16 LIFE OF 



been so generally adopted ; but this is a point on which some 

 diversity of opinion still exists among physiologists. Mr. Knight 

 considered the rays which are seen to diverge from the centre 

 of a horizontal section of the trunk of a tree, and which in 

 longitudinal sections is known as the silver grain, to be the 

 vessels through which the descending sap is conveyed from the 

 bark into the cellular cavities of the wood, there to remain till 

 it combines with the ascending fluid in the following spring. 



The following letter from Mons. Mirbel (though bearing date 

 a few years later) refers to the papers written by Mr. Knight 

 at this time ; and the candid and liberal spirit in which it is 

 dictated is so honourable to the writer, that the insertion of 

 it here reflects even more credit on him than on Mr. Knight. 

 The feelings with which it was received by the latter will be 

 seen by the following note, found among his papers : 



" M. Mirbel has changed his opinions respecting the transmu- 

 tation of bark into alburnum ; and in a private letter conceded 

 the point to me, in so manly and honourable a way, that I 

 really felt much more sorry that M. Mirbel should have 

 found himself called upon to make such a concession, than joy 

 at my own triumph, which I may be supposed to have felt. The 

 conduct of M. Mirbel greatly raises him in my esteem, and I 

 should feel proud to follow his example." 



" Paris, ce 20 Mai, 1816. 



" MONSIEUR : 



" J'ai re9U la lettre dont vous m'avez honore, et je prie rnon 

 ami, Mons. le Comte de Mosbourg, qui part pour TAngleterre, 

 de vous porter ma reponse. Mon ouvrage etoit deja imprime 

 quand j'ai eu connaissance de vos opinions ; elles m'ont d'abord 

 paru specieuses, et ensuite elles m'ont paru tres-bien fondees 

 Vous m'avez ouvert les yeux, et je vous en remercie, car de 

 meme que vous je ne cherche que la verite. Se refuser a 

 1'evidence est une folie dont un savant est tot ou tard puni par 

 la perte de sa reputation. II vaut mieux changer de route que 

 d'en suivre une qui nous egare. Je reconnois aujourd'hui que 

 le liber ne se change point en bois ; qu'il est constamment 



