NATURAL PHILOSOPHY. 201 



urea formed in the male sex in the ratio of 17 to 15. Although the 

 amount depends somewhat on the nature oi'food, Lehmann discovered 

 long ago that strong exercise of the bodily powers increased iN excre- 

 tion. I'rea was first, made artificially from inorganic substances by 

 Wohler. By the addition of two atoms of water, urea is changed to 

 the carbonate of ammonia, and thus becomes a valuable fertilizer. 



MAGNETISM OF IRON AND STEEL TURNINGS. 



Iron and steel " turnings," or the long spirals resulting from the 

 O;HT .lion of turning metal in the lathe, have been discovered by M. 

 Greis*, to possess magnetic properties. lie has communicated a 

 short, paper on the subject to a late number of Poggendorff's Annalen. 

 Both steel and soft iron turnings were found by him to have a very de- 

 cided and permanent polarity, and to behave in all respects like ordinary 

 magnets. He was particularly struck with the permanence of the mag- 

 netism in the case of the soft iron. His attempts to trace some con- 

 nection between the direction of the spiral which the turnings assume 

 when they leave the tool, and Ampere's law of the circulation of the 

 currents, were only attended with partial success. He found, however, 

 that the end of the turning at winch the tool began its work was inva- 

 riably a south pole; the end at which it left off, a north pole. A com- 

 parison of the strength of the magnetism in different specimens, led him 

 to notice that the turnings whose revolutions were in the opposite direc- 

 tion to the motion of the hands of a watch (the observer being at the 

 south pole) showed a much stronger magnetic power than those whose 

 revolutions corresponded in direction with that motion. 



DYNAMICAL THEORY OF ELECTRO-MAGNETISM. 



Prof. Maxwell, in a late number of the Proceedings of the Royal 

 Society, has published a "Dynamical Theory of the Electro-Mag- 

 netic Field,' 1 in which ho seeks for the origin of the electro-magnetic 

 effects in the medium surrounding the electro-magnetic bodies, and 

 assumes that they act on each other, not immediately at a distance, 

 but through the intervention of the medium. He considers the exist- 

 ence of this medium as probable, since philosophers believe that it is 

 in such a medium the propagation of light takes place, its properties 

 being: 1. That the motion of one part communicates motion to the 

 parts in its neighborhood (transverse vibrations). 2. That this 

 communication is not instantaneous, but progressive, and depends on 

 the elasticity of the medium as compared with its density. Prof. 

 Maxwell, after referring to the researches of Faraday, Thompson, 

 and others, on the subject, and expressing his own opinions, defines 

 light, according to his theory, as consisting "of alternate and oppo- 

 site rapidly recurring transverse magnetic disturbances, accompanied 

 with electric displacements ; the direction of the electric displace- 

 ment being at right angles to the magnetic disturbance, and both at 

 right angles to the direction of the ray." 



PHOTOGRAPHS OF THE ELECTRIC SPARK. 



Prof. Rood of Columbia College, N. Y., communicates to Silli- 

 in (Hi's Journal, No. 114, some researches on the electric spark ob- 

 tained through the aid of photography. This method consists in 



