APPENDIX TO NOTICE OF SCHOENBEIN. 



[The phenomenon called passivity of metals mentioned in the foregoing eulogy 

 as one of the discoveries of Schoenbein, consists in the fact that iron, for example, 

 which, under ordinary conditions, is readily disolvcd in nitric acid, may while in 

 a peculiar state remain for weeks in the same liquid without being acted on. 

 This phenomenon is, without doubt, due to a galvanic action, which, when the 

 iron is first plunged into the liquid deposits a coating of oxide which protects 

 the metal from tlie further action of the acid. To illustrate this let a piece of 

 clean iron wire be innnersed in strong nitric acid together with a slip of platinum, 

 the former being introduced first and the two connected with the ends of the wire 

 of a galvanometer, a powerful current will be inducted at the first comjdetion of 

 the circuit, the iron acting as the positive metal, but the strength of this current 

 will quickly decline to a small amount and then remain constant for several 

 days. The iron thus treated is no longer attacked when plunged alone into 

 nitric acid, and is said to be passive. Instead of using a galvanometer, which 

 was merely introduced to prove the existence of a galvanic current, the same 

 effect will be produced by touching the iron wire while the acid is acting on 

 it with a piece of gold or platinum, also immersed in the liquid, the action will 

 imraediatel}' cease and the mm become passive. When an iron wire in the 

 passive state is plunged into nitric acid and the upper end touched with another iron 

 wire, as soon as dipped into the acid the latter also becomes passive. In these experi- 

 ment, the iron, which is rendered passive, acts as the zinc element of a galvanic 

 pair, and is rapidly covered with an oxide which protects it from further action except 

 of a very feeble character. In this state it may serve as the copper or negative 

 metal of a galvanic pair, and really performs this part in the second experiment 

 in which a galvanic couple is formed by the contact, while in the acid, of the 

 passive and non-passive iron. The formation of an oxide sufliciently thick to 

 protect the iron is not produced in rutric acid, of ordinary strength, unless a gal- 

 vanic aiTangement such as we have described is adopted, but if it be plunged into 

 very strong acid the action, though violent for an instant, will soon cease, and the 

 metal assume the passive condition. If this wire be withdrawn from the acid 

 and exposed to the air for a short time, or rubbe<l with sand])aper, it will resume 

 its ordinary state. An iron wire nui}'^ also be rendered passive by holding it for 

 a few minutes in the flame of a spirit lamp. 



Other metals — namely, silver, coj)i)er, tin, aluminum, and especially bismuth, 

 may be brought into the passive state by the methods we have mentioned, but 

 the effect is not as marked as with iron. Dr. Hare constructed a galvanic 

 battery in which the platinum was represented by injn in the passive state, but 

 the action was capricious; tlunigh at one moment powerful at another it became 

 almost ncjthing. 



Another discovery of Schoenbein, mentioned in the foregoing sketch, is that 

 oi gun-cotton, n vary explosive substance, produced by steeping cotton-wool in 

 fuming nitric acid, or in a mixture of nitric and sulphuric acids, al'terwards washing 

 and drying the product. This discovery was. announced by Schoenbein in 1845, 

 but the ukkIc of j)reparation was kept secret. It was, however, soon rediscovered 

 independently by Bettger and Otto, while Kept improved the process of pro- 

 duction by the addition of sulphuric acid to the active rupiid. From the first it 

 was proposed as a su])stitute for gunpowder, over which it possesses the advan- 

 tages of burning without smoke, and leaving no rcsiiluuin to foul the clu'unber of 

 the caiUKJii, Large establishments were erected fur its j)reparation, hut the 



