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 formulae. 
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 ‘ causa 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 or flowing motion. 
5. Chemical A ffinity or clinging itiherent motion. 
B. Forces. 
1. Gravity. 
2. Magnetism. 
3. Electricity. ( Static .) 
4. Cohesion." 
This division is based on the property of introconversicn 
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 
different forms of motion, he states in these words : 
“ After due consideration of the matter in quest'on, I 
feel compelled to separate radiating from conducted heat 
Radiating heat is not really heat: it does not expand bodies; 
it does 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 the five 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 ot 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 (of 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 (force — gravity, for instance), acting by pressure or 
tension (supported or suspended weight), the question 
arises whether magnetism, gravity and cohesion are not differ - 
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 
