282 Prof. Tyndall on the Absorption and 



of matter. Starting from the idea that a gas absorbs such 

 vibrations as are isochronic with its own, in all cases the com- 

 pound gas reveals itself to the mind's eye with its molecules on 

 the whole swinging more slowly than the uncombined atoms of 

 which it is composed. Their absorption of the longer undula- 

 tions proves their general coincidence in period with those undu- 

 lations. We load the atom by the act of chemical union, and 

 thereby render its vibrations more sluggish, that is to say, more 

 fit to synchronize with the slowly recurrent waves of obscure 

 heat. 



In the foregoing Table I have given the absorption of nitric 

 oxide as 1590, which is less than that of nitrous oxide, though 

 the molecule of the former contains a greater number of atoms 

 than that of the latter. It will be noticed that those gases which 

 on combining suffer no condensation are less energetic absorbers 

 than those which suffer a reduction of volume. Whether this rule 

 is universal I am as yet unable to say. 



It is very difficult to operate with nitric oxide ; the affinity of 

 the gas for oxygen is so enormous that the slightest trace of this 

 substance gives rise to the brown fumes of nitrous acid. On first 

 sending this gas into the experimental tube, 1 inch of it gave an 

 absorption of 2040 ; but the needle slowly went up afterwards, 

 until it finally indicated an absorption of 5100. On looking 

 across the tube at this time, the brown hue of nitrous acid was 

 discernible. 



In a second experiment I made the vacuum as perfect as 

 possible ; on allowing nitric oxide to enter, the absorption was 

 1860, but the needle soon afterwards declared an absorption of 

 3060, the brown fumes appearing as before. 



On filling the experimental tube with nitrogen, then exhaust- 

 ing, and allowing nitric oxide to enter, the absorption of 1 inch 

 of the gas was 1680. On filling the experimental tube pre- 

 viously with hydrogen the absorption was 1590, which is that 

 given in the Table. On letting in a mixture of air and nitric 

 oxide till the tube was filled, the action last mentioned was 

 augmented nearly twentyfold. Nitrous acid is therefore an 

 extremely energetic gas. The difference between it and bro- 

 mine is enormous, when the colours of both are equally dense. 



A close inspection of Melloni's Table* reveals, I think, the 

 tendency of solid bodies also to become more transparent to heat 

 as their composition becomes more simple. After rock-salt 

 itself, comes the element sulphur, and after it fluor-spar. But 

 the case of lampblack will here occur to many, who regard it as 

 the most powerful absorber and radiator yet discovered. No 

 doubt the grouping of the atoms of an elementary substance may 



* La Thermochrose, p. 164. 



