40 ABSORPTION OF RADIANT HEAT PAKT i. 



gas was admitted produced a deflection of 75 or 360 

 units, consequently more than T Vfchs or 81 per cent, of 

 the whole heat was cut off by the olefiant gas. Such 

 opacity to heat in so transparent a gas is quite marvel- 

 lous. A current of it was sent into the open air between 

 the thermo-electric pile and one of the sources of heat, 

 and although it was perfectly invisible, it instantly 

 deflected the needle of the goniometer from to 41. 



In order to ascertain the relation between the density 

 of the gas and the quantity of heat extinguished or 

 absorbed, an ordinary mercurial gauge was attached to 

 the air-pump. The experimental tube was exhausted, 

 and the needle of the goniometer stood at zero. Then, 

 from a graduated glass vessel, measures of olefiant gas, 

 each amounting to the ^th of a cubic inch, were succes- 

 sively sent through the drying pipe into the exhausted 

 tube. The amount of the heat absorbed and the de- 

 pression of the mercurial column corresponding to each 

 measure of gas as it was introduced, was registered 

 from one to fifteen measures. This experiment showed 

 that for very small quantities of gas, the absorption is 

 exactly proportional to the density or tension. One 

 measure of the gas only produced a depression of the 

 mercurial column amounting to the -g-^yth part of an 

 inch, or about the -f^th ^ a millimetre. 



In many of the vapours of volatile liquids, the pre- 

 ceding law only prevails to a certain amount of pressure 

 differing in each case, beyond which increase of tension 

 produces diminished effects. In sulphuric ether the 

 change begins at the eleventh term. 



In bisulphide of carbon the law changes after the 

 sixth measure, &c. 



In order to adapt the apparatus for experiments on 

 coloured gases, a glass experimental tube 2 ft. 9 in. long, 

 and 2 ft. 4 in. in diameter, was substituted for the brass 

 tube, and, instead of boiling water, sources of radiant 



