Prof. TyndalFs Contributions to Molecular Physics. 525 



from the deportment of this substance its incompetence to enter 

 into rapid molecular vibration. When, however, we once quit 

 the visible spectrum for the rays beyond the red, the opacity of 

 the substance begins to show itself: for such rays, indeed, its 

 absorbent power is unequalled. The synchronism of the periods 

 of the water-molecules with those of the extra-red waves is thus 

 demonstrated. I have already proved that undried atmospheric 

 air manifests an extraordinary opacity for the radiation from a 

 hydrogen-flame, and from this deportment I inferred the syn- 

 chronism of the cold vapour of the air and the hot vapour of 

 the flame. The vibrating-period of a molecule is, no doubt, 

 determined by the elastic forces which separate it from other 

 molecules, and it is worth inquiring how these forces are affected 

 when a change so great as that of the passage of a vapour to a 

 liquid occurs. The fact established in the earlier sections of this 

 paper, that the order of absorption for liquids and their vapours 

 is the same, renders it extremely probable that the period of 

 vibration is not materially affected by the change from vapour to 

 liquid ; for, if changed, it would probably be changed in different 

 degrees for the different liquids, and the order of absorption 

 would be thereby disturbed*. The following Table, in which 

 the deportment of our series of liquids towards the radiation 

 from a hydrogen-flame is recorded, will throw additional light 

 upon this question : — 



Table XXXIX. — Radiation through Liquids. Source, 

 hydrogen-flame. Thickness of liquid layer O07 of an inch. 



Name of liquid. Absorption. Transmission. 



Bisulphide of carbon . . 27'7 72'3 



Chloroform 49-3 507 



Iodide of ethyle ... 75 -6 24-4 



Benzole 823 17*7 



Amylene 87"9 12-1 



Sulphuric ether . . . . 926 7'4 



Formic ether .... 93*5 6*5 



Acetic ether 93-9 6-1 



Water lOO'O ^00 



Through a layer of water 9*21 millimetres thick, Melloni 

 found a transmission of 11 per cent, of the heat of a Locatelli 

 lamp. Here we employ a source of higher temperature, and a 

 layer of water only one-fifth of the thickness used by Melloni, 



* The general agreement in point of colour between a liquid and its 

 vapour favours the idea that the period, at all events in the great majority 

 of cases, remains constant when the state of aggregation is changed. 



