22 MICROSCOPIC ILLUSTRATIONS. 



terminations of those vessels ! These are as distinctly 

 discoverable as in the finest preparations of a recent plant. 

 And what can be more amusing and instructive than the 

 examination of the silicified woods, when formed into 

 sections no thicker than the paper of a Bank-note? 

 Thus rendered pervious to light, the organic structure of 

 the wood becomes plainly distinguishable. And ema- 

 nating from this, what can be a more interesting subject 

 than the inquiry into the mode in which the silicifying 

 process has been carried on — by which the constituent 

 elements of the inmost and minutest portions are changed 

 —whilst their form and situation and colour remain the 

 same ? In investigating also that extinct genus of plants, 

 the Lepidodendra, a similar idea is raised in the mind, as 

 to what must have been the particular state of the earth 

 with respect to atmosphere and temperature at the pe- 

 riod of their growth, and what the changes which have 

 since taken place, in order to bring it to its present 

 condition. 



In our physiological inquiries into the animal and 

 vegetable productions of the present time, the assistance 

 of the microscope is essentially requisite. When Dr. 

 Harvey made his grand discovery of the circulation of 

 the blood, and first lectured upon it, in St. Bartholo- 

 mew's Hospital, in 1619, he was ridiculed, and lost his 

 practice, through maintaining what was then supposed 

 to be so absurd and wild a theory. The idea was 

 suggested to his mind by reflecting on the valves of the 

 heart and veins, which were evidently so planned as to 



