RADIANT HEAT. 331 



arise out of the slightest survey of the present state of our knowl- 

 edge, and on which accurate determinations are wanted before we 

 can be said to possess even the elements of a scientific theory. 



May it not be the law that if a body be placed in the rays from a 

 source of heat it will be acquiring and giving out heat till the inten- 

 sity of radiation at the points before and behind it resumes its original 

 proportionality ? 



The time in which this takes place will depend on the extent of the 

 bod}', Us thickness, its conduct in </ power, its capacity for heat, and the 

 state of both its surfaces. 



These may be such that the effect may be sensibly instantaneous, 

 and the radiation therefor appear to go on without interruption. In 

 this case, also, the distance of the screen from the source (within 

 moderate limits) may make no sensible difference, though if any of 

 the above circumstances retard the effect to a sensible amount, then 

 there will be a difference with the variation of distance. In this way 

 we may, as it were, regard the medium between the source and the 

 thermometer as merely a compound, of which the screen is one por- 

 tion and the air the other. 



Another class of questions respecting which little, if anything, is 

 accurately known may be put with regard to the modification (if any) 

 which radiant heat may undergo in passing through small apertures. 

 This will again be connected with the interceptive power of net-tcorl\ 

 Avery curious and delicate subject of inquiry is the repulsion exerted 

 between heated bodies at sensible distances, of which a short notice 

 is given in the Quarterly Journal of Science, xxxix, 164. 



The reflection of heat has been little examined, except in the single 

 case of its concentration by spherical reflectors; and here (according 

 to Leslie) it is not brought to the same focus as light; this requires 

 examination, as well as the simpler case of plane surfaces, and the 

 proportion of heat reflected at different incidences. There will prob- 

 ably in all cases be a very large deduction to be made for the heat 

 acquired by the reflector and radiated again. 



But another class of such questions yet remains in connexion with 

 that fundamental point which was the object of my first inquiries. 

 The conclusion from my experiments, viz: that luminous hot bodies are 

 sending forth at the same time tiro distinct species of heat distinguished 

 by different properties, is the unavoidable conclusion from the experi- 

 ments, depending on the mathematical truth, that if a ratio be altered 

 by the addition or subtraction of quantities from its terms, the quan- 

 tities added or subtracted must be in a different ratio from the origi- 

 nal one. I here repeat this, because the nature of the reasoning has 

 not been perceived by some persons. This conclusion undoubtedly 

 introduces a complexity into the view we must take of the phenomena; 

 whereas, if we were at liberty to adopt the simpler theory of De la 

 Roche and others, many of the apparent anomalies would be recon- 

 ciled. Hence the verification of my results becomes a point of con- 

 siderable importance. If any experimenter with more accurate ap- 

 paratus shall succeed in showing them to bo erroneous, he will achieve 



