340 On the Objections to Mr. Prevos^s Explanation [May, 



Pictet's explanation of the indication of decrease of tempprature in 

 the focus of the reflector, apply with equal force in cases of cooling 

 by direct radiation and absorption ; " which is certainly true. 



I will now describe also the experiment I have above alluded to 

 with the metallic cone. 



A metallic conical tube of about 18 inches in length, whose 

 respective apertures were one inch and five inches (the interior bei»g 

 polished so as to reflect powerfully and radiate little), being sup- 

 ported in a horizontal position, one ball of a diflertntial thermo- 

 meter was presented to the larger aperture, while a bottle of ice 

 was presented at the smaller aperture. The liquor in the tube of 

 the thermometer rose very little towards the bulli nearesr the aper- 

 ture, sligluly indicating a lower temperature. The relative situa- 

 tions of the thermometer and the ice bottle being reversed, (i.e. the 

 ice carried to the large end, and the bulb of the thermometer 

 placed before the smaller end,) the liquor rose much faster towards 

 the bulb, indicating in this position a much more rapid decrease of 

 temperature. 



In this case it was argued that since the rays admitted at the wide 

 opening were brought to a focus at the smaller one, if the thermo- 

 meter at the wide end radiated caloric, its rays must be lost upon 

 the ice in the focus, and it ought to indicate a loss of temperature ; 

 which was not the case, or ouly in a very small degree : but that 

 when transposed, as this was the only focus (the experiment in that 

 respect diftering from that of the mirrors), the loss of temperature 

 which it so rapidly indicated could arise only from a collection of 

 actual cold rays in the focus ; and that this experiment constituted 

 another objection to Mr. Prevost's theory. 



This objection I shall endtavour to remove with the rest. 



It will be convenient first to consider in what way a focus of rays 

 (whether of light, or of heat, or of cold,) is produced. 



Every point of a Luviinous body emits rays, proceeding in every 

 direction in straight lines diverging from that point at all angles. 

 Different parts of the same luminous body emit rays which cross 

 each other at, all angles; but which, after a few reflections, are 

 absorbed, or dissipated into boundless space. But since every body 

 possesses free caloric, more or less, and every point of these emits 

 rays of caloric, as the points of luminous bodies do those of light, 

 it follows that every body is surrounded on all sides by radiating and 

 by receiving points ; and may therefore be considered as surrounded 

 by a sphere, from every point of which rays proceed and fall on it : 

 and since the intensity is inversely as the squares of the distances, if 

 the temperature of the parts of which the whole imaginary sphere 

 consi--its is uniform, it receives equal quantities on all sides,- 

 alihongh some of the surrounding bodies are much nearer than 

 oihers ; because the quantity of surface of distant bodies, intercepted 

 by the intervening near bodies, is conversely in the same ratio ; 

 i. e. as the squ-res of the distances. Thus if a circular screen, 

 whose area we will suppose eoual to 36 sqy«re inches, be held at 



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