NATURAL PHILOSOPHY. 135 



this current is direct than when it is inverse; but a current of such low inten- 

 sity as to cause no appreciable contraction Avhcn transmitted in the latter 

 direction (inverse), will occasion very evident action when passed in the for- 

 mer (direct). Thus the difference betAveen these currents must be remem- 

 bered in testing irritability, as well as in testing power. It is sometimes a 

 source of fallacy in physiologic experiments; as, for example, in examining 

 the irritability of muscles in a paralyzed limb, by passing the current from 

 one arm to the other. In this case, it is, of course, direct in one arm, and 

 inverse in the other; and we have frequently seen the difference between the 

 irritability of the muscles on the paralyzed and non-paralyzed sides so slight 

 as merely to equal, or even fall below that which exists between the action 

 of the inverse and direct current respectively. "When such is the case, the 

 irritability appears greater in that limb through which the direct current 

 passes, whereas it may be really less." 



NEW AND CHEAP FORMS OF GALVANIC BATTERY. 



The following description of a new, cheap, and effective arrangement of a 

 galvanic battery, devised by Dr. John O'C. Barclay, U. S. X., is contained in 

 a note addressed to the editor by Dr. B., March 1859 : " Having in the years 

 1847-8 been engaged in some investigations touching the amount of work 

 done by constant batteries, and the comparative cost of running them, I was 

 led, in consequence of the well-known passive state enjoyed by cast iron in 

 contact with concentrated nitric acid, to construct a cheap form of battery, in 

 which both elements are of cast iron. The battery is powerful (perhaps in this 

 point rather inferior to Grove's), and is very constant. The following is the 

 form I adopted : In a vessel of convenient material is placed a hollow cylin- 

 der of cast iron, the metal being the black variety, or such as is used for gun 

 metal. The side of the cylinder should be split or cut through from top to 

 bottom by a slot one-fourth inch wide, or more. "Within this cylinder, and 

 in moderately close approximation, is a porous jar, which contains a solid, 

 or, at option, a hollow rod or cylinder, of the same kind of cast iron. To 

 charge this battery, I make a saturated solution of chloride of sodium, 

 which I pour into the outer vessel, and in contact with the larger iron cylin- 

 der. Into the porous jar I put a mixture of equal parts of concentrated 

 nitric and sulphuric acids, and the battery is now ready for use. Theoreti- 

 cally, the acids should not be equal in quantity; but the difference is so small 

 that, practically, it may be neglected. As some of your readers may wish to 

 know the changes undergone by the solids and fluids during the flow of the 

 electrical current, I will, in a line or two, state them. On uniting the poles, 

 water is decomposed, its oxygen uniting with the sodium of the chloride of 

 sodium, while its hydrogen combines with the fifth atom of the nitric acid 

 in the porous jar. The chlorine attacks the iron of the outer cylinder, form- 

 ing the very soluble chloride of that metal, and leaving the surface of the 

 iron always clean. The sulphuric acid in the porous jar unites with the 

 oxide of sodium to form sulphate of soda, thus completing the process. As 

 I before said, this form of battery is cheap, powerful, and constant, and is by 

 no means troublesome; and I offer it to your readers as a new and useful 

 instrument, particularly to those of them to whom the difference of cost 

 between iron and amalgamated zinc is worthy of consideration, or to those 

 who find the use of mercury injurious or troublesome. The black cast iron 

 should be used, this being passive in concentrated nitric and sulphuric acids. 



