512 Mr. A. Cayley on a Relation between 



thermometer introduced into the interior ultimately assumed. 

 Of course the experiments with this apparatus were not made 

 without numerous precautions ; it was more particularly neces- 

 sary that the whole apparatus should be always under the same 

 conditions, so as to give off the heat imparted to it always in the 

 same manner. For this it was necessary that the space sur- 

 rounding it should always be at the same temperature. In 

 these experiments the temperature of the surrounding space was 

 15 degrees. 



In this way the following results were obtained : — 



1. The temperature which a thermometer ultimately assumes 

 in a space heated from above, differs when this space is filled 

 with different gases. 



2. In hydrogen the temperature is higher than in any other 

 gas. 



3. In this gas the temperature is higher than in vacuo ; and 

 the denser the gas is, the higher is the temperature. 



4. Hence hydrogen conducts heat like metals. 



5. In all other gases the temperature is lower than in vacuo ; 

 and the denser they are, the lower is the temperature. 



6. It cannot hence be concluded that gases do not conduct 

 heat, but only that they do this in so small a degree that the 

 action of conduction is cancelled by their diathermancy. 



7. This remarkable property of hydrogen is evinced not only 

 when it moves freely, but also when it is contained between 

 eider down, or any loose substance which hinders its motion. 



8. The great conductibility of this gas is a further confirma- 

 tion of its analogy with metals. 



9. Hydrogen conducts not only heat, but also electricity, 

 better than other gases. 



LXIX. On a Relation between two Ternary Cubic Forms, 

 By A. Cayley, Esq.* 



THE cubic form 

 a^ + J^-f s 3 + Qlxyz 



is in general linearly transformable into the form 



(X + Y + Z) 3 + 27AXYZ. 

 In fact, writing 



X=2fo-y-*, 



Y = 2/y-<2r-o?, 



Z = 2lz-sc— y, 



* Communicated by the Author. 



