on the Surfaces of Solid Bodies. 247 



A plate of copper 1*5 millim. in thickness was first used. 

 Immediately on lighting the candle, after the lapse of an interval 

 of time which was hardly measurable, the deflection began. 

 Whereupon many such plates were employed, being placed one 

 close to the other up to eight, and which together formed a layer of 

 12millims. in thickness. The deflection began in about one minute. 

 After ten minutes the light was extinguished. At this time the 

 deflection amounted to from 150 to 200 divisions of the scale. 

 Thereupon this increased still during the following ten minutes 

 from 100 to 200 divisions further, and then began to decline. 

 When afterwards the plates, separated from one another, were 

 disposed as is figured (fig. 4), whereby their distances from 

 middle to middle of the plate amounted to 15 millims., and the 

 light stood at 25 millims. from the outermost plate, then the de- 

 flection commenced in little more than one minute after lighting 

 the candle. Yet in this instance, after the lapse of ten minutes 

 after extinguishing the candle, it amounted to 40 or 50 divisions 

 of the scale. After this it increased gradually for half an hour, 

 until it reached about 30 divisions more. 



Plates of wood behaved just like the copper plates. On taking 

 a board of beechwood of 27 millims. in thickness, the deflection 

 of the galvanometer began in two minutes after the lighting of 

 the candle. In nine minutes it amounted to 750 divisions of the 

 scale. On putting out the light, the deflection increased yet 

 during the next three minutes about 40 divisions, and then 

 became so great that the scale did not admit of further observa- 

 tion of it. After an hour the deflection was still 200 divisions. 

 Even through a beechen board of 65 millims. in thickness the 

 propagation of heat was still very plainly to be made out. The 

 deflection began eight minutes after lighting the candle, and 

 attained in ten minutes after the light had been extinguished 28 

 divisions on the scale, and still continued to increase until, 

 after the lapse of half an hour, it had attained to 290 divisions. 

 Not alone with the thermo-pile, but also with the above- 

 described air-thermometer could the passage of heat through 

 many plates be perceived. In a large box of pasteboard there 

 was a partition with an opening in it, and before the opening 

 two very thin plates of brass-foil were fastened at a distance of 

 15 millims. from one another. On one side of the same the 

 light was fastened at a distance of 60 millims. from the nearest 

 plate. On the other was placed the air-thermometer, of which 

 the blackened bulb was as near as possible to the other plate 

 without actually touching it. The height of the thermometer 

 was read off through an opening in the box by means of a tele- 

 scope. Shortly after lighting the candle the blackened bulb 

 began to rise in temperature, whereby an alteration of about 4 



