Imperial Instifuie of France. 67 



On the contrary, if light is only a motion impressed on 

 air. by the vibrations of the bodies which burn, iis quantity 

 will be in proportion not to ihe quantity of this body which 

 shall have been bnrned, hut to the vivacity with which the 

 combustion shall have been efiected, and particularly at the 

 time that each of its particles shall remain healed at the de- 

 gree proper for giving an impulse to those of air. 



Havins founded his experiments upon these ideas, with 

 lamps as well as candles, he found that the heat emitttd in a 

 given time was always in proportion to the quantity of oil 

 or wax burnt, white the quantity of light furnished at the 

 same time varied in an astonishing degree, and depended 

 particularly on the size of the flame, a size which retards 

 its cooling: a small rush-light, for instance, gives sixteen 

 limes less light than a common taper, although burning as 

 much wax and heating the same quantity of water to the 

 same degree. 



Thus every thing which supports the heat of the flame 

 contributes to increase the light, and we may attain some 

 most astonishing results. 



Count Rumford, who had ascertained by previous experi- 

 ments that every flame is transparent with respect to another 

 flame, has combined his two discoveries ; and having con- 

 structed lamps in which several flat wicks placed parallel to 

 each other, mutually preserved each other from cooling, he 

 made them procUice a light equal to fortv tapers; and he 

 thinks the intensity which we may reach has no bounds : 

 formerly it was considered inqiossible to carry the effect of 

 light beyond a certain length, because, by enlarging too 

 much the wicks with a double current of air, their light 

 diminished in virtue of the causes to be accounted for by 

 the following experiments : — 



What we have said above of the cooling of bodies by eva- 

 poration, is a particular case of the law according to which 

 every body which is dilated absorbs heat, whereas it is li- 

 berated by condensing. This law is nevertheless subject 

 to some exceptions, and some of them have been long since 

 known and explained: such as those of nitre, which pre- 

 serves on many occasions, in con<lensini>;, a great proporlioiv 

 of heal, the effects of which are sufficiently perce))tibk' at 

 the moment of the combustion of gunpowder; but tliere 

 are also some of these exceptions which depend upon more 

 obscure causes, (siich as that which has been made known 

 by M.Thillayo, pmfcssor in the Imperial School. 



The mixture of alcohol wiiii water is always accompanied 



by a rise in the lempcraiure, and there ii> in general a 



E 2 stronger 



