OK EVAPORATING FURNACES. 115 



ments have indeed been made in the conftruQion of furnaces On the com- 

 within thefe few years, but they are only advances towards ''"!^'°" ^'^ ^""i'^ij 

 perfedlion, and are yet very far from being carried to the ex-of furnaces. 

 tent of which they are capable. This will no doubt be the 

 cafe with the alterations I am about to propofe ; for thefe will 

 enable us to make new obfervations, which moil probably will 

 lead to further alterations ftill more important. 



Concerning Evaporatory Furnaces. 



The phyfical impoffibility of raifing the temperature in eva- 

 porating furnaces as they are at prefent conftrudted, is one of 

 the caufes which has always appeared to me moft ftrongly in- 

 imical to their improvement. For it muft not be imagined that 

 the intenfity of the heat will be in proportion to the quantity 

 of matter in ignition, or that caloric will not be more copi- 

 oufly produced by the fame quantity of combuftible under 

 certain circumftances than others; as for example, when the 

 temperature is already very high, the produfls of heat from a 

 combuftible fo fituated will be much more coniiderable than 

 thofe from the fame combuftible burned in a furnace, the tem- 

 perature of which is conftantly deprefled by the evaporation of 

 the liquid in the boiler. 



To prove that it is only by virtue of a temperature already 

 elevated that we can obtain an advantageous combuftion, I ftiall 

 take Argand's lamp, which will afford a comparative inftanc* 

 on a fmall fcale of the efFe6t produced by the intenfity of heat 

 during combuftion. When thefe lamps have their glafs chim- 

 ney, they afford a very brilliant light, and the oil will emit no 

 fmoke. But if the chimney be taken off, the oil will immedi- 

 ately burn duller, the light will be lefs intenfe, and the wick 

 will give out fmoke. This effect ftiews that it is the current 

 of air in the chimney, and the heat it keeps up round the wick, 

 which contributes to the effedl of the combuftion. What ftill 

 adds weight to this opinion is, that the perfection to which this 

 kind of lamp is brought depends principally on the form an4 

 proportions of its glafs chimney. 



This example muft then naturally lead us to think that eva- 

 porating furnaces, as they are made at prefent, cannot ad van- 

 tageoufly promote combuftion, ftnce the bottom of the boiler, 

 which is continually kept at the fame degree of heat by the 

 evaporation of the liquid in boiling, conftantly prevents the 

 I 2 rife 



