jfune 20, 1878] 



NATURE 



213 



which are now more appropriately called molecular mixtures. 

 The same fundamental cause further gives rise to the phenomena 

 of cohesion, adhesion, and capillary attraction, and it seems 

 therefore as if the supposition of special molecular forces is in no 

 way necessary any longer. 



Now as the attraction of atoms depends on their quality, it is 

 also clear that the molecular attraction caused by such atomic 

 attraction must,under favourable conditions, produce an orientation 

 of all molecules combining with one another, and must thus lead 

 to bodies of a regular molecular structure, therefore to crystals. 



Lastly, the question whether the properties of atoms are de- 

 pendent on their ludght has much occupied the chemists of 

 modern times. Positive results which could be rendered clear in 

 a few words have not yet been obtained, but it seems, according 

 to the observations made by Lothar Meyer and Mendelejeff, as 

 if not only the chemical properties and specially the chemical 

 quantivalence of atoms and the intensity of their mutual com- 

 bination, but also the physical properties, which at present are 

 ftill treated as constants for materially different objects, were a 

 function and indeed a periodic function of the atomic weight. 

 The mathematical form of this function is no doubt of a peculiar 

 nature, but one thing seems certain, viz., that the numerical value 

 of the atomic weight is the variable by which the substantial nature 

 and all properties dependent on this are determined. 



Thus there again seems hope that it will be possible to reduce 

 ail properties of matter, including gravity, to one and the same 

 force. 



The right of introducing all such speculations into the do- 

 main of exact science, has been questioned very much. It is 

 generally conceded, indeed, that the setting-up of hypotheses on 

 the domain accessible to exact investigation, as a method of 

 investigation, is useful, inasmuch as it often may accelerate the 

 progress of exact knowledge. But it is at the same time often 

 believed that speculations beyond a certain limit are not admis- 

 sible. ^The scientific value of all atomistic considerations particu- 

 larly has ever, and also in the most recent time, been very much 

 doubted. It has been pretended specially that the supposition 

 of atoms did not explain any properties of bodies which had not 

 first been ascribed to the atoms themselves. 



We must own that such remarks contain many truths, but just 

 for that reason it seems necessary that we examine the limit of 

 their correctness. 



It is generally acknowledged that the results of exact obser- 

 vation have the value of facts, therefore possess that degree of 

 certainty which human knowledge can attain at all. It is further 

 not contested that to all those laws which, independent of hypo- 

 theses on the nature of matter, are deduced from facts, nearly 

 the same certainty must be ascribed as to facts themselves. It 

 is just as incontestable, however, that the human mind in the 

 positive understanding of facts does not find complete satis- 

 faction, and that therefore natural sciences have to follow a yet 

 farther and higher aim, that of the knowledge of the essence of 

 matter and of the original connection of all phenomena. 



But the essence of matter is not accessible to any direct inves- 

 tigation. We can only draw conclusions regarding it from the 

 phenomena which are accessible to our observation. And thus 

 it is evident that there is a certain limit which, moreover, is 

 influenced by the state of our knowledge at any given time, 

 beyond which positive investigation loses ground and where the 

 path is only open for speculation. 



If, therefore, the single investigator, following the inclinations 

 of his nature, rests satisfied with positive investigations and 

 renounces all speculations, it is yet clear that to science as such 

 this is not permitted. 



By way of hypothesis, based upon what is known as facts, 

 ideas must be formed on the nature of matter ; the consequences 

 of these ideas must be developed logically, and, if necessary, by 

 the aid of calculation, and the results of these theories^must be 

 compared with the phenomena accessible to observation." 



Of course, the complete truth will never be reached in this way, 

 or there will, at least, never exist complete certainty that our 

 conceptions are really identical with truth. But that conception 

 which is simplest in itself, and which in the simplest manner 

 accounts for the greatest number of phenomena, and finally for 

 all, will have to be considered not only as the best and most 

 probable one, but we shall have to designate it as relatiyely, and 

 we may say, humanly, true- 

 By this the scientific right of existence of speculative in- 

 vestigation is no doubt proved, also for the so-called exact 

 sciences, because beyond a certain limit these indeed cease to be 

 «xact. 



Simultaneously, however,' the scientific value of the present 

 atomic theory is also proved, because it has not been contested 

 that, even in its present and still extremely incomplete form, it 

 accounts satisfactorily for an uncommonly large number of facts, 

 better than any other conception. 



It will certainly require further extension, and also a deeper 

 fundamental structure ; but at present there is very little proba- 

 bility that it will be completely superseded by essentiall;^ 

 different conceptions. ' 



There are other reproaches which have been. made to chemistry 

 specially, and still more to chem-sts, now and since the time of 

 Lord Bacon ; and even chemists cannot deny that they were not 

 altogether undeserved. 



It has been said that chemistry wilfully makes innumerable 

 single hypotheses which are neither in connection with one 

 another nor with the whole ; that the value of hypotheses is 

 over-rated by her disciples, far to3 great certainty being ascribed 

 even to such as are only little justifiable, and that they are treated 

 as if they had been actually proved ; and finally, that her 

 hypotheses are always gradually raised to articles of faith, and 

 that everybody who sins against such dogmas is prosecuted as a 

 heretic. 



Recent times have also, in this direction, brought about a 

 considerable improvement. The justification and the value of 

 hypotheses are now recognised in chemistry, but at the same time 

 the true value of hypotheses is also understood by chemists. 



In chemistry also, as in all domains of science, blind faith 

 in authorities has been crushed, and by this alone the danger 

 of dogmatising is lessened. And should perhaps any one, who 

 holds antiquated views, try to attach his dogma upon pro- 

 gressing science as a restraint, he will always find the striving 

 young generation, the representatives of the future, ready to 

 remove unjustified impediments. If others, in the fiery zeal of 

 youth, should be inclined to look upon daring flights of fancy 

 as scientific hypotheses and to give them out as such, then those 

 who are more moderate by themselves or by the riper experience 

 of age, will always feel it a duty to step in as regulators. 



The school of independent, and at the sime time quiet 

 thinkers, is now so numerously represented also among chemists, 

 that a constant development of the science may be confidently 

 expected, and an overgrowth of weeds need no longer be 

 feared. Also in chemistry we are now well aware of the con- 

 tinuity of human mental work ; the present generation no longer 

 looks with despising contempt upon the work of their prede- 

 cessors ; far from thinking themselves infallible, they know that 

 at any time it remains to the future to continue the work of 

 preceding generations. 



ON THE CA USES OF THE ASCENT OF SAP 



IN TREES ^ 



'T'HE question as to what forces cause water to rise to such 



-'■ a remarkable height (frequently) in trees has had very 



various answers given to it. But these have mostly failed to 



account adequately for the phenomenon. 



Capillary action is perhaps the oldest cause adduced. The 

 view was long popular that water rose in trees like oil in a wick, 

 the connected vessels of the wood forming capillary tubes. This 

 view lost force when it was known that the wood of coniferse 

 was without vessels ; and it did not explain the weakening or 

 stoppage of the rise of sap produced by amputation of the roots, 

 nor the presence of air in the columns of sap. 



Shortly after Dutrochet's thorough study of difhision, this 

 phenomenon was called in to account for the rise of sap. One 

 grave objection to such a theory is the rapidity of the ascent of 

 sap (it has been carefully measured) as compared with the slow- 

 ness of diffusion, which depends simply on molecular motion ; 

 another is the inevitable consumption of the osmetic force of 

 tension. So that other problematical forces had to be called in. 



When Jamin found that the imbibition of water through fine 

 porous substances {e.g. blocks of gypsum) took place with great 

 force, and that the air could thus be compressed to several atmo- 

 spheres, an effect of this nature was afhrmed to occur in living 

 plants, the cell membrane being considered a porous substance. 

 But in fact the natural saturated cell membrane has no air-filled 

 pores, but only pores already filled with water, and even the 

 hollow spaces, bounded by the cell membrane, are partly filled 

 with water ; besides, the fact that a branch, immediately after 

 being cut off, loses in great measure the power of raising water, 

 is against this theory. 



' Abstract of a recent paper In Der Naturforicher. 



