4'8 M. MELLONI OX THE IMMEDIATE TRANSMISSION 



against the principle as the zero is never followed by appretiable trans^ 

 missions. 



The same principle holds in respect to all the liquids that I have been 

 able to submit to experiment. It will be recollected that, in my mode of 

 operating, the rays of heat, before they reach the liquid layer, must pass 

 through a plate of glass. Now this substance becomes more and more 

 interceptive in proportion as the sources employed are of a less elevated 

 temperature, and consequently acts upon the calorific rays with an effect 

 the same as that which a screen of variable transparency would produce 

 in respect to light. The process therefore which I pursued in my first 

 Memoir could not enable me to determine the exact ratios of the ca- 

 lorific transmissions through the same liquids when the source is changed ; 

 but it was possible to make it available for the purpose of establishing, 

 in the greatest number of cases, the general law of decrease which we 

 have just determined in respect to solid bodies. 



Let us suppose that a thick plate of glass being submitted to the suc- 

 cessive action of an equal quantity of heat, emanating from our four 

 sources, gives these transmissions : 



30, 18, 2, 0. 



Let us suppose a parallelopiped, with sides parallel to the faces of the 

 plate, to be cut out of the glass, and the cavity thus made to be filled 

 with a given liquid : let us then suppose that the transmissions of the 

 system become all respectively inferior to the preceding, and are re- 

 duced, for instance, to 



20, 8, 1, 0, 



it will be immediately concluded that the liquid acts on the calorific rays 

 from different sources in the same manner in which its glass case does ; 

 that is, that it exhibits an order of decrease similar to that exhibited 

 by the glass and by solid bodies in general. Now this is precisely the 

 result furnished by the liquids contained in my glass vessels *. 



• la many instances I was unable to obtain any transmission, even by em- 

 ploying a very powerful radiation. It is thus that water, which transmits six 

 or seven hundredths of the rays from a Locatelli lamp, completely intercepts the 

 heat of the last three sources. Calculating the limit of error for the case least 

 favourable to interception I found it -^-^ : the source was then brought very close 

 to the liquid and an equal layer of oil employed, which caused in the index of 

 the galvanometer a deviation of several degrees. Now if the water allows a 

 passage to the radiation from bodies heated even to incandescence or brought 

 to lower temperatures, the part transmitted must be less than ^5- of the incident 

 quantity. I here speak of a layer of 3""" or 4""" in thickness : for it is possible 

 and even very probable that layers much thinner than these may be in some 

 slight degree permeable to rays of this kind. Thus we have seen glass of 0"""-07 

 in thickness transmit -rinT of the rays emanating from boiling water, while a 

 plate of l""" intercepted them totally. But as, in order to compare different 

 transparencies, we must operate on a certain thickness of each medium (for the 



