Radiant Heat to Aqueous Vapour, 31 



certain to have numerous and important applications. I there- 

 fore thought it right to commence my investigations this year 

 with a fresh series of experiments upon atmospheric vapour, and 

 I now have the honour to lay the results of these experiments 

 before the Royal Society. 



Rock-salt is a hygroscopic substance. If we breathe on a 

 polished surface of rock-salt, the affinity of the substance for the 

 moisture of the breath causes the latter to spread over it in a 

 film which exhibits brilliantly the colours of thin plates. The 

 zones of colour shrink and finally disappear as the moisture 

 evaporates. Visitors to the International Exhibition may have 

 witnessed how moist were the pieces of rock-salt exhibited in the 

 Austrian and Hungarian Courts. This property of the substance 

 has been referred to by Professor Magnus as a possible cause of 

 error in my researches on aqueous vapour ; a film of brine depo- 

 sited on the surface of the salt would produce the effect which I 

 had ascribed to the aqueous vapour. I will, in the first place, 

 describe a method of experiment by which even an inexperienced 

 operator may avoid all inconvenience of this kind. 



In the Plate which accompanies my former paper, the thermo- 

 electric pile is figured with two conical reflectors, both outside 

 the experimental tube ; in my present experiments the reflector 

 which faced the experimental tube is placed within the latter, its 

 narrow aperture, which usually embraces the pile, abutting 

 against the plate of rock-salt which stops the tube. Fig. 1 is a 

 sketch of this end of the experimental tube. The edge of the 



Pig. 1. 



inner reflector fits tightly against the interior surface of the tube 

 at a b ; cd\s the diameter of the wide end of the outer reflector, 

 supposed to be turned towards the " compensating cube " situated 

 at C f *. The naked face of the pile P is turned towards the plate 

 of salt, being separated from the latter by an interval of about 

 ^ n th of an inch. The space between the outer surface of the 

 interior reflector and the inner surface of the experimental tube 

 is filled with fragments of freshly-fused chloride of calcium, 



* I here assume an acquaintance with my two last contributions to the 

 Philosophical Transactions, in which the method of compensation is de- 

 scribed. [These memoirs will be found in vols. xxii. and xxiv. of the 

 Philosophical Magazine.] 



