Description of a Htjdro-pneiimalic Taile. 247 



he says, *' In hoc autcm opuseulo nullus erit fictioni locus, 

 cam si excipias, qiue ad lacum Mexicanum canentes poeta$ 

 indncit. jQua; vidi rctero, quoeque milii testes oculati, c^- 

 teroquin yeracissimi, retulere. Pra;terea curae niihi fuit 

 oculatorum testiuni auctoritate subscripta, quse rariora 

 sunt, confirmare." I am, sir, with much respect. 

 Your obedient servant, he, 



Philadelphia. Nov.l, 1809. BENJAMIN SMITH BaRTON. 



XLTI. Description of- a Hydro-pneumatic Table for coU 

 lecting ctfid traiufei'ring Gases, and for experimenting on 

 permanently elastic Fluids. By Fredrick Accum, 

 Operative Chemist, Lecturer on Chemistry and on Mi' 

 neralogy and Pharmacy, M.R.LA., F.L.S.^ ^c* 



J- HERE is but one sure road to arrive at truth in all de- 

 partments of experimental knowledge, namely, to consult 

 nature herself by operative experiment ; and this becomes 

 the more necessary as the subjects to be studied are more 

 complicated and extended. 



In mathematics, the propositions necessary to be known 

 for the acquisition of knowledge, are self-evident, and ad- 

 niilted as soon as announced, and from their ready admis- 

 sion the student is led to the explanation of the most com- 

 plicated truth. In the system of chemical science it is 

 otherwise. This branch of inquiry, it may be said, opens 

 with a detail of various processes which are by no means 

 self-evident, nor easily to be repeated by those who have 

 not yet a claim to the title of a chemist. 



To awaken the ardour of chemical research, as well as to 

 give facility to the practical acquisition of knowledge at the 

 least possible cost, and in the most advantageous manner, 

 must therefore be an object of some importance to the ad- 

 vancement of science ; for to the abridgement of labour, and 

 the superior aid of moderii instruments of research, does 

 the science of chemistry owe its rapid strides towards 

 perfection. 



It is well known that before the invention of the ther- 

 mometer, men were accustomed to judge of the different 

 degrees of heat and cold, by the sensations produced oil 

 their organs of feeling, and the estimates nmst have b«en 

 often highly exaggerated, and always vague and fallacious. 

 It must be acknowledged that a number of valuable facts 



* Communicated by Mr, Accum. 



(^ 4 would 



