178 Prof. Tyndall on the Absorption and 



their absorptive powers, though I have made several hundred 

 experiments with the view of doing so. Their proper action is 

 so small that the slightest foreign impurity gives one a predomi- 

 nance over the other. In preparing the gases, I have resorted 

 to the methods which I found recommended in chemical trea- 

 tises, but as yet only to discover the defects incidental to 

 these methods. Augmented experience and the assistance of 

 my friends will, 1 trust, enable me to solve this point by and by. 

 An examination of the whole of the experiments induces me to 

 regard hydrogen as the gas which exercises the lowest absorp- 

 tive power. 



We have here the cases of minimum gaseous absorption. It 

 will be interesting to place in juxtaposition with the above results 

 some of those obtained with olefiant gas — the most highly ab- 

 sorbent permanent gas that I have hitherto examined. I select 

 for this purpose an experiment made on the 21st of November. 

 The needle being steady at zero in consequence of the equa- 

 lity of the actions on the opposite faces of the pile, the admis- 

 sion of olefiant gas gave a permanent deflection of . . 70 o, 3. 

 The gas being completely removed, and the equilibrium re- 

 established, a plate of polished metal was interposed between 

 one of the faces of the pile and the source of heat adjacent. 

 The total amount of heat passing through the exhausted tube 



was thus found to produce a deflection of 75°. 



Now a deflection of 70°'3 is equivalent to 290 units, and a 

 deflection of 75° is equivalent to 360 units ; hence more than 

 seven-ninths of the total heat was cut off by the olefiant gas, or 

 about 81 per cent. 



The extraordinary energy with which the needle was deflected 

 when the olefiant gas was admitted into the tube, was such as 

 might occur had the plates of rock-salt become suddenly covered 

 with an opake layer. To test whether any such action occurred, 

 I polished a plate carefully, and projected against it for a con- 

 siderable time a stream of the gas ; there was no dimness pro- 

 duced. The plates of rock-salt, moreover, which were removed 

 daily from the tube, usually appeared as bright when taken out 

 as when they were put in. 



The gas in these experiments issued from its holder, and had 

 there been in contact with cold water. To test whether it had 

 chilled the plates of rock-salt, and thus produced the* effect, T 

 filled a similar holder with atmospheric air and allowed it to 

 attain the temperature of the water; but its action was not 

 thereby sensibly augmented. 



In order to subject the gas to ocular examination, I had a 

 glass tube constructed and connected with the air-pump. On 

 permitting olefiant gas to enter it, not the slightest dimness or 



