JEFFREY. — NATURE OF SOME SUPPOSED ALGAL COALS. 275 



have been used which admit of the securing of very numerous and 

 even serial thin sections of the various coals studied. Previous ob- 

 servers have labored under the disadvantage of having to form their 

 views from isolated and not very thin microscopic sections, obtained 

 by the grinding methods of the lapidary. In the present research it 

 has been found possible to adapt the more precise and delicate meth- 

 ods of the Biologist to the remains of organisms long extinct, which 

 compose the mass of some of our most important if not most abundant 

 coals. It will be well to give some account of the methods adopted 

 before beginning the account of the observed facts. 



Methods used in the Investigation. 



It was found in all cases more advantageous to work with material 

 softened by various treatments so that it could be cut successfully on 

 the microtome. Control sections were used in many cases, prepared 

 by the usual method, to make sure that the process of softening had 

 not essentially modified the microscopic structures found in the various 

 coals examined. The same process of softening was not found to be 

 applicable to all kinds of coal. In the case of cannel or canneloid coals, 

 treatment for a week or more with alcohol of seventy jyer cent, saturated 

 with caustic soda or potash and kept at a temperature of from sixty to 

 seventy centigrade, was found sufficient for the preliminary softening. 

 After careful removal of the caustic alkali by repeated treatments with 

 hot alchohol, it was generally found expedient to treat for two or three 

 weeks with the strongest hydrofluoric acid. After washing out the 

 acid, the small fragments of cannel are embedded in the usual way in 

 Schering's celioidin and cut into thin sections on the sliding microtome. 

 (The Jung-Thoma was found very useful for this purpose, on account 

 of its rigidity.) The sections must in many cases be at least five micra 

 thin. Thicker sections do not show details of structure with sufficient 

 clearness nor can they be as advantageously photographed by the 

 microscope as the thinner ones. In well-prepared material it is quite 

 possible to cut serial sections of five micra. Individual sections thinner 

 than five micra may be readily obtained. The preparation of serial 

 sections has the advantage of making it possible to follow the structures 

 observed through a number of sections, by which means their real 

 nature can be more clearly and accurately elucidated. 



In the case of the more resistent cannels and particularly in the case 

 of those coals known as bogheads and oil-shales, more vigorous methods 

 had to be adopted. It was found necessary to treat some of the Amer- 

 ican cannels and particularly American bogheads from Kentucky, with 



