12S jORt-fOtlOS. 



hare bronght in fupport of the general principle " that the 

 particles of gafes prefs only on thofe of their own kind;" and 

 in fo applying them, I trufl I have conformed to the rigorous 

 laws of philofophical induction. Such general principles are 

 not uncommon in natural fcience; and though, like the law of 

 gravitation and f )me other iefs comprehenfive ones, they may 

 be mathematically purfued and inveftigated, yet the^y are not 

 derived from mathematical reafoning, but from a method of 

 enquiry which Newton himfelf did not difdain (o employ,— 

 that of induction on the bafis of experiment and obfervation. 

 Tiie fpeculations of Mr. Dalton, being founded on wellafcer- 

 (ained fa6ls, appear therefore to me to be fairly entitled to the 

 appellation of theory, and not to be included within the defi- 

 nition of hypothefia* handed down to us from the father of ex- 

 perimental philqfophy. I remain, very truly. Sir, 



Your's, &c. 



WILLIAM HENRY. 



XVL 



Defer iption of a very fimple and cheap Contrivance for making 

 Fort Folios qf large Dimenfions. By the late James Mal- 

 TON, Efq.f 

 SIR, 



Method of con- Jl\S I well know the great inconvenience experienced hy 

 ftruaing large artifts and collectors of prints and drawings, from the want 

 ^' ' of portfolios of dimenfions capable of inclofing large fubje^s, 



and as I alfo well know that the means ufed by the Society for ' 

 the Encouragement of Arts, &c. to promulgate knowledge 

 and ufeful information are earnefl as they are extenfive, I am 

 induced to lay before that body a port-folio of my conflru6lion, 

 which I perfuade myfelf pofleiTes every advantage that can be 

 withed. 



The difficulty, or rather the impoffibility of obtaining cafes or 

 port-folios, as large as are fometimes requifite, has given rife 

 to many expenfive contrivances, to the fame end ; or larger 



* " Quicquid ex phaenomenis non deducitur, bypothefis vdcand* 

 eft." Princ. LIII. in Bruckeri Hift. Crit. Phil. Tom. IV. p. 646. 

 fSacicty of Arts, 1803. 



prints. 



