THE SMITHSONIAN INSTITUTION. 415 



To prevent a strong disturbance of tlie solution during the develop- 

 ment of gas, a film of turpentine oil, about one line thick, is poured 

 upon tlie surface. 



A voltameter, with moderately large electrodes, can be made by 

 means of iron plates, in the same manner, at little cost. Such a 

 Toltameter is capable of yielding a large quantity of gas in a short 

 time ; yet the development is not near so great as might be expected 

 from the magnitude of the plates, probabl}^ because the potasli solu- 

 tion is a worse conductor than the dilute sulphuric acid of the ordinary 

 platinum voltameter. 



Such an iron plate voltameter, according to my observations, is ijot 

 well adapted to exact experiments. I have noticed that the maximum 

 development of gas takes place some time after the closing of the bat- 

 tery, and that the appearance of the gas bubbles does not cease witb 

 the interruption of the current, but lasts considerably longer. This 

 is due to the absorption of the gas by the liquid. 



While with the use of alkaline solution in water and jierfectly neu- 

 tral salts, iron is passive, however the circuit may be closed, on the 

 other hand iron never becomes passive, however the closing may be 

 effected, if the iron electrodes be immersed in a solution of an elec- 

 trolytic compound not containing oxygen, whose negative component 

 lias a great affinity for iron, such as the hydracids, halogen salts, sul- 

 phurets, &c. In such solutions iron is always attacked, and free 

 oxygen is never liberated at its surface. 



In the experiments described in section 48, the primary passive and 

 secondary passive ends of the wire were dipped in the same vessel 

 filled with acid. Schonbein has extended the phenomena by using 

 two vessels filled with acid, connected in different ways. 



The vessels A and B, Fig. 41, are filled 

 with nitric acid from 1.3 to 1.36 sp. gr. 

 Dip the end of the wirep, rendered passive 

 by red heat, in A, and the unheated end a 

 in B ; then a will be attacked. If a second 

 fork of iron wire, both ends of which have 

 not been heated, be now immersed, d being 

 first put in B, and then p' in A, p will be- 

 come passive, p and p' will remain free 

 from attack, while at a and d a lively development of gas will occur. 

 This is not essentially difterent from the form of the experiment 

 represented in Fig. 37. 



Fig. 42. Let the vessels A and B, Fig. 42, filled 



with acid of the sp. grav. 1.3 to 1.37, be 

 connected by an asbestos cord saturated with 

 the same acid. Immerse in A the passive 

 end of an iron wire, and then the other end 

 a in B ; a will not become passive, but will 

 be briskly attacked. 



Here, evidently, the current is on account of 

 the great resistance, too weak to render a passive. The correctness 

 of this^ view is proved by the fact, that if the negative pole of a battery, 

 formed of platinum or passive iron wire, be dipped in A, and then, 



