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THE POPULAR SCIENCE MONTHLY 



will ensue, but whether at different geo- 

 logical eras the physical conditions attend- 

 ing the cooling of eruptive masses have 

 been substantially identical. That this has 

 sometimes been the case will scarcely be 

 denied. The problem with which geologists 

 have to deal, however, is not precisely that 

 just stated, for, since the earlier formations 

 have been deeply eroded, while the degrada- 

 tion of comparatively recent rocks is, as a 

 rule, correspondingly small, upper portions 

 of more recent eruptions have to be com- 

 pared with lower portions of more ancient 

 eruptions. The lithological problem is, 

 therefore, greatly complicated. The main 

 purpose of lithology being, as the author 

 believes, to trace the physical conditions 

 through which a mass of readily ascertain- 

 able chemical composition has passed, in 

 the present state of ignorance concerning 

 the effects of temperatures and pressures, 

 the most rational method is to study and 

 record every peculiarity of every occur- 

 rence and every perceptible difference be- 

 tween rocks. . . . Rocks can, of course, 

 never be classified with the sharpness of 

 minerals. Rocks are essentially mixtures, 

 and therefore pass into one another insensi- 

 bly. The wonder is, that rocks not only 

 conform in some degree to a system, but 

 that certain lithological types exhibit such 

 an extraordinary persistence, being met 

 with at the most remote quarters in typiwd 

 development. "While the very nature of 

 things thus excludes a rigid classification 

 of rocks, observation clearly indicates the 

 possibility of reducing them to a natural 

 system. 



rnfonnded Fears. Something is occa- 

 sionally said about the expediency of avoid- 

 ing heavy work in very warm weather. The 

 "Lancet" evidently thinks there has been 

 too much talk of the kind, and expresses 

 views of its own on the matter. It says : 

 " After it has been proved to demonstration 

 that nothing whatever should be done, in 

 schools, particularly, during hot weather, we 

 shall be quite prepared to make common 

 cause with the school-children in the en- 

 deavor to demonstrate that it is undesirable 

 to do anything in cold weather either. In 

 short, let us give up eating, drinking, work- 

 ing, playing, sleeping, and living, in the last 



issue, lest some harm should befall. It is 

 rank nonsense to write and speak, as too 

 many who ought to be better informed are 

 writing and speaking just now, as though 

 every act of existence were beset with peril. 

 It would be better for all of us and the world 

 at large if less ingenuity were spent on the 

 invention of scares, and in the pitiful task 

 of working on the fears of the public." 



Caries of the Teeth. Caries of the teeth 

 has been but little investigated scientifical- 

 ly, according to Professor Busch, of Berlin, 

 because in studying it the external hurtful 

 processes have been alone considered, where- 

 as the second important factor, the resist- 

 ing power of the teeth, has been overlooked. 

 It appears to be peculiar to man, having 

 been observed in no animal ; but not to be 

 characteristic of civilized man alone, for it 

 has been observed in large collections of 

 skulls, including those of prehistoric time. 

 Some races are more disposed to it than 

 others. Certain families are particularly 

 predisposed to it. General habit of body 

 has a pronounced influence upon its devel- 

 opment, as well as certain peculiarities and 

 crises of physical condition and disease. 

 Disposition to caries shows itself even in 

 the developing tooth in the composition of 

 its enamel, which is undulating, whereas 

 teeth with quite smooth enamel have much 

 greater power of resistance. The enamel 

 appears to be the only tissue in the body 

 which is subject to no metabolism, and 

 which remains quite unchanged. Every al- 

 teration in it which is caused by external 

 influences, and every defect of the enamel, 

 remains during the whole of life, and can 

 never be repaired. Dentine also shows dif- 

 ferences in structure as regards its dispo- 

 sition to caries. The dentine tubes cither 

 run regularly closely side by side, when the 

 teeth have greater power of resistance ; or 

 they branch and surround cellular bodies, 

 or even small air-vesicles, when the teeth 

 fall an easy prey to caries. If dentine has 

 been decalcified at any place by the action 

 of acids, it undergoes putrefaction under 

 the influence of bacteria which do not seem 

 to belong to any particular species. Den- 

 tine is sensitive, though nerve-filaments 

 have not as yet been traced into it. Actual 

 toothache does not occur in the course of 



