On the Distillation of Wood. 269 



In ordef to render the expression — — integrable, 



« being given, let both the numerator and the denominator of 

 the fraction be multipKed by S * -f {I— a) '' ; the resulting ex- 



pression will be r- , -1 '— . The 



first member will evidently be made rational by putting S = x% 

 and the second by putting l — a = z"- ; so that both are then 

 susceptible of integration by the well known methods for rational 

 fractions. 



Still, however, as the final expression for the time thus deter- 

 mined, is in some cases complex, Bossut, Prony, and others, 

 w^ho have treated this class of questions, recommend the ap- 

 proximative method of squaring a dependent curve, of which I 

 have spoken in the article of my Mechanics*, cited by Mr. 

 Harvey. 



In reply to the optical query of your Correspondent A. M. 

 allow me to refer him to "Harris's Optics," at pp. 120-124 of 

 which he will find detailed some obvious experiments for the de- 

 termination of " the minimum visibile," They would, I fear, 

 occupy more room than could well be devoted to them in the 

 Philosopliical Magazine, unless Harris's book should be out of 

 print. It was published by White, Fleet Street. 

 I am, &;c. 



Rota] Mil. Aradenn-> OlINTHUS GreGORV. 



March 3, 1815. 



XLVIII. On certain Products obtained in the Distillation of 

 Wood, ivitk some Account of bituvwious Substances, and Re- 

 marks on Coal. By J. MacCulloch, M.D. F.L.S. Chemist 

 to the Ordnance, and Lecturer on Chemistry at the Royal 

 Military Academy at Woolwich. 



[Continued from p. 217.J 



-L HERE is a wide interval between the external characters of 

 the lignites and of coal, and though we cannot presume to state 

 the period which Nature has used in her operations, nor during 

 how long a space the causes have continued to act, before the 

 vegetable matter has undergone its ultimate change into coal, 

 nor therefore whether the long continued agency of water and 

 pressure may not have produced the required changes; yet, since 



• A new edition of tliis work, with considerable improvements, is now 

 in the press, and will be published about midsummer, 



philosoj^herl 



