276 PROCEEDINGS OF THE AMERICAN ACADEMY. 



aqua regia and in some cases with a similar combination of acids in 

 which the hydrochloric acid was replaced by hydrofluoric. The last 

 reagent is recommended as the most effectual one which has been found 

 for softening and disintegrating even the hardest coals, such as anthra- 

 cite, etc. In this fluid, both the nitric and hydrofluoric acid are used 

 of full strength. The last method was found particularly advantageous 

 in preparing the bogheads of Autun and the so-called oil-shale of New 

 South Wales for microscopic examination. Sometimes it is an advan- 

 tage to return the coals to alkaline alcohol after treatment Avith the 

 various acids described. In such cases care must be taken to wash out 

 all the acid and to thoroughly dehydrate the pieces of coal before 

 transferring them to alkali in alcohol, as otherwise they suffer disastrous 

 swelling. In all instances, no matter what devices were used for soft- 

 ening the coals, they were embedded in celloidin before cutting. The 

 presence of any free acid or alkali in the material is disastrous to the 

 knife in the first instance or to the consistency of the celloidin in 

 the second. After the sections are cut they are dehydrated in a mix- 

 ture of absolute alcohol and chloroform, to avoid softening the celloidin 

 matrix. After clearing in benzole or xylol, they are mounted in balsam. 

 In a few instances, such as the oil-shale of New South Wales, where 

 the sections are very light colored, it was found advantageous to mount 

 in glycerine jelly. In the case of serial sections, the best procedure is 

 to lay the sections on a slide as they come off the microtome knife and 

 then dehydrate and clear them carefully in their order. The usual 

 methods of cutting celloidin series do not answer in the case of coal, as 

 from the nature of the material frequently sections become folded or 

 torn in transferring to glass. As not above four or five sections are 

 ordinarily needed in series in a given case, to elucidate structural 

 features, the method described above is not so laborious as it might 

 appear. 



The Structure of Cannel Coal. 



It will be well as a preliminary to the observations on so-called algal 

 coals (bogheads, oil-shales, bituminous schists, etc.) to describe the 

 structure shown by ordinary cannel coal. It has long been known and 

 has been made particularly clear by the monumental investigations of 

 Renault, that what is usually known as cannel coal is composed very 

 largely of the flattened spores of vascular cryptogams. Figure 1, 

 Plate 1 shows the structure of cannel coal as seen in sections vertical to 

 the layering in a moderately high microscopic magnification. The 

 linear light bodies in the dark matrix are the flattened spores. The 

 remains of the originally rotund central cavity can be seen as a dark 



