Mathematics and General Phy.sics. 355 



pcrty of the light than any separate agent. The length of the 

 undulations which Professor Forbes has shewn may by analogy 

 be calculated, would belong to this species of radiant heat, and 

 be simply such undulations in the etherial medium as are too 

 large to affect our visual organs ; whilst the other portion may 

 very probably be, as Sir J. Leslie conjectured, merely a convey, 

 ance of heat by the air ; this would agree with what is observed 

 of its rapidly diminishing as the distance increases, and being 

 incapable of permeating screens except by conduction ; this, 

 perhaps, may account for the irregularities found by Sir J. Les- 

 lie, in the position of the forces of a reflector for heat ; whereas 

 the other sort is conveyed unimpaired to the same distances as 

 the light, and only excited when it is absorbed. A vacuum ab- 

 solutely perfect, demonstrably, can never be formed with the 

 air-pump, and the Torrecellian, even if free from air and the va- 

 pour of mercury, is not large enough for satisfactory experi- 

 ments, but even if it should be contended that the radiation 

 through vacuum has been proved, we must now say to which 

 sort of heat it applies. By this distinction the theories of Les- 

 lie, De la Roche, and others, may very probably be brought in- 

 to accordance, since they were perhaps not always speaking of 

 the same thing when they spoke of radiant heat. 



Another question of importance which has occurred to the 

 author is this : — Whether, in the polarization apparatus, sup- 

 posing one glass, or pile of mica, heated, it will radiate the same 

 quantity of heat to the other in the two rectangular positions ? 

 The question is purely a mathematical one, and has been in 

 some degree considered at the author's suggestion by IMr JMur- 

 phy of Cambridge ; the integration has not been completed, but 

 Mr Murphy thinks it clear that there will be a difference. 



4. Dr H Hudson read a paper on the phenomena usually 

 classed under the denomination of radiation of heat. 



He exhibited experiments with the reflecting and differential 

 thermometer, to illustrate the radiating powers of surfaces, in- 

 terposing occasionally screens with plain and coated surfaces. 

 He tried the diathermancy of rock salt, and shewed several ex- 

 periments with Melloni's thermo-multiplier, in which the effects 

 were much smaller than those of Melloni. 



5. Captain Sir John Ross detailed a new theory of the aurora 



