased plant food production as a 

 result of bacterial activity, and of direct chem- 

 ical action, in highly heated soils remains to 

 be ascertained. 



The vtrriter will be pleased to obtain sug- 

 gestions or criticisms on the point of view 

 presented in this paper. 



James Johnson 



IJniveksity of Wisconsin 



note on the interferences of parallel 

 and crossed rays 



After perfecting the design (Fig. 1) of my 

 last article^ thus obtaining an apparatus which 

 is free from transmission through glass and in 

 which all the rays are guided by reflection from 

 metal surfaces only, I have secured definite 

 evidence showing that the strands of interfer- 

 ence patterns obtained are actually referable to 

 the intersection of two grids, due to the two 

 sodium lines, respectively. One of the grids 

 is retarded in rotational phase with respect to 

 the other. Why in the case of a transmitting 

 grating, the nature of the phenomenon is so 

 effectively concealed, I have not been able to 

 make out; but with mercury light, but one set 

 of striations is obtained, as anticipated. 



With this definite understanding of the phe- 

 nomenon, the resolving power works out as 



1 Science, February 25, p. 282. 



