>66 



THE CIVIL ENGINEER AND ARCHITECT'S JOURNAL. 



[April, 



RBVICWS. 



FARADAY AND THE ELECTRICAL SCIENCES. 

 Lectures on Electricity, comprising Galnanism, Magnetism, Electro- Magnetism, 

 Magneto and Tkermo-Electricitg. By Henry M. Noad, Author of Lec- 

 tures on Chemistry, &lc. London : George KniglU & Sons, Foster Lane. 



Mr. Noad has published a new edition of his Lectures on Electri- 

 city, which give succinctly an account of each department of elec- 

 trical science, with the forms of illustrative experiments and the 

 history of the mi)st recent discoveries and improvements. In peru- 

 sing this work we have seen v/ith gratification how much Dr. Faraday 

 has contributed to every branch of these sciences, and we thought we 

 could not give a much better example of the value of Mr. Noad's 

 book, nor a more interesting selection for our readers, than to extract 

 some account of Dr. Faraday's valuable labours. 



It will not he forgotten that it was by means of an apparatus, in some de- 

 gree resembling the Electrophorus in principle that Faraday succeeded in 

 demonstrating that induction is essentially a physical action, occurring be- 

 tween contiguous particles, and never taking place at a distance without 

 polarizing the molecules of the intervening dielectrics. The conclusion seems 

 to be that induction is not through the metal of the apparatus, but through 

 the air in curved lines. In fact, as Mr. Noad puts it, it is an action of the 

 contiguous particles of the insulating body thrown into a state of polarity 

 and tension, and capable of communicating their forces in all directions. 

 With regard to the theory of electro-chemical decomposition, in his second 

 ecture, page 77, Mr. Noad remarks — 



"The following beautiful experiments, made by Faraday (See Exp. Research, 

 series v. 462 et seq.), prove that, so far from electro-chemical decomposition 

 depending upon the simultaneous action ot two metallic poles, air itself may 

 act as a pole, decomposition proceeding therewith as regularly and truly as 

 with metal. 



" A piece of turmeric paper, not more than 0'4 of an inch in length, and 

 0'3 of an inch in width, was moistened with sulphate of soda, and placed 

 upon the edge of a glass plate opposite to and about two inches from a point 

 connected with a discharging train arranged by connecting metallically a 

 sufficiently thick wire with the metallic gas pipes of the house, with those of 

 the public gas works of London, and with the metallic water pipes of London. 

 A piece of tin-foil resting upon the same glass plate was connected with the 

 machine and also with the turmeric paper by the decomposing wire a (Fig. 1.) 



rig. 1. 



The machine was then worked, the positive electricity passing into the tur- 

 meric paper at the point p, and out at the extremity n. After forty or fifty 

 turns of the machine (a plate fifty inches in diameter), the extremity n was 

 examined, and the two points or angles found deeply coloured by the pre- 

 sence of free alkali. 



" A similar piece of litmus paper dipped in a solution of sulphate of soda 

 (Fig. 2) was now supported upon the end of the discharging train a, and its 



extremity brought opposite to a point p, connected with the conductor of the 

 machine. After working the machine for a short time, acid was developed 

 at both corners towards the point, that is, at both corners receiving the elec- 

 tricity from the air. Then a long piece of turmeric jiaper, large at one end 

 and pointed at the other, was moistened in the saliue solution and immedi- 

 ately connected with the conductor of the machine, so that its pointed ex- 

 tremity was opposite a point upon the discharging train. When the machine 

 was worked, alkali was evolved at that point; and even when the discharging 

 train was removed, and the electricity left to be diffused and carried off al- 

 together by the air, still alkaU was evolved where the electricity left the tur- 

 meric paper. 



" Arrangements were then made in which no metallic communication with 

 the decomposing matter was allowed, but botti poles formed of air only. 

 Pieces of turmeric and litmus paper, a and b, (Fig. 3,) moistened with solu- 

 tion of sulphate of soda, were supported on wax between the points, con- 

 Fig. 8. 



nected with the conductor of the machine and the discharging train, as 

 shown in the fijure ; the interval between the respective points was about 

 half-an-inch. On working the machine, evidence of decomposition soon 

 appeared, the points b and a being reddened from the evolution of acid and 

 alkali. 



"Lastly, four compound conductors of litmus and turmeric paper were ar- 

 ranged as shown in Fig. 4, being supported on glass rods ; and on working 



Fig. 4. 



the machine carefully, so as to avoid sparks and brushes, evidence of decom- 

 position was obtained in each. 



" Notwithstanding, then, the absence of metallic poles, we have here cases 

 of electro-chemical decomposition precisely similar to those effected imder 

 the influence of voltaic battery ; and we appear to have direct proof also 

 that the power which causes the separation of the elements is exerted not 

 at the poles, hut at the parts of the body which is suffering decomposition. 



" The arrangement shown in Fig. 5 was employed by Faraday for effecting 

 electro. chemical decomposition by common electricity. On a glass plate, 

 raised above a piece of white pig. 5. 



paper, two small slips of tin- 

 foil, a, b, were placed : one was 

 connected by the insulated wire 

 c with an electrical machine, 

 and the other by the wire g 

 with a discharging train, or 

 with the negative conductor. 

 Two pieces of line platinum 

 wire, bent as in Fig. 6, were 

 provided, and so arranged that the part d,f, was nearly upright, while the 

 whole rested on the three hearing points,/), e,f. 



The points JO, H, thus became the decomposing (/^ Fig. 6. 

 poles. They were placed on a piece of filtering 

 paper wetted wilh the solution to be experi- 

 mented upon. When litmus paper, moistened '^V^l \j j 



in solution of common salt or sulphate of soda, 

 was employed, it was quickly reddened at 7; ; a 

 similar piece, moistened in muriatic acid, was 

 very soon bleached at the same point, but no effects of a similar kind took 

 place at n. A piece of turmeric paper, moistened in solution of sulphate of 

 soda, was reddened at n by two or three turns of the machine ; and in twenty 

 or thirty turns, plenty of alkali was there evolved. On turning the paper 

 round, so that the spot came under ;u, and then working the machine, the 

 alkali soon disappeared, the place became yellow, and a brown alkaline spot 

 appeared in the new part under n. When pieces of litmus paper and tur- 

 meric paper, both wetted with solution of sulphate of soda, were combined, 

 and put upon the glass, so that p was on the litmus, and n on the turmeric, 

 a very few turns of the machine sufficed to show the evolution of acid at the 

 former and alkali at the latter, exactly in the manner effected by a volta- 

 electric current. (See Exp. Researches, third series, 309 et seq.) 



" In these experiments the direct passage of sparks must be carefully 

 avoided. If sparks be passed over moistened litmus paper, it is red- 

 dened ; anil if over paper moistened with solution of iodide of potas- 

 sium, iodine is evolved. But these effects must carefully be distin- 

 guished from those due to electro-chemical powers, or true electrolytic 

 action, and must be carefully avoided when the latter are sought for. The 

 effect just mentioned is occasioned by the formation of nitric acid by the 

 chemical union of the oxygen and the nitrogen of the air: the acid so formed, 

 though very small in quantity, is in a high state of concentration, and there- 

 fore reddens the litmus paper, and decomposes the iodide. 



" It does not appear that Faraday was more successful than Wollaston in 

 effecting a true electro-polar decom|)osition of water. He says, ■ there is 

 reason to believe that when electro-chemical decomposition takes place, the 

 quantity of matter decomposed is not proportionate to the intensitj, but to 

 tlie quantity of electricity passed; hut in Wollaston's experiment this is not 

 the case. If wilh a constant pair of points the electricity be passed from 

 the machine in sparks, a certain proportion of gas is evolved ; but if the sparks 

 he rendered shorter, less gas is evolved ; and if no sparks be passed, there is 

 scarcely a sensible portion of gases set free. On substituting solution of 

 sulphate of soda for water, scarcely a sensible quantity of gas could be pro- 



