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 



r O 



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, , 

 on the blackened bulb, 5, forms an image upon it, and 

 the differential thermometer yields a corresponding in- 

 dication. In trying different solutions the same trough 



*J O O 



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 e, 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, was in a 

 stratum of the same thickness, permitted much more 

 heat to pass. 



