104 



ABSORPTION OF HEAT AND CHEMICAL RAYS. 



the yellow light, being operated upon in like manner, was found to transmit twenty 

 rays out of every hundred that fell upon it. 



404. There is, however, considerable difficulty in obtaining these numerical results 

 with accuracy, arising partly from the difficulty of obtaining specimens of exactly the 

 same composition, but more especially owing to changes taking place in their colour. 

 In process of time, most vegetable solutions undergo spontaneous changes, and no lon- 

 ger give the same results. But, where the same sample is operated on, under the same 

 circumstances, repeated experiment assures me that this arrangement gives comparable 

 indications. 



405. Vapours and gases may also be put under trial. The vapour of iodine, the 

 spectrum of which is remarkable as containing only the extreme rays, and wanting those 

 of medium refrangibility (fig. 49), absorbs two thirds of the heat that impinges on it. 

 The vapour of nitrous acid, which stops the violet, blue, indigo, and yellow light (fig. 

 51), has a similar effect on the heat. To experiment upon these bodies, a cubical bot- 

 tle (fig. 52) is very convenient to generate the vapour in, and also to transmit the 

 light through ; it will then replace the trough of (389). Nitrous acid vapour is best 

 made for these purposes from nitrate of lead. 



406. Having prepared a variety of solutions for the purpose of experiment, and using 

 for each the same trough, thoroughly cleansed after each trial, the following table will 

 give an estimate of the results obtained; it is arranged according to the power of each 

 solution, the first on the list being the most energetic : 



TABLE OP THE THERMO-ABSORPTIVE POWER OF SOLUTIONS. 



Decoction of logwood in alum water, 



Solution of sulphate of copper and ammonia, 



Litmus water, 



Decoction of Brazil wood, 



Decoction of cochineal, 



Solution of tannin, 



Solution of chloride of chromium, 



Tincture of turmeric, 



Tincture of saffron, 



Sulpho-cyanate of iron, 



Hydro-sulphate of ammonia, 



Muriate of cobalt, 



Bichromate of potassa, 

 Hydro-sulphate of lime, 

 Muriate of iron, 

 Oil of turpentine, 

 Prussiate of potassa, 

 Sulphate of copper, 

 Chloride of platinum, 

 Chloride of gold, 

 Oil of bergamot, 

 Linseed oil, 

 Nitrous ether, 

 Water. 



407. Still more powerful effects are produced by making binary or ternary arrange- 

 ments. If, for instance, a beam of the sun falls upon a very thin, transparent stratum of 

 pitch, and then passes through a solution of sulphate of copper and ammonia, or through 

 linseed oil, not more than one fortieth part of the caloric is transmitted. 



408. A question here naturally arises : What becomes of the heat thus lost ? does 

 it enter into such a combination with these media as to be detected in them by the 

 thermometer as sensible heat ? This is a question of much difficulty ; there are so 

 many disturbing causes in operation, the general results of experiment have not yet 

 furnished actual proof that the heat missing is to be found in the fluid solutions. I 

 would not, however, be understood to deny that such is the case ; only that, at present, 

 our information does not warrant such a conclusion. It might be supposed that these 

 solutions do not act by a proper absorptive power, but merely offer that kind of obstacle 

 to the transmission of heat that turbid media do to light. Not only, however, does 



