SCIENCE. 



245 



The work, as may be gleaned from the few quotations 

 given above, abounds in a variety of new and original ideas, 

 many of them elaborated, others rather fragmentary, but 

 all of them bearing the stamp of the author's genius and 

 containing an inexhaustible source of information and 

 elucidation on the somewhat abstract subject of chemical 

 theory. It may be added that Mohr does not approve of 

 the modern valuation of atoms, but uses exclusively the 

 old formula?. 



The Allgemeine Theorie, etc., is written in the same vein, 

 in fact it is introduced by the author as a supplement to the 

 former, the ' Mech. Theory of Chem. Aff.' Its purport is 

 to lay out the different regions of physics as far as they re- 

 late to motion and force ; it also gives an outline of the 

 method by which the correlation of forces and especially its 

 grandest practical achievement, the ' Mechanical Theory of 

 Heat,' should be made the basis of natural philosophy. 



Mohr insists particularly on the complete eradication of 

 the wrong use of the two words : motion and force : 

 " Motion (actual energy of the English writers) implies a 

 change of place, and consequently force {potential energy) 

 comprises those states in which a change of place does not 

 obtain." (1. c. p. 12). 



" Steam force is therefore a correct expression and desig- 

 nates the tension (potential energy) of the steam. As soon 

 as the piston moves, the tension disappears (potential is 

 converted into actual energy). In the flying wheel and the 

 balancier we have motion (actual energy) and not force 

 (potential energy) when ' running empty,' but motion and 

 force, when there is some work done at the same time " 

 (Ibid.) 



The translation of these few passages will suffice to 

 show the principal aim of the author: To obtain perfect 

 clearness of expression by distinguishing between the two 

 forms, to either of which every ' causa efficiens' in nature 

 belongs, viz., force (potential energy) and motion (actual 

 energy). Prompted by the same desire, he makes these 

 divisions in classifying those ' catisce efficientes,' to wit : 

 "A. Motions. 



1. Motion of bodies ox progressive motion. 



2. Light and Heat in the state of radiation or radiating 

 motion. 



3. Common, conducted heat or conducted motion. 



4. Galvanism ox flowing motion. 



5. Chemical Affinity or clinging inherent motion. 

 B. Forces. 



1. Gravity. 



2. Magnetism. 



3. Electricity. (Static.) 



4. Cohesion." 



This division is based on the property of introconversion 

 which is peculiar to the former, not to the latter. The five 

 'motions' are introconvertible and also convertible into 

 either of the latter (the forces), while these are only conver- 

 tible into the former. 



The reason for his peculiar view that heat does exist in two 

 diffei-ent forms of motion, he states in these words 1 



" After due consideration of the matter in question, I 

 feel compelled to separate radiating from conducted heat 

 Radiating heat is not really heat: it does not expand bodies; 

 it doe* not act on the thermometer nor on the thermo-elec- 

 tric column. That is what conducted heat does. The fact 

 that radiating heat is converted into conducted heat, as soon 

 as it strikes a body which does not reflect it, is no reason 

 why we should declare the heat-rays to be heat already, for 

 the luminous rays and the galvanic current must undergo 

 the same change (before being capable of acting on the 

 thermometer or on the thermo-electric column.) The mode 

 of motion obtaining in the heat-ray differs so radically from 

 that in conducted heat, that the separation adopted above 

 seems to me fully justified. The circumstance that the 

 heat-ray does not penetiate the different fluids of our eye- 

 ball, or if it do penetrate them, is not transmitted through 

 the optic nerve, constitutes only a mechanical difference as 

 compared with the visible ray of light, and we justly con- 

 clude that this depends entirely on their different wave- 

 length." 



The manner in which the various modes of motion and 

 of force are measured by means of the application to 



them of the mechanical theory of heat, is particularly 

 dwelled upon. From this it appears that of theySW modes 

 of motion, two only — mechanical motion (of bodies) 

 and heat are reducible to absolute measurement. These 

 two may be compared by means of the unit of heat, ex- 

 pressed in kilogrammometers. 



Another line of thought which Mohr has first dared to 

 follow and to pronounce upon, is the one dealing with the 

 real value of the use of mathematical expressions in natural 

 philosophy. 



He holds that the value of mathematics is only second- 

 ary, and is, as a rule, greatly overrated. He says: 



. . . "For, such propositions are proved by logical 

 reasoning only, not by applied mathematics. A formula is 

 nothing but the mathematical expression of the true inward- 

 ness of certain phenomena, as it has been previously found 

 by intellectual observations and reasonings. What is ob- 

 tained as a result through the formula, is in it from the begin- 

 ning ; it is not at all a discovery made by the mathemati- 

 c an. If the first equation is wrong, the conclusions arrived 

 at will be wrong. The ancient mathematicians who knew 

 nothing of algebra, logarithms and the differential calculus, 

 had to reason logically exclusively ; even the lack of the 

 Arabic numerals was a great drawback to them. That we 

 are able, by the help of all these advantages, to deduce 

 quicker and more accurately a number of relations from a 

 given equation, only diminishes the intellectual merit of 

 our work but not the practical value of the result. Mathe- 

 matics have only one object, to wit: to evolve from certain 

 given conditions the unknown quantity." 



It cannot be denied that there has been, of late, an 

 obvious tendency to overestimate the value of matnematics 

 in the philosophical investigation of physical problems. 

 And it is not the least of Mohr's many merits that he has 

 conclusively shown that their importance is secondary 

 only, and that their intrinsic value is far below that of logi- 

 cal deductions. "A mathematician can never discover a 

 new law in physics ; that is done by observations only and 

 by the logical reasonings, based on such observations." 



By thus defining the proper limits and the legitimate 

 domain which mathematics should occupy in natural phi- 

 losophy, Mohr has certainly rendered an invaluable service 

 to numerous students of nature. Great numbers of such 

 have been frightened by the many hundreds of pages filled 

 with mathematical formulae, with which we now see treatises 

 on physics so profusely interspersed. 



A perusal of this somewhat fragmentary work does not fail 

 to leave the impression in the mind of the reader what a 

 great misfortune it was that the author should have been 

 called away before he had given us a more complete and 

 comprehensive treatise on the subject ; numerous requests 

 addressed to him with this view were invariably met, as we 

 are informed, by^ a modest decline, on the ground that 

 much was to be done before this could be ventured upon. 



His immense capacity for progressive thought is best 

 illustrated by quoting from one of the "Appendices," happy 

 after-thoughts, to this work. There he says : 



" In determining the conception of the word force in con- 

 tradistinction to motion : gravity, cohesion and magnetism 

 seem to be without any inherent motion, as we have no in- 

 dication that there is such motion. On the contrary, the 

 tension of compressed air, the action of an explosive mix- 

 ture, etc,, appear in the form of inherent molecular motion, 

 which could appear dead only in comparing it to mechan- 

 ical motion (cf solid bodies). In a compressed gas there 

 is perceptible heat and chemical affinity (motion). The 

 perceptible heat acts on the thermometer only, but not as a 

 force on the walls (of the enclosing vessel), while that 

 amount of heat which is available for expansion, and which, 

 while the (movable) wall of the enclosure (piston in cylin- 

 der of steam engine) is receding, is actually used for ex- 

 pansion, implies tension directed outwardly. Since, there- 

 fore, we recognize in this the effects of actual energy (mo- 

 tion) as being of the same nature as those of potential en- 

 ergy (/era — gravity, for instance), acting by pressure or 

 tension (supported or suspended weight), the question 

 arises 'whether magnetism, gravity and cohesion are not diff'et- 

 ent forms of an inherent molecular motion of which we have, 

 as yet, not the slightest conception." 



This idea has occupied his mind during the last years of 

 his life particularly, and he has published one of a series 



