TEANSMISSION OF HEAT. 809 



applying considerable pressure, or by combining both 

 means. Gases which have been actually liquefied are 

 called 'coercible;' those which have hitherto resisted 

 all attempts to liquefy them are called ' permanent ' 

 gases, but there is every reason to believe that these 

 gases, of which oxygen, hydrogen, and nitrogen may 

 be mentioned as instances, will be liquefied if the means 

 of producing great pressure and a very low temperature 

 which are at present at the disposal of physicists should 

 be still further increased. 



Ammonia, carbonic acid, and sulphurous acid are 

 coercible gases. The liquefaction of sulphurous acid, 

 which, as seen from the above table, has its boiling 

 point at 10. 8, is most easily effected. An adequate 

 lowering of temperature is quite sufficient for the lique- 

 faction of sulphurous acid; but in the case of ammonia 

 or carbonic acid great pressure must be resorted to. 



Experiments on the liquefaction of gases can only be undertaken 

 by experienced hands and with the help of suitably constructed 

 apparatus, because the tension of the vapours produced at ordinary 

 temperatures by the liquids obtained is so enormous (for example, in 

 carbonic acid it amounts to from 50 to 60 atmospheres'), that most 

 dangerous explosions may occur if the experiment is conducted by 

 unskilled hands. 



57. Transmission of Heat. Radiation. Conduction. 

 Heat may be transmitted from one body to another in 

 two ways. One body may send out 'heat ra}^s,' or 

 thermal rays, in all directions and excite heat in another 

 body at a distance without sensibly heating the interven- 

 ing space, just as 4 light rays ' are given out by luminous 

 bodies and pass through transparent substances. This 

 mode of transmitting heat is called radiation. Or heat 

 may be transmitted from one body to another by the 



