xxm 



experiments the conclusion that planets, even at a great distance 

 from the sun, might have atmospheres of such a character as to 

 maintain upon their surfaces sufficient solar heat for the maintenance 

 of life, such as we know it on the surface of oar earth. 



Tyndall's first paper communicated to the Royal Society on this 

 subject was received on May 26, 1859. This was, however, only a 

 preliminary note, and his first formal paper on the investigation 

 formed the subject of the Bakerian Lecture delivered on the 7th 

 February, 1861. In this lecture he enumerated the enormous diffi- 

 culties he had to contend with in devising methods by which trust- 

 worthy results could be obtained. He relates how, for seven weeks, 

 he worked from eight to ten hours daily, and had to abandon all the 

 results as liable to certain errors. It was only after much labour 

 and many failures that he constructed an apparatus which yielded 

 consonant and trustworthy numbers. Shortly summarised, the 

 results of this classical investigation may be thus stated: 1. Ele- 

 mentary gases scarcely absorb any perceptible amount of radiant 

 heat. 2. All compound gases absorb proportions varying directly 

 with the complexity of their molecules. Thus the vapour of ether was 

 found to absorb, for equal volumes at maximum density, 100 times 

 the quantity of radiant heat intercepted by the vapour of carbonic 

 disnlphide. The molecule of carbonic disulpbide vapour contains 

 only 3 atoms, whilst that of ether contains no less than 15. Never- 

 theless, the quality of the atoms constituting the molecule has also a 

 profound influence upon the absorptive power. Thus, carbonic acid 

 contains in its molecule the same number of atoms as carbonic di- 

 sulphide, but at a tension of 1'2 in. its absorption is represented by 

 the number 37, whilst the vapour of bisulphide of carbon at a tension 

 of only 1 in. is represented by the number 62. Again, ethylic borate, 

 which contains in its molecule no less than 25 atoms, has an absorp- 

 tive coefficient, at only O'l in. tension, represented by the number 620. 



Of all the molecules experimented upon, boric ethy late was the most 

 complex and exercised the most powerful absorptive effect upon 

 radiant heat from a source at 100 C. Whilst elementary and diffi- 

 cultly liquifiable gases exercise, as already stated, a scarcely sensible 

 absorptive effect, easily liquifiable elementary gases and vapours, like 

 those of chlorine and bromine, exert a very sensible action. Thus, 

 whilst oxygen, nitrogen, and hydrogen are represented by the absorp- 

 tion coefficient of unity, chlorine is represented by the number 60, and 

 bromine vapour, at the same tension, by 160. Compound molecules, 

 though no more complex than elementary ones, exert a much more 

 powerful absorptive action ; thus bromine and hydrobromic acid both 

 contain only 2 atoms in their molecules, nevertheless the absorptive 

 coefficient of hydrobromic acid is more than six times as great as that 

 of bromine. 



