RECENT PROGRESS IN PHYSICS. 433 



with a smooth body, and under the microscope had the appearance 

 of scratches on glass with rough sand. When tested by the electrome- 

 ter, the points of contact being held between the fingers, it was found 

 that the glass at the marked, as well as at several unmarked places, had 

 become conducting. By breathing on the plate all the conducting 

 places became visible, they remained unmoistened and showed more 

 or less numerous ramifications ; even after the glass plate had been 

 washed with nitric acid and dried, the stripes appeared to conduct. 



Other glass plates gave exactly similar results. 



With mica the appearance of the electrical marks was quite dif- 

 ferent. A serpentine stripe, of uniform width, passed from the point 

 of contact on both surfaces to the place of puncture, which, by trans- 

 mitted light, was light gray in color, but in oblique reflected light 

 appeared as a delicately colored band, bounded by two sharply defined 

 dark lines, bordered by a clear brilliant fringe ; the inner part of the 

 band, between the fringe, contained blurred zones of yellow, blue, red 

 and green colors. 



The pieces of mica used in this experiment were good insulators 

 both before and after use, though when breathed upon they appeared 

 covered with innumerable reticulated ramifications, which were not 

 moistened, indicating the places where the electricity had touched the 

 surface. 



There was no essential difference between the two surfaces of the 

 plate of mica, either in regard to the colored stripes, or the reticulated 

 figures. 



The electricity appeared to penetrate only by a sort of crack into 

 the substance of the glass, and even to separate the alkali, which was 

 indicated by the circumstance, that the injured places became more 

 perceptible after a while, than immediately after the experiment. 



A plate of mica having been smeared with oil, a discharge, which 

 without the oil would have produced colored stripes, penetrated it at 

 the place of contact. An irregular hole ffppeared with fused edges, 

 about which there was a slight splitting of the mica. 



By careful diminution of the electrical accumulation, R less ohtained 

 repeatedly, in spite of the coating of the oil, discharges without pene- 

 trating, and colored stripes of considerable length and size towards 

 the edge of the plate, or towards a previously pierced place, which 

 seemed to indicate that the mica conducts electricity better in the 

 direction of its lamina than perpendicular to them. 



In general the electrical marks on glass and mica are altogether 

 dissimilar, though there are kinds of glass which, at their surface 

 conduct electricity quite well, on which stripes appear similar to those 

 on mica, 



§ 42. The aiu thermometer. — The air thermoneter, which Bies» 

 used in his researches, is represented in fig. 53, from an instrument 

 made by Kleiner of Berlin. 



28 s 



