ON THE MICROSCOnC STRUCTURE OF FLINTS, &C. 235 



interesting, indeed, when first discovered, but all appearing to be 

 foreign bodies intruded into the situations they occupy rather than 

 cognate with the substances that enclose them. 



I was very soon dissatisfied with this, as I felt that the bodies 

 imbedded in them threw no light, or scarcely any, on the nature of 

 the nodules themselves. So I reflected upon the matter, and soon 

 saw that, in their own proper substance, either they were homo- 

 geneous or they were not. If they were homogeneous a solvent 

 would act equally upon them in every part ; if they were not, the 

 solvent would probably act unequally, and possibly throw some 

 light on their constitution. In arranging the experiment, it was 

 obviously advisable to let the action of the solvent be as gentle as 

 possible, but at the same time efficient. 



The first examined was a Nodule of black Flint from the chalk, 

 the study of this substance having indeed first drawn my atten- 

 tion to the general group. I cut a conveniently thin slice of it, 

 say gLth inch in thickness, and placed it for about an hour in 

 hydrofluoric acid ; then removed it, washed it gently by aspersion 

 with distilled water, and dried it. Originally black and translu- 

 cent, it was now white and opaque ; it could not have undergone 

 any chemical change, the acid merely acting as a solvent, so that 

 the alteration must be due to a change in the physical character of 

 the surface. On examining it by the microscope as an opaque 

 object the appearance j) resented is what I have shown in this draw- 

 ing (PI. IX., Fig. 1), a structure apparently consisting of fibres 

 ramifying in all directions, the organic character of which can 

 scarcely be doubted. This was very striking and suggestive, par- 

 ticularly as I found the same structure to pervade the whole mass 

 almost uniformly. 



Having made this discovery, it occurred to me to try similar 

 experiments with other nodules. Accordingly I set to work upon 

 Coprolites, Septaria, and several others. I had no difficulty in 

 finding solvents, dilute hydrochloric acid acting upon them readily. 

 I submitted them to the prolonged and gentle action of this acid, 

 washed, and dried them, and on examination by the microscope was 

 not a little disappointed to find them converted into a sort of mud, 

 cracked in all directions. I repeated the experiments several 

 times, with weaker acid, stopping its action at an earlier stage, but 

 with no better result, and I had almost relinquished the idea, when 

 somehow or other I thought I might as well examine them in the 



