498 -Art of Sricl-maihtg. — Combiijlkn of Phofphartis. 



what rtiore velocity than that of the wheel itfclf. If the ftream can be brought on with the 

 velocity of three feet at the top of the fall, it will, undoubtedly, be befl ; but if the watef 

 (hould flow from a pond, having no perceptible current, the portion or fpace required to pro- 

 duce this velocity will be about one foot and a half. 



Thefe remarks are equally applicable to bucket or overfhot wheels. 



The art of brick-making is concifely defcribed in the French Encyclopedie. Bergmanrt 

 has attended to this fubjed in a direft chemical way. A tranflation of his treatife intcS 

 Englilh, may be fecn in the third volume of his Phyfical and Chemical -Eflays ; or an abridge- 

 ment of the fame in my Chemical Diftionary, article Brick. As the art itfelf is of extenfive 

 utility, and of confiderable fimplicity in the pradice, I fhould, in this place, have defcribed 

 the procefs, with remarks, if I were not defirous of again infpefling the methods which are 

 pradifed in the neighbourhood of this metropolis. When I fhall have done this, the account 

 he defires will appear. 



Ui 



sm. 



VII. 



On the Combiijlion of Phofphorous, 



To Mr. NICHOLSON. 



PON putting a fmall piece of phofphorus in a {hovel over the fire, in fome experiments 

 I lately made for the entertainment of a company of friends, that fubitance fmoked, began 

 to melt, and took fire as ufual. When the combuftion had ceafed, and we were looking at 

 the ruddy coloured refidue, or acid, one of the company wetted the refidue by fpltting upon 

 it i at which inltant, the combuftion was renewed, with a crackling noife, at every part where 

 the moifture had reached. I fuppofe the refidue to have been oxygenated phofphorus, or 

 phofphoric acidj and that the addition confifted chiefly of hydrogen and oxygen. What then 

 was the new combination, which was attended with the farther extrication of heat ? Will 

 you have the goodnefs to explain this faii, or fubmit it to your correfpondents. 



I am, Sir, 



Your obliged reader, 

 January 4, 1 7^9. , R. S* 



I have complied with the requeft of this correfpondent, by publifliing his letter ; upon 

 which my crther friends will make their remarks, if the fubjcdt fhould appear to require it. 

 If phofphoric acid, at an elevated temperature, in contaft with iron, can be made to emit heat 

 and light by the addition of water, as R. S. apprehends, the true explanation of the play of 

 affinity mull be fought from varied experiments, in which all the products, as well gafeous 

 as fixed, Ihould undergo examination. But in the inftance before us, I am much inclined 

 to' think the renewed combuftion is produced by a portion of phofphorus, which efcaped 

 the atmofpheric action in the firft burning. That is to fay, when a piece of phofphorus i& 



burned. 



