366 RECENT PROGRESS IN PHYSICS. 



clean, but leaves it unchanged if it is soiled or tarnished. Thisistlie 

 case, in forming breath images on metals. A very small number of dis- 

 charges having passed between a metallic surface and one of m'ca 

 covering it, the intermitting discharge begins in the foreign stratum 

 on the surface of the metal, and the metal remains uninjured ; but 

 when the stratum is destroyed, and the breath image is produced, and 

 the discharges are continued, the latter then begin on the metal itself, 

 which is thus changed. Such images, appearing without breathing, 

 and representing some parts of the stamp in brownish colors, Riess 

 produced on silver with from fifty to sixty revolutions. 



§ 78. Electrolytic images. — If the blunt point of a platinum needle 

 be placed on a paper moistened with a solution of iodide of potassium, 

 and lying on a metallic plate connected with the ground, a brown 

 spot will appear under the point if the needle is electrified positively, 

 but there will be no spot if it be negatively electrified. Using positive 

 and negative electricity one after the other in any order, the coloring 

 remains even when the quantity of negative electricity far exceeds 

 that of the positive. 



This fact explains the electrolytic images, which Riess has invented 

 for proving the correctness of the view presented above, on the forma- 

 tion of breath images by alternating discharges. 



A piece of card paper, moistened on one surface with a solution of 

 iodide of potassium, was laid on a metallic plate connected with the 

 ground, and then covered with a plate of mica. A stamp was placed on 

 the mica, and, being loaded with a weight of 2 to 14 ounces, was con- 

 nected with the spark micrometer, whose knobs were \ a line asunder. 

 After twenty revolutions of the machine, positive electricity continuing 

 to pass between the knobs, a very sharp image appeared on the paper 

 in which the letters of the stamp appeared with a brown color. 



The explanation of this phenomenon, according to the above, is easy. 

 As in breath images, the stamp being charged with positive electricity, 

 it passes from the lower surface of the mica to the metal plate, and 

 thence through the moist paper ; by this passage of the -j- ^^ f o the 

 metal plate the iodide of potassium is decomposed ; as soon as a dis- 

 charge takes place between the knobs of the spark micrometer, an op- 

 posite current sets in between the metal plate and the mica ; the -{- E 

 now returns to the mica, and the — E through the moist disk to the 

 metal. While the -f- E goes to the metal the iodide of potassium is 

 decomposed, and this eft'ect is not destroyed by the discharge in the 

 opposite direction. 



It is to be remarked that the passage of the -\- E from the mica to 

 the metal takes place gradually, while the discharge in the opposite 

 direction happens instantaneously. 



The same experiment being repeated in the same manner with — E, 

 no image is obtained, but only irregular brown spots. 



This also may be easily explained ; the negative electricity goes 

 gradually to the metallic plate, while the passage in the opposite di- 

 rection is instantaneous ; thus, a greater quantity of positive elec- 

 tricity returns at once to the metal plate, and passes more readily to 

 such points as lie beyond the image surface. 



To obtain an image with negative electricity, care has only to be 



