242 HEAT. 



principle bodies naturally vibrating in a given period are set vibrating 

 by receiving waves of that period from different sources. 



2. Bodies exchanging Radiations at different Temperatures. 



We can apply what we have learnt from uniform-temperature 

 enclosures to cases of unequal temperature by the aid of two general 

 principles : 



(a) Bodies in the same physical state continue to absorb the same kind 

 of rays independently of change of temperature. 



The experimental evidence in favour of this is that bodies keep the 

 same colour through wide ranges of temperature, that is, they absorb the 

 same constituents of the radiation falling on them. If they change 

 colour, we usually find some evidence of chemical action or of change of 

 molecular aggregation, as when yellow phosphorus changes to red phos- 

 phorus, a change which might almost be considered chemical. 



(6) The radiation of every kind emitted from a body increases as the 

 temperature rises, as long as the physical state remains the same. 



We may verify this for light rays by heating an opaque body till it 

 becomes incandescent. When first visible, it only gives out the longest red 

 rays in sufficient quantity to be seen and these only in small quantity ; but 

 as the temperature rises, the spectrum gradually extends towards the blue 

 till the body becomes white-hot, when it gives out a full spectrum. But 

 as fresh rays are added to the spectrum, those previously existing become 

 stronger the red, for instance, being much more intense than it was 

 when the body first began to glow. The same holds good for the dark 

 long wave radiations, as has been shown for blackened copper by Langley. 

 He investigated the wave-lengths emitted at various temperatures from 

 40 C. up to about 800 0., and found that, as the temperature rose, the 

 spectrum became extended towards the shorter wave-lengths, and that 

 the previously existing radiations also became more intense. 



In the case of solids and liquids, the spectrum is always more or less 

 continuous, and we may lay down the general rule that, at low tempera- 

 tures, only the very long wave-lengths are sensible, but that, as the 

 temperature rises, the spectrum gradually extends to the shorter wave- 

 lengths, and ultimately comes into the visible range. 



Gases, however, only emit, at least in the visible spectrum, waves of 

 definite length or in definite groups, and their spectra consist of bands or 

 lines. As the temperature rises, these bands or lines become brighter. 

 We may easily show this by comparing the sodium flame in a spirit-lamp 

 with the hotter sodium flame in a Bunsen burner. 



Application of the foregoing Principles to Special Cases. We 



know that a black surface such as lampblack absorbs nearly all the radia- 

 tions which fall upon it. If we raise its temperature, it will still continue 

 to absorb. It must, therefore, also emit rays of nearly all kinds, and at a 

 given temperature it will emit more radiation than other bodies, or when 

 incandescent will shine more brightly. In illustration of this, if a piece 

 of platinum-foil with inkmarks on it, which will withstand heat, is heated 

 to incandescence, the inkmarks shine more brightly than the rest of the 

 foil. If a piece of white porcelain has a dark pattern on it, the white 

 porcelain reflects a great part of the light-radiations falling on it, and the 



