OF RADIANT HEAT THROUGH DIFFERENT BODIES. 25 



as the losses suiFered by the rays in successively passing through the four 

 plates of glass ; it being carefully kept in mind that these values are not 

 referred to the initial quantity, but to the number of rays which arrive 

 at the surface of each screen. 



Thus the proposition of Delaroche is true as far as the third and the 

 fourth screens ; for in the transition from one loss to another a dimi- 

 nution of each loss is observable. 



It will have been observed that the losses were not so great in re- 

 spect to the four equal layers of the screen of a fourfold thickness ; and 

 that this should happen will be easily conceived if we consider that in 

 the latter case there is a solution of continuity which causes a greater 

 dispersion of the heat by reflexion. But we see that in both cases the 

 difference between two successive losses becomes less in proportion as 

 the distance from the surface, at which the rays entered, is greater. 



Let us now proceed to consider the influence exercised on calorific 

 transmission by the composition of the substance of the screen. 



M. Prevost had concluded from the experiments described in a me- 

 moir already quoted, that water and glass ought to transmit rays of heat 

 in different quantities ; for by causing the sheet of water to fall between 

 the lighted candle and a very delicate air-thermometer, he obtained no 

 indication of heat being transmitted unless when he had blackened the 

 ball of the thermometer, and even then the increase of temperature was 

 extremely small ; whilst a plate of glass substituted for the sheet of water 

 produced effects sufficiently manifest*. But it was objected to him that 

 the difference between the action of the water and that of the glass was 

 owing to the conductible caloric which was perceptible in the latter 

 case only. Delaroche subsequently observed that a square of gi-eenish 

 glass transmitted more heat than a plate of another species of glass 

 perfectly pure. However, as the first flake was much thinner than 

 the second, it was insisted that the difference in the effects was owing 

 to the difference of thickness. At length, some time after the in- 

 vention of the thermomultiplier, M. Nobili and myself made some ex- 

 periments on olive oil, alcohol, water, and nitric acid ; whence we in- 

 ferred that water opposed a greater resistance than any of the three 

 other liquids did to the passage of rays of heat emanating from a hot 

 ironf . But these experiments are to be regarded only as mere trials, 

 tending to show the facility with which the thermomultiplier may be 

 employed in all sorts of inquiries relative to calorific radiation ; for we 

 did not take sufficient precautions to prevent the heat from passing by 



• His own words are : " It appears, therefore, that water does not allow so 

 much caloric to pass immediately as glass does. At least it affords a passage of 

 thai kind only to a quantity of caloric more minute than that which passes 

 through tile glass." (Mem. already (juotcd, § 48.) 



t Sec the note in page 1. 



