SOIL CONDITIONS AFFECTING PLANT GRO WTH 89 



the nature of the plant. Parkinson ^ has tested the method 

 with satisfactory results ; spirea, rhubarb, seakale, etc., 

 steeped for twelve hours in water at 95°, at the end of 

 November, or early in December, made rapid growth when 

 subsequently forced. 



Effect of Radium. — R. J. Strutt has shown ^ that the 

 typical soil deposits contain measurable amounts of radio- 

 active substance equivalent to 0*25 (but more usually i) to 

 5*8 X io~^* grms. of radium per gram. Zircon and apatite 

 are much richer, the figures being 75 to 865 x lo"^^ and il 

 to 30 X 10"^^ respectively. Joly^ finds that the amount of 

 radium emanation in soil air is many thousand times greater 

 than in the atmosphere ; J. Satterly * at Cambridge gives a 

 lower estimate, viz. 2 x io~^^ curie per litre, or 2000 times 

 the usual amount in the atmosphere ; while J. C. Sanderson ^ 

 in America estimates the amount in I c.c. of soil air as the 

 quantity in equilibrium with 2*4 x io~^^ grms. of radium. 

 Among the many remarkable properties of radium it was 

 perhaps natural to expect that it might have some definite 

 effect on plants or micro-organisms. The suggestion has 

 even been made that radium emanations might, under suitable 

 conditions, cause sufficient increase in the amount of growth 

 to justify its use in horticulture and agriculture. The early 

 observations of Dixon and Wigham ® at Dublin, however, did 

 not seem very promising; 100 seeds of cress {Lepidium 

 sativum) were uniformly distributed on an even surface of 

 moist quartz sand, and after germination had taken place, a 

 sealed tube containing 5 mgms. of radium bromide was set 

 I cm. above the central seed. The seedlings grew up, but 

 without any curvature indicating positive or negative " radio- 

 tropism," and the only noticeable effect was a slight depression 



^Journ. South-Eastern Agric. Coll., 1909, 19, 245-257. 



'Proc. Roy. Soc, 1906, 77a, 472, and 1907, 78a, 150. 



^Sci. Proc. Roy. Soc, Dublin, 1911, 13, 148. 



*Proc. Camb. Phil. Soc, 1912, 16, 514. The soil air was taken at a depth 

 of 100150 cms. For his estimates of the amount in the atmosphere see Phil. 

 Mag., 1910 (vi.), 20, I. 



''Amer. y. Sci., 1911 (iv.), 32, 169. 



^ Proc Roy. Soc, Dublin, 1904, 10, 178-192. 



