of the Magnetic Metals. 479 



on this point. The enormous cohesive power of iron, nickel, 

 and cobalt in the solid state signalizes these substances as the 

 most tenacious of metals. To overcome this cohesion a very- 

 high and somewhat similar temperature is required, and their 

 melting -point is only exceeded by the platinum group of me- 

 tals. Their refractory character renders them not volatile even 

 at the temperature of the hottest furnace. When, however, 

 they are volatilized by means of the electric spark, their in- 

 candescent vapours yield a spectrum which has a close and curious 

 resemblance. This teaches us that the molecules of these bodies, 

 freed from the thrall of cohesion, vibrate in periods which are 

 closely akin. 



A comparison of the chemical properties of the same metals 

 furnishes a similar result. The ratio of the combining weight 

 of the metallic elements ranges from lithium 7, to bismuth as 

 210, or a difference of 203. When we compare the magnetic 

 metals, we find the combining weight of iron is 56*0, nickel 

 58*5, and cobalt 58*5, or a difference of only 2*5. Chemists 

 class these three metals in the same group from the similarity of 

 their chemical behaviour, and also the identity of their combi- 

 ning energy or atomicity. 



In strong nitric acid iron becomes endowed with a so-called 

 passive condition, not acted upon, as it is in the dilute acid. 

 Likewise I fiud nickel is capable of assuming a passive state in 

 strong nitric acid. Cobalt, it is true, was violently acted upon 

 under similar circumstances ; but that, I believe, was due to the 

 fact that the cobalt contained iron largely, and so an electrolytic 

 action was probably set up. I have been unable to obtain pure 

 cobalt — a very difficult matter, I believe. 



A series of very similar chemical compounds are formed by 

 these metals, mostly characterized by the brilliancy of their co- 

 lour. The protosalts of iron are generally bluish green, of 

 nickel emerald green, and of cobalt of a rose-colour. It is 

 moreover a well-known fact that this rose-colour of certain co- 

 balt salts passes into a bright green when they are warmed. 

 Now, when the metal cobalt is moderately heated it increases 

 in magnetic power, thus differing from its congeners, iron and 

 nickel, which are in their maximum magnetic condition at the 

 ordinary temperature, and at ordinary temperatures present the 

 green-coloured salts*. 



What has been said concerning the likeness of iron, nickel, 

 and cobalt, in many respects holds true of manganese and chro- 

 mium, also feebly magnetic metals. Placed in the same group 



* The therapeutic effect of the salts of these metals one would expect to 

 be somewhat similar ; and in confirmation of this I hear that nickel has 

 lately often been used advantageously to replace the medicinal properties 

 of iron. 



