200 



EXPERIMENTS MADE IN VIRGINIA. [MEMOIR XIII. 



Fig ' 31 ' 



following apparatus was used. The mirror being placed 



on the shutter, as in Fig. 31, a 

 plano-convex lens, $, is screwed 

 into the tube so as to bring the 

 rays to a focus on one of the 

 bulbs of a delicate differential 

 thermometer ; this gives the heat 

 of the sunbeam as concentrated 

 by the lens. To find the effect 

 of any liquid medium in absorbing heat-rays, the trough 

 filled with the substance under trial is placed as at <?, 

 Fig. 31. The cone of rays, converging from the lens, a, 

 on the blackened bulb, &, forms an image upon it, and 

 the differential thermometer yields a corresponding in- 

 dication. In trying different solutions the same trough 

 is always to be used, so that the solutions may always 

 be of the same thickness. It is also requisite to cover 

 the thermometer with a very thin shade of clear glass, 

 e 0, to prevent disturbance from air-currents. 



I may select as examples the following : A solution of 

 sulphate of copper and ammonia, absorbing the red and 

 yellow light, transmitted twenty rays out of every hun- 

 dred that fell on it. 



A thin stratum of pitch enclosed between two plates 

 of crown-glass, and transmitting a homogeneous red 

 light, but absorbing all the other colors of the spec- 

 trum, allowed only 19 per cent, of the heat to pass 

 through it. 



In examinations of the transmissive powers of vapors 

 and gases, a cubical bottle was used instead of the 

 trough. The vapor of iodine was thus found to absorb 

 two thirds of the heat falling on it ; but the same bottle 

 filled with nitrous acid, and which, therefore, \vas in a 

 stratum of the same thickness, permitted much more 

 heat to pass. 



