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THE POPULAR SCIENCE MONTHLY.— SUPPLEMENT. 



perfectly well what I do not know of chemistry. 

 If I did not know that, then, of course, I should 

 always be wavering to and fro. But as I imagine 

 that I atu tolerably well aware what I do not know, 

 I say to myself every time I am obliged to enter a 

 domain which is still closed to me : " Now I must 

 begin again to learn ; now I must study afresh ; 

 now I must do as anybody does who enters the 

 domain of science." The great error, which is 

 shared even by many educated people, consists 

 in not remembering that, with the enormous ex- 

 tent of natural science and with the inexhaustible 

 quantity of its details, it is impossible for any one 

 person to master the whole. 



We must acquire such familiarity with the 

 fundamentals of natural science and the gaps 

 which exist in our knowledge, that every time 

 we find a gap of this kind we shall say to our- 

 selves, " Now you enter a domain which is un- 

 known to you." If every one had this much 

 knowledge, many a one would strike his breast 

 and own that it is a ticklish thing to draw uni- 

 versal inferences with regard to the history of all 

 things, so long as he is not entirely master of the 

 material on which the inferences are based. 



It is easy to say: "A cell consists of small 

 particles, and these we call plastidules; and plas- 

 tidules are composed of carbon, hydrogen, oxy- 

 gen, and nitrogen, and are endowed with a spe- 

 cial soul ; this soul is the product or the sum of 

 the forces which the chemical atoms possess." 

 This may be all so; I cannot judge of it exactly. 

 This is one of those points which are yet unap- 

 proachable for me ; I feel there like a navigator 

 who gets upon a shoal, the extent of which he 

 cannot guess. But yet I must say that until the 

 properties of carbon, hydrogen, oxygen, and ni- 

 trogen, are so defined that I can understand how 

 a soul results from their combination, I cannot 

 admit that we are justified in introducing the 

 plastidule-soul into the educational programme, 

 or in asking every educated man to recognize it 

 as a scientific truth, from which logical conclu- 

 sions may be drawn, or on which he may base 

 his conception of the universe. We really can- 

 not demand any such thing. On the contrary, I 

 think that before we put forward such theses as 

 the expression of science, before we say this is 

 modern science, we must first make a whole 

 series of laborious investigations. We must 

 therefore sag to the schoolmasters, " Do not teach 

 this." This, gentlemen, is the resignation which, 

 in my opinion, they ought to exercise who deem 

 such a solution in itself to be the probable out- 

 come of scientific investigation. We can cer- 



tainly not differ on that point for a moment, that, 

 if this doctrine of the soul were really true, it 

 could only be confirmed by a long series of sci- 

 entific researches. 



In the history of the natural sciences is re- 

 corded a multitude of facts which go to show 

 that certain problems remain a long time in sus- 

 pense, awaiting solution. And if this solution is 

 found at last, and found in a direction of which 

 there was a presentiment perhaps centuries ago, 

 it does not follow that during those times which 

 were occupied only by speculation or presenti- 

 ment the problem might have been taught as a 

 scientific fact. 



Herr Klebs spoke of contagium animatum the 

 other day, i. e., the idea that in diseases the trans- 

 mission takes place by means of living organ- 

 isms, and that these organisms are the causes of 

 contagious diseases. The doctrine of contagium 

 animatum is lost in the obscurity of the middle 

 ages. This expression has been handed down to 

 us by our forefathers, and it was very prominent 

 in the sixteenth century. Certain works of that 

 period exist, which propound contagium anima- 

 tum as a scientific dogma with the same confi- 

 dence, with the same kind of justification, with 

 which the doctrine of the plastidule-soul is now- 

 adays advocated. Nevertheless, the living causes 

 of diseases could not be found for a long time. 

 The sixteenth century could not find them, nor 

 could the seventeenth nor the eighteenth. In the 

 nineteenth century we have begun to find con- 

 iagia animata one by one. Zoology and botany 

 have both contributed their contingent ; we have 

 found animals and plants which represent conta- 

 gia, and a special part of the theory of contagia 

 has been confirmed in zoology and botany, quite 

 in the sense of the theories of the sixteenth cen- 

 tury. But you will already have seen from the 

 address of Herr Klebs that the proof is not yet 

 all in. However much we may be disposed to 

 admit the general validity of the old doctrine, 

 now that a series ot new living contagia have 

 been found, now that we know cattle-disease and 

 diphtheria to be diseases which are caused by 

 special organisms, still we may not yet say 

 that all contagious or even all infectious dis- 

 eases are caused by living organisms. After it 

 has appeared that a doctrine, which was pro- 

 pounded as early as the sixteenth century, and 

 which has since obstinately emerged again and 

 again in the ideas of men, has at last, since the 

 second decade of the present century, obtained 

 more and more positive proofs for its correct- 

 ness, we might really think that now it was our 





